School Science Lessons
13. Fluid dynamics, atmospheric pressure, barometers, Bernoulli force, hydraulics, non-Newtonian
fluids, turbulence, vortex
2012-05-04b SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
13.0.0 Fluid dynamics
13.1.0 Atmospheric pressure
13.7.0 Barometers
13.2.0 Bernoulli force
13.3.0 Hydraulics
13.6.0 Non-Newtonian fluids
13.4.0 Turbulent and streamline flow
13.5.0 Vortex
13.1.0 Atmospheric pressure
13.1.0 Atmospheric pressure, barometer, Torricelli
4.220 Atmospheric pressure (experiments)
13.1.1 Atmospheric pressure causes liquids to rise in a sipping straw
13.1.4 Card on inverted glass
13.1.2 Measure atmospheric pressure with a bicycle pump
13.1.3 Measure atmospheric pressure with a rubber suction cup
24.2.4 Pressure and boiling point of water
13.1.5 Pressure drop along a line
13.3.4 Syringes for investigating air pressure
4.153 Three holes can, 3-hole can, a vase with three holes, spouting cylinder
23.2.5 Torricelli tube
13.7.0 Barometers
4.230 Aneroid barometer, barograph
13.1.02 Barometer in a bell jar
6.3.1 International system of units (SI) the 7 base units
4.229 Mercury barometer, barometric pressure, atmospheric pressure
15.3.8 Oxidation can affect air pressure
13.1.01 Simple barometer
13.2.0 Bernoulli force
13.2.0 Bernoulli force, Bunsen burner, air streams
13.2.8 Aerofoils, parts of an aircraft
13.2.8.01 Aerofoils, Comments on diagram 13.2.8
13.2.23 Air flow from a hair dryer or vacuum cleaner
4.242 Air streams, Bernoulli theorem
1.43 Air streams (Primary)
13.2.2 Attracting ping-pong balls, attracting aluminium or paper sheets, attracting drink-cans
15.0.4.1 Axis of rotation
13.2.11 Ball in a water stream
13.2.13 Bernoulli cups
13.2.9 Bernoulli force and shower curtain
13.2.12 Bernoulli loop the loop, Bjerknes' tube
13.2.14 Bernoulli pen barrel
13.2.15 Blow ping-pong ball from cup to cup
13.2.4 Card and cotton reel spool
18.3.5.1 Cardboard boomerang
13.2.27 Coanda effect, spoon touches a water stream
13.2.26 Deep breathing exercise
12.3.27 Egg in a bottle (See 4.)
13.2.28 Flettner rotator
13.2.1 Flowing air can do work, flow pipe of uniform cross-section
13.2.10 Hydrodynamic attraction
4.8 Kites, Flying kites (Primary)
13.2.7 Lift from spin, "swerve ball" curve ball, golf balls, Bjerknes' tube
13.2.6 Lift water by blowing, flowing air decreases the pressure, Venturi tube, fly spray, atomizer
3.40 Paper plane (Primary)
13.2.18 Pitot tube
37.4.0 Pressure tube anemometer, wind speed indicator
4.187 Propeller
13.2.3 Push down paper cards by blowing, lift paper strips by blowing
13.2.20 Rayleigh's disc
4.186 Reduce friction with air stream
18.3.5.6 Ships' stabilizers
13.5.8 Soccer balls
13.2.22 Spinning spring scale
13.2.21 Spinning ball
13.3.0 Hydraulics
13.3.0 Hydraulics, hydraulic lift, Pascal's hydraulic press, Bramah press
13.3.3 Hydraulic ram, water ram, water hammer
13.3.10 Pascal's diaphragms
13.3.2 Raise weights by water pressure, hydraulic lift
13.3.1 Water cannot be compressed
13.6.0 Non-Newtonian fluids
Order online: Colour changing heat sensitive putty, PVA (polyvinyl alcohol)
13.6.1 Cornstarch, cornflour slime, isotropy and thixotropy
13.6.2 Density balls in beans
13.6.3 Reynolds' dilatancy
13.6.5 Rising stones, granular mixtures, Brazil nut effect
13.6.0.4 Shear-thickening, stir-thickening, dilatant fluids, rheopectic fluids
13.6.0.3 Shear-thinning, stir-thinning, thixotropy
3.4.11 Slime ball, "Silly putty", silicone polymer to amuse children
13.6.4 Tomato sauce, ketchup, catsup
13.6.0.1 Viscosity of Newtonian fluids
13.6.0.2 Viscosity of non-Newtonian fluids
13.4.0 Turbulent and streamline flow
13.4.8 Mariotte' bottle
13.4.5 Poiseuille flow
13.4.6 Rayleigh-Taylor instability
13.4.7 Reynold's number
13.4.1 Turbulent and streamline flow, shadows, weather maps
13.4.9 Water stream from a tap, faucet
37.42 Weather maps, Buys Ballots law, geostrophic wind and gradient wind
13.5.0 Vortex
Order online: Tornado Tube, joins plastic drink bottles, spiralling vortex
13.5.0 Vortices, vortex
13.5.4 Detergent vortex
13.5.1 Grow a large drop
13.5.2 Liquid vortices
13.5.3 Ring vortices on liquid
13.5.6 Smoke ring
13.5.7 Tornado tube
13.5.5 Tornado vortex
13.1.0 Atmospheric pressure, barometer, Torricelli
Evangelista Torricelli (1608-1647) was the first to explain why mercury rises in a barometer. The space
above the mercury in the barometer tube has been called the "Torricellian vacuum" but that space is
saturated with mercury vapour.
Atmospheric pressure is the pressure due to the weight of the atmosphere on the surface of the earth.
You can measure it from the height of mercury in a barometer. One standard atmosphere, 1 atm. =
101.325 kPa, 101325 n / m2, 1013.25 millibar of pressure is the pressure at the base of a column of
mercury 760 mm high at 0oC. You can measure atmospheric pressure with a Fortin barometer, Aneroid
barometer, or a Bourdon Gauge. The Fortin barometer measures the height of a column of mercury. The
aneroid barometer measures changes in the volume of a vacuum chamber. The Bourdon Gauge measures
the change in shape of a flexible tube. The millibar, mbar or mb, is used in meteorology for recording
barometric pressure. Use a plastic Torricelli barometer made of Lucite (Perspex).
13.1.01 Simple barometer
Barometers are used to measure pressure in gases, air pressure, density and buoyancy of gases, gas
pressure, pressure in a compressed gas bottle, blood pressure with a sphygmomanometer.
the simple barometer and mercury barometer, also known as the Fortin barometer for measuring
atmospheric pressure.
Stretch rubber from a balloon across the mouth of a small jar and fix it in place with elastic bands around
the rim of the jar. The jar now contains a volume of air at air pressure. Lay one end of a drinking straw
over the rubber and fasten it to the centre with glue. Press lightly on the this end of the drinking straw and
see how the other end magnifies the movement and acts as a pointer with the edge of the jar acting as a
fulcrum. Draw a scale on paper and fix it vertically. When air pressure increases the volume of air in the
jar decreases the rubber dips down and the end of the pointer moves up. When air pressure decreases
the volume of air in the jar increases the rubber moves up down and the end of the pointer moves down.
13.1.02 Barometer in a bell jar
Put a simple mercury barometer in a tall bell jar. Evacuate the bell jar and note the mercury level in the
barometer.
13.1.1 Atmospheric pressure causes liquids to rise in a sipping straw
See diagram 37.117
Use a flask with a "straw " of glass tubing and a short glass right angle elbow held in a rubber. When the
end of the bent tube is closed with the finger sipping liquid up through the straw is difficult, but it is easy
when you remove the finger. To show that pressure on the surface of the water is the factor that causes
the liquid to rise in the tube, blowing through the right angle tube can raise the pressure. For a variation of
this demonstration, completely fill a flask with water and close with a rubber stopper containing a length
of glass tubing. Try to drink the water through the "straw "! If you completely exclude air from the bottle,
you will be unsuccessful.
13.1.2 Measure atmospheric pressure with a bicycle pump
See diagram 12.309: Measure atmospheric pressure with a bicycle pump
The average atmospheric pressure at sea level is 1.033 kilogram per square centimetre,
(14.7 pounds per square inch.)
1. Use a bicycle pump with the washer reversed to measure atmospheric pressure. Make the piston
airtight by adding thick oil to the barrel of the pump and sealing the hole for the valve connection. Find
the weight supported by atmospheric pressure by hanging various loads from a hook attached to the pump
handle. Record the weight F. Take apart the pump to measure the inside diameter of the pump. Record
the inside diameter, d. Calculate the cross-section area of the pump, Area = pi x (d / 2)2. To calculate
the pressure, pressure = F / A, where the force is equal to the weight hanging from the pump handle.
2. Make the piston airtight by adding thick oil to the barrel of the pump and sealing the hole for the valve
connection. Find the weight supported by atmospheric pressure by hanging various loads from a hook
attached to the pump handle. Record the weight, F. Take apart the pump to measure the inside diameter
of the pump. Record the inside diameter, d. Calculate the pressure of the air. The cross-section area of
the pump, Area = pi × d / 22. Pressure = F / A The force is equal to the weight hanging from the pump
handle = Atmospheric pressure, Po = 4F / pi ×d2.
13.1.3 Measure atmospheric pressure with a rubber suction cup
See diagram 12.6.3: Measure atmospheric pressure with a suction cup
See pdf: Lil' Sucker
1. Measure the area of the suction cup by pressing it down onto a sheet of glass above a sheet of graph
paper and drawing its outline. Measure the diameter to calculate the area. Record the area A. Attach
the hook of the spring balance to the neck of the sucker. Use a spring balance to find the force required
to pull the sucker away from a smooth surface, e.g. a sheet of glass. Record the force, F. Calculate the
average atmospheric pressure. When there is no air inside the sucker, by fully pressing the sucker on
the smooth surface the force shown on the spring balance equals the force because of atmospheric
pressure acting on the sucker. Pressure = force / area. Repeat the experiment with different size suckers,
then calculate the average atmospheric pressure. The average atmospheric pressure at sea level is
1.033 kilogram per square centimetre, 14.7 pounds per square inch.
13.1.4 Card on inverted glass
Put playing card on full glass of water then invert the glass. Replace the glass by a 50 cm glass tube,
when half filled it cannot be inverted.
Bernoulli's law, principle and effect, (Daniel Bernoulli, 1700 -1782, Switzerland), Bernoulli tubes, uniform
flow pipe, aerodynamics, turbulence, streamlines, air streams, fluid dynamics, streamline and turbulent
flow
13.1.5 Pressure drop along a line
Open tubes along a drain pipe show pressure drop along a line.
13.2.0 Bernoulli force, Bunsen burner, air streams
See diagram 13.242.3: Bernoulli tubes, Venturi tubes
Order online: Balloon Helicopter, Bernoulli'sprinciple.
Daniel Bernoulli, (1700-1782), showed that when the speed of a fluid increases the pressure of the fluid
decreases.
The four forces are as follows: 1. Lift, 2. Weight, 3. Drag, 4. Thrust.
Bernoulli's law, Bernoulli's principle, Bernoulli theorem, applies the law of conservation of energy to fluids
and is the basic law of fluid mechanics. It states that at any point in a fluid flowing with constant speed,
the sum of the pressure, potential energy and kinetic energy per unit volume is constant. Bernoulli's
principle states that when the speed of a fluid increases, the pressure in the fluid decreases. You can
explain aerodynamic lifting force, lift, as a reaction force of the air stream pushed down by the aerofoil.
The longer path length of air passing over an aerofoil does not cause lift. Bernoulli tubes have air flowing
through a tube and a restricted tube and manometers show the pressure of flowing air at points along both
tubes. You can blow air through a constricted tube and measure the pressure with a manometer of
flowing air at points along the restricted tube. Also, you can use open vertical pipes show the drop in
pressure as water flows through a constriction in pipes by placing three pressure-indicating manometers
with bright wood floats located at and on either side of a constriction in a horizontal tube with water flow.
Similarly, when air flows through a restricted tube, manometers show the pressure differences.
Bernoulli's theorem can be regarded as a statement of the principle of conservation of energy for a special
case of an incompressible liquid flowing through a pipe of non-uniform cross section, viz. for any fluid
flowing steadily along a pipe of non-uniform cross section area, the total change in energy per unit mass
taken along a steam line is equal to the work done against the pressure.
.13.2.1 Flowing air can do work, flow pipe of uniform cross-section
See diagram 3.1.4.2: Bunsen burners
The Bunsen burner is an application of Bernoulli's law. It has a small jet at the base of the burner that
delivers gas under pressure. The drop in air pressure around the gas in the tube cause air to be pushed in
through adjustable inlets by atmospheric pressure.
13.2.2 Attracting ping-pong balls, attracting aluminium or paper sheets, attracting drink-cans,
attracting sheets
See diagram 13.1.2: Ping-pong balls
13.2.2.1 Attach threads to two ping-pong balls with adhesive tape. Hold the ends of the threads so that
the ping-pong balls hang suspended 2 cm apart. Blow between the ping-pong balls. They move together
into the region of decreased pressure caused by your blowing action. If you blow hard enough the
ping-pong balls click together.
13.2.2.2 Blow between two suspended parallel and vertical sheets of aluminium foil or sheets of paper.
The suspended sheets move together.
13.2.2.3 Put 10 plastic drinking straws 2 cm apart and parallel on a flat table. Place two empty
drink-cans 2 cm apart on the drinking straws. Blow between the drink-cans. They move together over
the rolling drinking straws.
13.2.2.4 Blow an air stream between two parallel cards on bifilar suspensions.
13.2.2.5 Close a paper envelope by blowing. Hold an envelope end-on and parallel to your mouth with
the flap slightly open. Blow along the envelope under the flap and it closes. A stream of air blown between
paper and a surface will cause the paper to cling to the surface.
13.2.3 Push down paper cards by blowing, lift paper strips by blowing
See diagram 13.1.3: Blowing card
1. Make cards by cutting out two pieces of light cardboard 6 cm × 10 cm.
Fold the cards as follows:
1. about the centre line and,
2. 1 cm from the edge.
Put the folded cards on the edge of the table and blow underneath them. The cards become pressed down
against the table. According to Bernoulli's principle, the faster the air flow, the lower the pressure it exerts.
When you blow underneath the cards, you lower the air pressure underneath them. so the cards are
pressed down against the table by atmospheric pressure.
2. Cut out a strip of paper 20 × 2 cm. Bend it down at one end and hold the bent end in front of your
mouth. Blow over the paper strip and it rises. Some electric fans have paper strips attached to the safety
grille in front of the fan blades. Turn on the fan and the paper strips stream out. This can be a useful safety
device to show that the fan is turned on.
13.2.4 Card and cotton reel spool, lifting plate
See diagram 13.1.4: Cotton reel
1. Insert a thumbtack (drawing pin) through the centre of a 7 × 7 cm piece of cardboard or a playing
card. Place the card on the table with the pin pointing vertically up. Blow through the centre hole of a
cotton reel spool to remove any obstruction. Hold the cotton reel spool over the card so that the pin
points up through the centre of the hole in the spool. Pick up the card and spool, holding the spool with
your left hand and holding the card up against the spool lightly with your right index finger. While blowing
down constantly through the spool, remove your right index finger. Raise the spool and the card lifts as
well. The card appears to stick to the cotton reel. Air moving through the inside of the spool is at a lower
pressure than the air outside the spool. Atmospheric pressure pushes the card against the end of the
spool.
2. Show lifting plates by blowing air out radially out between two horizontal plates. The bottom plate
supports weights hung from it.
3. Spin out the air. Mount a disc hanging horizontally from a spring scale just above an identical disc.
Start the lower disc spinning and the spring scale shows an increase in force.
13.2.6 Lift water by blowing, flowing air decreases the pressure, Venturi tube, fly spray, atomizer
See diagram 13.1.6: Venturi tube, fly spray
1. Observe the action of an atomizer by blowing a jet of air across one end of a U-tube.
2. Make a Venturi tube. (G. B. Venturi 1746 - 1822, Italy) Use two glass tubes or two transparent
drinking straws. Put one tube in a half glass of coloured water. Put the second tube at a right angle with
the first one so that the ends of the two tubes are close together. Blow through the horizontal tube and
observe the water level in the second tube. Moving air has less pressure than stationary air. Since air is
moving over the top of the vertical tube, the pressure in this region is less than atmospheric pressure. Thus,
atmospheric pressure pushes water up the tube.
3. Use a Venturi meter. Use a manometer to measure the pressure difference between the restricted and
unrestricted flow in a tube
4. Fill the beaker about 3/4 full with water and add a few drops of ink and stir. Cut a long drinking straw
half across, fold it over the uncut side. Hold the shorter part with your left hand and insert it into the
coloured water. Hold the longer part with your right hand horizontally. Hold white paper vertically
opposite the horizontal straw. Blow into the horizontal straw and adjust the distance until the coloured
water can wet the white paper. Water goes up the vertical straw when you blow. By blowing through the
horizontal straw, the speed of flowing air gets faster and the pressure gets lower, Bernoulli's principle. As
the pressure at A is smaller than that at B in the diagram, then this pressure difference makes the water
move upward in the vertical straw.
13.2.7 Lift from spin, swerve ball, curve ball, golf balls, Bjerknes' tube
See diagram 13.2.7: Swerve ball, curved ball
1. When there are different airs at different flowing speeds between the two sides of an object, there
will be difference in pressure, thus the travel path of the object will become inclined. A ball can spin
without wind. A ping-pong player may make the ping-pong ball spin and move ahead at the same time
when the player "peels" or "pulls" the ball with the bat. A badminton player may make the shuttlecock
spin and move ahead at the same time by use the racquet. A footballer may kick a "banana" ball for a
corner kick, "bend it". A volleyball player may serve a "floating" ball. When a ball spinning moves ahead,
the difference in pressure between the two sides of the ball, because of the different flowing speeds of
the air changes the travel path of the ball. It may not only prolong the distance the ball travels in the air
and may confuse the opponent who may not even catch the ball.
2. Observe a curved ball trajectory. Throw a curve ball with a Bjerknes' tube. Cut 20 cm down the
centre of a cardboard cylindrical mailing tube. Remove one side of the cut tube and close the other end
of the tube. In the cut end attach a 20 cm long lining of sandpaper. A track covered with sandpaper helps
give a ball lots of spin. Put a Styrofoam ball into the tube and let it fall down inside the tube to the closed
end. Grab the tube near the closed end then swing the tube to make the ball travel up the tube. When the
ball reaches the sandpaper at the other end of the tube the ball will begin to spin as it leaves the tube.
Throw a ping-pong ball with a paddle covered with sandpaper. Use a shaped launcher lined with
Styrofoam to launch curved balls. Throw a polystyrene ball with a shaped launcher lined with emery cloth.
3. Throw a polystyrene ball with a V shaped launcher lined with emery cloth.
4. Wrap 5 turns of string around a cylinder made of cardboard. Do not to leave any space between turns.
Lay the cylinder at the edge of a tabletop and the end of the string under the cylinder. Hold the end then
quick jerk the string so that the cylinder falls and spins at the same time. Observe the motion of the
cylinder. Repeat the experiment by letting the cylinder fall from the edge of the table without spinning.
Observe the motion and compare with the previous experiment. The above experiment shows that a
spinning cylinder may increase upward force. When the cylinder spinning falls, the friction between it and
the air, at one side, hinders the flowing of the air at the other side. Thus, the flowing speeds of the air at
the two sides of the cylinder are different. According to Bernoulli's principle, the pressure in a moving
fluid decreases as the velocity increases. So there is difference in pressure between the two sides of the
cylinder and the upward force forms.
5. Wrap one metre of cloth tape around the middle of a mailing tube and give a jerk. The tube does a
loop-the-loop.
13.2.8 Aerofoils
See diagram 13.2.8: Parts of an aircraft | See 13.4.7: Reynold's number
13.2.8.01 Comments on diagram 13.2.8
Some people say that the Bernoulli's principle is incorrectly applied to understanding aircraft lift and that
Newton's three laws contribute to lift. So the shape of the wing has nothing to do with the physics of lift,
only to the efficiency of lift (reduced drag). Diagram 13.2.8 of an asymmetrical aerofoil does not produce
lift because there is no down wash off the trailing edge of the wing. Down wash is caused by the angle of
attack of the wing and viscosity. Air flow in a venturi tube is not air flow over a wing and a wing is not
half a venturi tube.
However, other people say that two separate and quite different processes create aerodynamic lift for
modern aircraft, Reaction lift and Bernoulli lift. Reaction Lift is the effect of the pressure of moving fluid,
e.g. air, against the bottom of a tilted surface. Consistent with the action and reaction of Newton's laws
of motion, the air that hits the bottom of that tilted surface is deflected downward (action) to create an
equal and opposite reaction, upward lift, in the wing itself. The process of Reaction Lift is naturally
unstable. Bernoulli Lift is entirely created due to the shape of the wing. The upper surface of the wing is
always bulging out more than the lower surface and Newton's Conservation of Energy causes any fluid
flow to have (slightly) lower pressure if it is moving faster. The air that meets the front edge of a wing
must get past it, to meet up again after the wing has gone by. The bigger bulge of the top side of a wing,
(aerofoil), means the air has to move a little faster, to cover the longer distance, than air that went under
the wing where the path was straighter. Bernoulli Lift is simply the effect of this slight difference of
pressure above and below a wing. It only depends on the shape of the wing, the velocity of the air and
the density of the air. It has no dependence on the angle of the wing to the air motion.
1. See diagram 13.2.2: Blow across a wing
Cut out a 12 × 12 cm piece of paper. Draw a line parallel to one side and 1 cm from the edge. Fold the
paper over so that the top edge is along the line. Now the paper is divided into two parts. The bottom
part is 5.5 + 1.0 = 6.5 cm long. The top part is 5.5 cm long. Use adhesive tap or a stapler to attach the
two edges. to make an aerofoil, i.e. a wing. The wing now has a curved edge and a sharp edge. Lay the
wing on the desk so that the shorter 5.5. cm long side is flat down on the desk. The top 6.5 cm side is
curved because it is longer. Hold a round pencil horizontally through the aerofoil so that it hangs down
from the pencil with the longer curved side away from you. Blow sharply across the wing just above the
pencil. The sharp edge of the wing rises. Instead of blowing, repeat the experiment with a vacuum cleaner.
The air you blow along the curved side has further to go and moves faster than air you blow along the
straight side, so its pressure is less. The higher pressure under the straight side pushes up the sharp edge
of the wing.
2. Hold one edge of a sheet of paper horizontally, let the rest hang down. Blow across the paper and
watch the sheet rise.
3. Hold the wing in front of a globe of paper or Plasticine (modelling clay) hanging from a piece of cotton.
Hold the end of the cotton so that the globe of paper or Plasticine is hanging near the vacuum cleaner
outlet. Turn on the vacuum cleaner and note the angle of the cotton to the vertical. Repeat the experiment
with the aerofoil wing between the vacuum cleaner outlet and the hanging globe. By adjusting the position
of the aerofoil wing you can get the globe to move backwards towards the wing because turbulence, eddy
currents, around the wing obstruction creates a partial vacuum. Turbulence produces frictional drag which
slows an object travelling in air or water. The ideal shape is an airfoil which keeps turbulence to a minimum.
When standing on a bridge, you can see that the water below flowing around a pier is turbulent behind the
pier. Air around an obstruction also behaves in the same manner.
4. Blow an air stream between two suspended parallel cards on bifilar suspensions. A stream of air blown
between a paper and a surface will cause the paper to cling to the surface.
5. Connect slant manometers to holes on the top and bottom of an airfoil
6. Strong winds raise the roof. Blow air over a model house to raise the roof. During hurricanes or
typhoons the contents of house may be "sucked" out of a broken window because of high pressure inside
the house and low pressure because of the strong winds outside. In Hong Kong SAR, China, the houses
have locking security windows that leave a small gap to allow pressure to equalize inside and outside the
house.
7. Observe the aerofoil of a Formula 1 racing car at the back of the car behind the driver. Compared to
the aerofoil wing of an aircraft it is inverted because it is not designed to produce lift but a "down force" to
keep the racing car on the road, especially when rounding a corner.
8. Detach the hose from a vacuum cleaner and connect it to where air can come out so the vacuum
cleaner can act as a blower. Remove any nozzle and aim the hose vertically upwards. Turn on the vacuum
cleaner and place a ping-pong ball in the airflow. The ping-pong ball moves to the centre of the airflow.
Tilt the nozzle to one side and the ping-pong ball stays in the middle of the air flow. 4. Direct a stream of
air from a vacuum cleaner at a balanced model aircraft and observe the lift.
9. When a boomerang is thrown, it is held nearly vertically, slightly tilted to the right. The cross sectional
shape is asymmetric as in an airfoil. As the boomerang is thrown, it spins. If the side that is more "bulged"
is on the left side as it is held, a Bernoulli Lift force acts toward the left. This nearly horizontal force vector
constantly acts to curve the path of the boomerang so that it may follows an entire horizontal circle and
returns to the thrower. The rotational spin creates the Bernoulli force vector that is slightly upward of being
straight horizontal to the left. This small vertical component of the force vector overcomes the vertical
weight vector of the boomerang, which keeps it from crashing down. Aerodynamic drag slows down the
boomerang's spin, the Bernoulli force vector also reduces. If the vertical component of the Bernoulli force
drops to less than the weight of the boomerang, it falls and crashes.
13.2.9 Bernoulli force and shower curtain
Use a shower made of light plastic material. Turn on the shower and the curtain moves towards the falling
water due to an Bernoulli effect. The falling water causes a reduction in air pressure under the shower.
13.2.10 Hydrodynamic attraction
Move a small sphere in water and another in close proximity will move because of hydrodynamic
attraction.
13.2.11 Ball in a water stream
Drill out a clear Plexiglas tube to different diameters connect water and show that the ball sits at the
change of diameter despite being tipped upside down
13.2.12 Bernoulli loop the loop, Bjerknes' tube
See diagram 13.2.7: Swerve ball, Bjerknes' tube
1. Pulling a cord wrapped around a mailing tube spins it into a loop. Jerk out cloth webbing wrapped
around a mailing tube to cause the tube to spin through a loop. Wrap one metre of cloth tape around the
middle of a mailing tube and give a jerk so that the tube does a loop-the-loop.
2. Tie one 125 cm of heavy cotton string to the end of a meter stick. Wrap the string tightly around the
exact middle of the Bjerknes' tube, a 90 cm long, 10 cm diameter mailing tube with duct-taped ends,
leaving 30 cm of unwound string between the tube and the meter stick. Start with plenty of slack. Rapidly
jerk the stick to the side at an angle of 20o above the horizontal. The tube spins as the string unravels.
The Magnus effect (Bernoulli effect) will make the tube take off and fly in a "loop-the-loop". Wind the
string so that the tube will have a backspin when the string is snapped. Be careful! Practice the jerk in an
empty room. You can substitute a 2 m long, 4 cm wide strip of cloth in place of the string.
13.2.13 Bernoulli cups
Glue the rims of two or four Styrofoam cups together and launch by letting them roll off the fingers while
throwing.
13.2.14 Bernoulli pen barrel
Remove the filler from a ball point pen place under your thumbs at the edge of the bench. Pop the barrel
out from under your thumbs to give it lots of spin.
13.2.15 Blow ping-pong ball from cup to cup
Put two cups 2 cm apart on the table. Put a ping-pong ball in one cup. Blow obliquely into the cup
containing the ping-pong ball towards the side nearest the second cup or blow in air from a vacuum
cleaner outlet. The blown air with high pressure pushes the ping-pong ball up out of the cup. The low
pressure flowing air above the two cups guides the elevated ping-pong ball towards the second cup.
13.2.18 Pitot tube
A small glass Pitot tube, connect it to a water manometer and measure the varied air stream velocity.
13.2.20 Rayleigh's disc
A lightweight disc turns perpendicular to the air flow.
13.2.21 Spinning ball
Direct a high speed stream of air at a ball spinning on a rotating rod free to pivot perpendicular to the air
stream.
13.2.22 Spinning spring scale
When hanging from a spring, a scale is mounted just above an identical spinning disc, the spring scale will
show an increase in force.
13.2.23 Air flow from a hair dryer or vacuum cleaner
Air flow from a hair dryer or vacuum cleaner
An electric hair dryer and a vacuum cleaner can be used as a laminar flow gas propulsion devices to show
Bernoulli forces. However, the hair dryer should be used only under adult supervision and never in or near
water, not switched on for more than two minutes and not blocked where the air exits. The vacuum
cleaner can be used instead of a hair dryer only if the flexible hose can be attached to the air exit. The
vertical air stream from the hair dryer mover around the balloon to create a partial vacuum above it. The
balloon tends to move up into the partial vacuum but air moving around the balloon come together as jets
of air above the vacuum that keep the balloon in place.
1. Direct the air flow from the hair dryer vertically upwards and balance a ping-pong ball or a balloon on
the flow so that they do not move but remain suspended in the air.
2. Move the hair dryer and vertical air flow next to a wall or a corner to increase the height of the
suspension.
3. Throw the ping-pong ball or balloon into the air and catch them on the vertical air flow from the hair
dryer.
4. Ask someone to hold a balloon with both hands half in the vertical air flow. Hold the palm of the hand
around and above the balloon and feel the air flow jet.
13.2.26 Deep breathing exerciser
See diagram 13.2.26: Deep breathing exerciser
A patient sucking air into the mouth at a rate of 600 cc per second can raise the first ball to the top of the
cylinder. Air rushing past the ball creates a partial vacuum above the ball so it moves up. At the top of the
cylinder the ball where it blocks the outlet and remains at the top of the cylinder because of the partial
vacuum in the passage leading to the tops of the three cylinders. Similarly, a patient sucking air into the
mouth at a rate of 900 cc per second can raise the second ball to the top of the cylinder and a patient
sucking air into the mouth at a rate of 120 cc per second can raise the third ball to the top of the cylinder.
13.2.27 Coanda effect, spoon touches a water stream
See diagram 13.2.27: Coanda effect
The Coanda effect, Henri-Marie Coanda (1885-1972), is the tendency of a fluid stream to attach itself
to an adjacent surface and follow its contour. The fluid stream follows a gently curving surface when it
emerges from a nozzle. Pockets of low pressure turbulence form between the curving surface and the
fluid stream to cause the stream to stick to the wall. The fluid stream jet is pulled onto the curved surface
by the low pressure region that develops as entrainment pumps fluid from the region between the jet and
the surface. (Entrainment is to pull or drag along in a current). The jet is held against the wall by the
resulting pressure gradient that counterbalances the jet's internal resistance to turning. The Coanda effect
has been used to increase the lift of aircraft by using flaps on the wings to bend down and accelerate the
air flow and decrease pressure due to the Bernoulli's principle. The Coanda effect has also been used to
make spinning "flying saucers", e.g. the "Vectron UFO Flying Saucer" and windshield washers without
moving parts. The Coanda effect allows aircraft wings to to bend airflow and change the amount of lift.
Also, in hydroelectric power stations, the Coanda effect allows water to keep close to the curved
surface of a ramp before reaching the turbines, leaving potentially damaging objects and tree branches to
fly off the end of the ramp and not reach the turbines.
1. Hold a spoon by the end of the handle so that it hangs down with the convex side of the spoon bowl
close to a water stream from a tap. Move the spoon so that the bowl starts to enter the water stream.
You can feel a force pulling it further into the water stream. The accelerated flow of water over the spoon
bowl creates the force. The curve of the spoon is similar to the side of a venturi.
2. Hold a finger near a jet of the water stream as it emerges from a tap. Observe the deflection of water
around the finger.
3. Turn on a laboratory tap so that the falling water stream drops into a shallow container. Close the tap
to produce the thinnest possible water stream before the end of the water stream breaks into drops. Hold
your index finger with nail vertical or a cylindrical rod, so that just touches the side of the water stream near
where it falls into the container. The water stream curves around the finger.
4. Place a drink-can or drinking glass between yourself and a lighted candle on the bench. Blow air on the
middle of the drink-can. The air curves around the drink-can and the candle flame flickers and may be
extinguished. Replace the drink-can about the same width. Blow air on the middle of the drink-can. The
candle flame does not change.
5. Blow air from a vacuum cleaner at a large cylindrical can, e.g. a waste paper can or garbage can. Use
a light piece of paper to find the direction of air around the large can.
13.2.28 Flettner rotator
If you direct an air stream at a rotating vertical cylinder on a light car, the car will move at right angles to
the air stream. A car with a spinning Styrofoam cylinder moves perpendicular to an air stream.
13.3.0 Hydraulics, hydraulic lift, Pascal's hydraulic press, Bramah press
See diagram 12.5.0: Hydraulic press
Hydraulics is the study of the flow of fluids.
The principle of Pascal's hydraulic press is used in hydraulic machines, car jack, car brakes, wool press
and to raise freight and passenger lifts.
Pascal's principle states that pressure applied to an enclosed fluid is transmitted equally and undiminished
in all directions throughout the fluid. So a force applied to a small area piston is transmitted through an
enclosed liquid to a large area piston that moves with a larger force. In a car brake, foot pressure on the
brake pedal is transmitted mechanically to the piston of a master cylinder that transmits pressure to front
and back brakes in the wheels through the brake fluid. Pistons in the wheels push the brake shoe with
attached brake lining against the brake drum to increase friction between the brake lining and break drum
and slow or stop the car. Machines may use different forms of petroleum oil as the working fluid.
1. Connect a rubber hose to a motor car hand pump and bind the connections with wire and adhesive
tape. Connect the other end of the hose to a water tap. Fix a weight to the handle of the pump. Turn on
the water tap and see the water pressure lift the weight.
2. Connect a 2 m vertical glass tube to a hot water bottle and sit on the hot water bottle and watch water
move up the vertical glass tube.
3. Use a hydraulic press with a pressure gauge to break a board or compress a large spring.
4. Blow into the mouths of two hot water bottles that support a person to lift up the person.
13.3.1 Water cannot be compressed
See 34.5.03: Bulk modulus, modulus of incompressibility, K
Fit a bottle with a one-hole stopper fitted with a glass tube or a medicine dropper passing through it. Fill
the bottle with water until overflowing. Insert the stopper tightly until the water rises slightly in the glass
tube or medicine dropper. Grasp the bottle in your hands and squeeze as hard as you can. Water rises in
the tube because you cannot compress water. By simply filling the bottle and inserting the stopper so that
water rises in the glass tube would adequately show that water cannot be significantly compressed. [Some
teachers have tried this experiment several times but cannot get the desired effect.]
13.3.2 Raising heavy weights by water pressure, hydraulic lift
See diagram 12.5.2: Raise a girl, raise books
See diagram 12.274: Raise heavy weights with water pressure
Use a rubber hot water bottle. Put a one-hole stopper carrying a short glass tube tightly in the neck.
Punch a hole in the bottom of a plastic container and make it large enough to take a one-hole stopper.
Put a short piece of glass tubing through the stopper. Connect the water bottle and the container with
1.25 m of rubber tubing. Wind wire or adhesive tape firmly around the connection at the bottle. Fill the
bottle, tube and can with water. Place the bottle on the floor and put a heavy object or a little girl on it.
Raise the plastic container above the level of the floor. The heavy objects or the little girl rise. Note how
heavy a weight you can lift by raising the plastic container.
13.3.3 Hydraulic ram, water ram, water hammer
See diagram 12.277: Hydraulic ram | See diagram 12.2.18: Hydraulic ram
A hydraulic ram or impulse pump is a device which uses the energy of falling water to lift a much smaller
amount of water to higher elevation than the source Water flows from the source through the drive pipe
and escapes through the waste valve, "clack valve", until it creates enough pressure to suddenly close the
waste valve. Water then flows through the delivery discharge valve into the air pressure vessel where it
compresses trapped air. When the pressurized water reaches equilibrium with the trapped air it closes
the delivery discharge valve. Pressurized water then flows from the air pressure vessel and up the delivery
pipe to the destination storage tank. The closing of the delivery discharge valve causes a slight vacuum
that allows the waste valve to open again. The cycle repeats many times per minute, depending upon the
flow rate. A hydraulic ram is cheap to build, easy to maintain, and very reliable. It does not need any fuel.
The air pressure vessel contains air between the pump and the delivery pipe to cushion the shock when
the waste valve closes and improves the efficiency by allowing a more constant flow through the delivery
pipe. The difference in height between the water source and the pump site is called vertical fall or delivery
head. The difference in height between the pump site and the point of storage or use is called the lift or
supply head. The hydraulic ram is suitable for flowing streams with steep slope where some water is
needed at a higher place.
A water hammer is a pressure surge caused by the kinetic energy of water when it is forced to stop
suddenly. Moving water in a pipe has kinetic energy proportional to the velocity of the water × mass of
water in a given volume. For this reason, pipe flow velocity below 1.5 m / s are recommended. In home
plumbing, you may hear a loud bang resembling a hammering noise. Stand pipes open at the top may be
added to water systems to provide a cushion to absorb the force of moving water. Stand pipes open at
the top may be added to water systems to provide a cushion to absorb the force of moving water.
Hydraulic rams are sometimes used to raise water from a low level to a higher level. A flowing stream of
water operates them. A large quantity of water that falls from a small height pumps a small quantity of
water through a large height. A hydraulic ram lifts water higher than the supply. Hydraulic rams are used
to raise water from a low level to a higher level. They are operated by a flowing stream of water so need
no power source but they may be noisy!
1. Make a model hydraulic ram. Remove the bottom of a plastic drink bottle. Fit the bottle with a
one-hole rubber stopper carrying a short length of glass tubing. Connect the glass tubing to a glass or
metal T-tube that has a piece of rubber tubing on one end and a jet tube connected to it with a rubber
tube. Fill the bottle with water and pinch the tube at the end. Let the water run from the end of the tube.
Stop the flow suddenly by quickly pinching the tube, and note the height to which the water squirts from
the jet tube. Let the water flow and stop alternately, and you have a working model of the hydraulic ram.
2. Use a plastic bottle from which the bottom has been removed. Fit the bottle with a one-hole rubber
stopper carrying a short length of glass tubing. Connect this to a glass or metal T-tube that has a piece
of rubber tubing on one end and a jet tube connected to it with a rubber tube. Fill the bottle with water
and pinch the tube at the end. Let the water run from the end of the tube. Stop the flow suddenly by
quickly pinching the tube, and note the height to which the water squirts from the jet tube. Let the water
flow and stop alternately, and you have a working model of the hydraulic ram.
4. Use a plastic bottle with the bottom has been removed. Insert a one-hole stopper fitted with a glass
tube bent at a right angle. Use a rubber tube to connect to one arm of a glass T-tube. The open end of the
centre part of the T-tube points upwards. Connect the other arm of the T-tube to a rubber emission tube.
Pour water into the plastic bottle while holding the end of the emission tube. Observe the water spouting
from the T-tube. Let go of the emission tube so that water flows through it and not water is flowing up
through the T-tube. Hold the emission tube suddenly again. This stops water flowing up through the
T-tube and water spouts up from the centre of the T-tube again. The height of the flowing water is higher
than the original height and flows with more force. Let the water flow and stop alternately and rapidly, you
have a working model of the hydraulic ram. If elasticity of the emission tube exerts a great influence on
height and strength of flowing water, you can remove the emission tube, use your index finger and middle
finger to squeeze the T-tube, then press the T-tube tube with the thumb directly.
13.3.4 Syringes for investigating air pressure
See diagram 12.5.4: Model syringe force pump | See diagram 12.4.3.2: Force pump
1. When the tip is sealed, use the syringe to compress the air or to produce a partial vacuum. Attach a
small piece of plastic tubing to allow you to seal the tip with pinch clamps. Seal a syringe by pushing the
tip into a wooden or plastic block that has been drilled to the proper size. With such a base as a platform
use the syringe in a vertical position for applications such as serving as a balance for measuring weight by
air compression.
2. Fill the syringe with a small amount of air and hang it inverted to serve as a "spring-type" balance.
3. Compress moist air within a syringe to cause water condensation and form "artificial rain".
4. Attach a piece of plastic tubing 20 or 30 cm long to make a simple syringe pump. Put water in the tube
to make an air thermometer or use 12 metres of tubing to make a water barometer.
5. Couple two syringes with a piece of tubing to show pressure changes within closed systems. You can
easily quantify all these experiments because syringes are already graduated.
6. To show compressibility of liquids, fill a syringe with water and then, having made sure no air is present,
seal the outlet with the finger and try to move the plunger. You cannot you move it. Draw some air into
the syringe. Seal the outlet with the finger and try to move the plunger. You can move the plunger because
air can be compressed.
7. Connect two syringes of different sizes and you can feel the pressure difference.
8. Compare water / air compression in a syringe. You can compress a syringe filled with air with a large
weight but you cannot similarly compress a syringe filled with water
13.3.5 Machine moving up and down, "perpetual motion" machine
See diagram 12.5.5: Moving up and down
To investigate the transfer of mechanical energy, fix two pulleys on a wall or a metal stand. Use a coffee
jar lid with a hole, about 1 cm diameter. Attach the end of a string to the middle of the cover and the edge
of the lid. Attach a weight to the other end of the string. Use two pulleys to hoist the lid at the left side and
hoist a weight at the right side. Connect a hose to a water tap. The hose should be just long enough ft to
reach the lid. Turn on the tap and let water stream into the lid. The lid moves down and hoists the weight.
The water from the tap cannot reach the lid. Water streams out of the cover through the hole in the lid.
The lid will rise to the original position by the pull of the weight until water from the tap can reach the lid
again. The lid will move up and down continually if the water tap is not turned off.
13.4.1 Turbulent and streamline flow
1. Construct a streamline flow apparatus that uses several potassium permanganate tracers from a source
point to a collection point.
2. Show laminar and turbulent flow. Introduce an ink jet at different rates into a tube of flowing water.
Vary the velocity of a stream of ink in smoothly flowing water. 3. Observe laminar and turbulent flow
shadows. See how rising warm air flowing around objects produces shadows. Put a hot iron ball in
slowly or rapidly moving air in laminar and turbulent flow. Use the Krebs apparatus to show flow of
water around objects.
5. Use streamline flow to blow out a candle. Place a lighted candle on one side of a beaker and blow on
the other side of the beaker to extinguish the candle.
13.4.5 Poiseuille flow
Poiseuille's formula for volume of liquid per second, Q, with viscosity n, flowing with laminar flow through
a capillary tube length L and radius R under pressure P is Q = πPR4 / 8 L n. The formula is used to
describe the apparent viscosity of non-Newtonian fluids, e.g. fluid polymers. (The CGS (cgs) unit "poise"
comes from J. L. M. Poiseuille, 1799-1869, France, not French: pois, weight). Drop coloured glycerine on
top of clear glycerine in a square cross-section tube and open a stopcock at the bottom to adjust the flow.
Watch the interface between clear oil on the bottom of a glass tube and coloured oil on top as you drawn
oil off the bottom.
13.4.6 Rayleigh-Taylor instability
Rayleigh-Taylor instability occurs when heterogeneous fluids with very different physical properties are
placed over another, e.g. honey over milk. Air bubble rising in a tube of Prell shampoo shows
Rayleigh-Taylor instability
13.4.7 Reynold's number
Reynold's number (Osbourne Reynolds 1842 - 1912) (no dimensions), is the ratio of pressure forces to
viscosity forces in a fluid flow. It is important in the study of fluid dynamics. Re = d v d / n, where d =
 density of fluid, viscosity n, travelling at velocity v in a pipe diameter d. If Re < 2000, flow is laminar.
If Re > 2000 flow, is turbulent. In laminar flow, streamline flow, the layers do not mix except at the
boundaries In turbulent flow, the motion of particles varies rapidly, often in eddies, e.g. liquids with high
Reynold's numbers and in boundary layer of aircraft where high drag occurs. Reynolds realized that the
tendency of water to form eddies increases with temperature, which in turn is related to viscosity.
1. Introduce tracer fluid into a tube at the bottom of a reservoir with tapered nozzle. Vary the flow in a
tube and introduce a tracer into the flow. Use a funnel to feed methylene blue into a vertical tube with
adjustable water flow.
2. Let water with potassium permanganate flow through a vertical tube. Vary the flow is and find the rate
by timing or by collecting water for a given time.
13.4.8 Mariotte' bottle
See diagram 13.4.8: Mariotte's bottle 1 | See diagram 13.2.27: Mariotte's bottle 2
A reservoir designed by Edme Mariotte (1620-1684) is closed by a stopper and has an air inlet and a
siphon connection. The pressure at the bottom of the air inlet is always at atmospheric pressure. The
entrance to the siphon is at the same level as at the bottom of the air inlet. so it supplies water at
atmospheric pressure and gives a flow of water under constant head height despite any changes of water
level inside the reservoir.
Based on "Wikipedia"
13.4.9 Water stream from a tap, faucet
Observe how the continuous water stream from the tap becomes thinner with distance from the tap. It may
become so thin as to break into water drops. The water molecules bind together to form the column of
water because of hydrogen bonds. However, when the thinning water column breaks into droplets the
droplets keep their "muffin shape" because of hydrogen bonds. If filling a narrow necked container,
e.g. a bottle, from a tap it is easier to fill the bottle by holding is lower down where the water stream is
thinner and the air can easily leave the bottle as the water displaces it.
13.5.0 Vortices, vortex
A vortex is an eddy where part of a fluid rotates with intense spiral motion.
13.5.1 Grow a large drop
A vortex is formed in an air stream allowing one to form a large water drop.
13.5.2 Liquid vortices
A drop of inky water is allowed to form on a medicine dropper above a beaker of water. This height
because the vortex will rebound if the beaker is less than 10 cm deep.
13.5.3 Ring vortices on liquid
Bursts of coloured water are expelled from a glass tube in a beaker of water. Also, a drop of aniline sinks
in a beaker of water.
13.5.4 Detergent vortex
A few drops of detergent in a jar of water are shaken and given a twist to form a vortex lasting several
seconds.
13.5.5 Tornado vortex
1. Open tap at base of column of water. Water starts to spin then pressure drop at centre of surface.
curves downwards then a vortex forms almost down to the bottom.
2. Vortex forms in a large cylinder of water on a magnetic stirrer.
13.5.6 Smoke ring
Tap smoke rings out of a coffee can through a 2 cm diameter hole. Tap smoke rings out of a can with a
rubber diaphragm on one end and a hole in the other end. A rubber sheet at the back on a large wooden
box is struck with a hammer to produce smoke rings capable of knocking over a plate. Fuming HCl and
concentrated ammonia produce the smoke rings with LP gas.
13.5.7 Tornado tube
Couple two soft drink bottles and spin the top bottle so the water forms a vortex as it drains into the
bottom bottle.
13.5.8 Soccer balls
The "Teamgeist" soccer ball used in the 2006 World Cup was almost perfectly round with grooves
between the panels. The grooves scatter air particles and create turbulence close to the ball. The
"Jabulani" soccer ball used in the 2010 World Cup is similar except that the grooves have tiny ridges that
keep the turbulence close to the ball for a longer time. So instead of air leaving the ball at an angle of 90o
from the ridges it leaves at 120o from the ridges producing a smaller wake and less drag so the ball holds
its speed longer. However, some players report that when kicked for a long distance the ball behaves in
odd ways that cannot be predicted.
13.6.0.1 Viscosity of Newtonian fluids
Fluids can flow and to take on the shape of their container. High viscosity fluids do not flow easily. Low
viscosity fluids do flow easily, e.g. honey and water have different viscosity. A fluid does not have a fixed
shape. Liquids and gases are both fluids. The behaviour of fluids can be explained in terms of the
arrangement and energy of the particles of which they are composed.
Viscosity is the rate at which a fluid flows. Different fluids have different viscosity. The viscosity of
Newtonian fluids is affected only by temperature. With Newtonian fluids, e.g. water and solutions of low
molecular weight solutes, viscosity is independent of shear strain rate. A graph of shear strain rate against
shear stress is linear and passes through the origin, so you can call Newtonian fluids "linear fluids". The
relationship between viscosity with concentration is generally linear up to viscosity values of about twice
that of water. This dependency means that more extended molecules increase the viscosity to greater
extents at low concentrations than more compact molecules of similar molecular weight, e.g. amylose,
carboxy methyl cellulose, arabinoxylans and guar.
Let two flat plates, area A, separated by a layer of fluid, thickness D, move with velocity V, relative to
each other. The rate of shear, shear rate = V / D. If a force F is applied to each flat plate, the
shear stress = F / A. In a Newtonian fluid, F / A = mu × V / D, where µ = Newtonian viscosity.
13.6.0.2 Viscosity of non-Newtonian fluids
The viscosity of non-Newtonian fluids is affected by shear forces (stirring) as well as temperature. The
gel and flow properties of hydrocolloids may change. Thermogelling materials gel above a temperature
and are usually reversible. Above certain concentrations, hydrocolloid solutions show non-Newtonian
behaviour where their viscosity depends on the shear strain rate. The viscosity depends on the
cross-section area in the direction of flow. At low flow rates, long and thin solute molecules have
effectively large cross-sections because of them tumbling in solution but at high shear strain rate the
molecules align with the flow, giving much smaller effective cross-sections and hence much lower
viscosity.
Many hydrocolloids are capable of forming gels of various strength dependent on their structure and
concentration plus factors such as ionic strength, pH and temperature. The combined viscosity and gel
behaviour (viscoelasticity) can be examined by determining the effect that an oscillating force has on the
movement of the material. With viscoelastic hydrocolloids, some of the deformation caused by shear
stress is elastic, e.g. contortion of the chains into high energy conformations, and will return to zero when
the force is removed. The remaining deformation, i.e. the sliding displacement of the chains through the
solvent, will not return to zero when the force is removed. Under a constant force, the elastic
displacement remains constant whereas the sliding displacement continues, so increasing.
13.6.0.3 Shear-thinning, stir-thinning, thixotropy
See 17.3.0: Viscosity, Stokes' law, fluid friction, falling ball in liquid
Fluidity is the reciprocal of the viscosity. Pseudoplastic materials instantaneously decrease in viscosity
with increase in shear strain rate, i.e. they flow, and are therefore easier to pump and mix. They are
shear-thinning. This is often a consequence of high molecular weight molecules being untangled and
oriented by the flow. This behaviour usually increases with concentration. Thixotropy is reduction of
viscosity due to applied stress. Thixotropic liquids show a time-dependent response to shear strain rate
over a longer period than that associated with changes in the shear strain rate. They may liquefy on being
shaken or stirred and then solidify (or not) when this has stopped. Applied stress lowers viscosity that
return to normal when stress is releases, i.e. gel to sol then sol to gel. The thinning depends on the rate of
force applied. Some clays and polymer fluids and mixtures are thixotropic. Hair gel flows when it is
stirred, and thickens when it is not stirred to produce a hairdo. Ink in ball point pens is a stir thinning
liquid that thins under pressure. Toothpaste flows when force is applied but thickens when it is not under
pressure. Toothpaste is designed to flow from a tube, but not flow off the brush. Non-drip paint is a stir
thinning liquid.
13.6.0.4 Shear-thickening, stir-thickening, dilatant fluids, rheopectic fluids
Some sols gel rapidly when gently agitated. The stir-thickening depends on the rate that force is applied.
Shear-thickening, dilatancy, shows an increase in viscosity with shear stress and strain, e.g. uncooked
corn starch paste where shear stress squeezes the water from between the starch granules allowing them
to grind against each other. This property is utilized in tomato sauce where flow is prevented under small
shear stress but then catastrophically fails, producing too great a flow, under greater stress (shaking).
"Shake, shake the ketchup bottle,
First none'll come, and then a lot'll". William Richard Willard Armour (1906 - 1989)
Bouncing putty, silly putty, is both stir thinning and stir thickening. Whether it thickens or thins depends on
the rate at which force is applied. The application direction of force determines the effect the force has on
fluids.
The pumping and storing of stir thickening liquids presents different problems to normal fluids in factories
which produce products such as soups and paints, tar which melts and flows on hot days, paints designed
to be non-drip and hair gel.
Dilitant fluids, when stressed, increase resistance to further stress by increasing the shear rate, e.g. wet
beach sand, polyvinyl chloride plastisol.
Rheopectic fluids have a time-dependent change in viscosity so the longer the fluid undergoes shear, the
higher its viscosity. The more you shake it the thicker it becomes, e.g. some clays containing gypsum,
printers inks and lubricants. Asphalt splinters when smashed but flows gradually.
13.6.1 Cornstarch, cornflour slime, isotropy and thixotropy
See: Cornflour
Isotropy is when a fluid becomes firm when agitated, e.g. running over wet sand makes the sand mixture
firmer but when you stop running your feet sink into the sand. Thixotropy is when a fluid mixture becomes
less firm, i.e. more fluid, when agitated, e.g. strike the end of a tomato sauce (ketchup bottle) to make the
sauce become more liquid and run out. Also, if you agitate quicksand, the sand mixture becomes more
liquid and you sink more quickly.
Cornflour slime, is cornstarch dissolved in water to form a viscous near solid white fluid which you can
picked up. However, it flows easily when not under pressure. Cornflour slime is a dilatant fluid, a
shear-thickening fluid (STF) but it is not a rheopectic fluid because it does not show time-dependent
change when sheared. The more pressure is applied the more resistance to deformation. Stir thickening
(shear-thickening) mixtures become viscous when pressure is applied.
1. Put 1 cup of cornflour in a bowl, add 1/4 cup of water then stir to form a thick paste. Knead the
mixture to make it firm as long as you keep mixing. If you punch the mixture you can hurt your hand
because the pressure on the cornflour paste causes tit to become solid and even crack. After you have
stopped kneading push your fingers very slowly through the mixture. Raise your fingers and see the
mixture pour through your fingers. Cornflour slime ("gloop", "oobleck") can be stirred, punched, poured
and rolled into a ball. It is an example of a non-Newtonian fluid. The rate at which it flows is affected by
shear forces as well as temperature.
2. Add water to corn starch in an aluminium basin. Sir it slowly then quickly, then pour it out of the basin
then back into the basin. Punch it and hit it with a hammer. Pull out a handful then throw it to shatter
against the wall, then collect the pieces.
3. Mix custard powder with water while stirring until it feels strange when you squeeze it with your hand.
The custard should stick to your finger. Push a spoon handle through the custard mixture so that it leaves
a clean cut groove that swiftly fills with liquid custard again. Pick up some custard mixture and roll it into
a ball between your hands. It feels slimy if you mixed it well. Keep moving the custard so that it forms a
ball. Stop moving the custard ball and becomes a liquid. Custard powder contains finely ground cornflour,
colouring and flavouring. The cornflour particles link together if you put pressure on them but will separate
when the pressure stops. If you keep squeezing, the links join and the custard stays in a ball. The pressure
of the spoon handle causes a clean cut, but as soon the spoon handle passes, the custard becomes liquid
again.
3. Walk on cornflower paste! The structure of cornflour paste is irregular-shaped particles separated by
water. The particles can move around if the mixture is gently stirred. However, if pressure is applied to the
mixture some of the water moves sideways and the particles touch, lock together and the mixture behaves
as a solid. So you can walk quickly or run on cornflour paste. Cornflour is used to thicken soups because
the cornflour grains open when heated to release long starch molecules that tangle together forming a
gel-like structure.
4. Convert cornflour paste from being shear-thickening to being shear-thinning, thixotropic. Dilute a thick
paste with water and heat the mixture. Starch molecules are released and the paste become thixotropic.
13.6.2 Density balls in beans
A ping-pong ball in the middle of a beaker of beans will rise when the beaker is shaken. The size of an
aluminium ball determines whether it goes up or down in a shaking bowl of beans.
13.6.3 Reynolds' dilatancy
Compacted granular material, e.g. sands and soils, may expand in volume when sheared. The compacted
grains interlock and cannot move around. When sheared, a lever motion occurs between grains to cause
a bulk expansion of the material. However, loose granular material may initially compact when sheared.
13.6.4 Tomato sauce, ketchup, catsup
Tomato sauce used to be a stir thinning liquid. Fill a super soaker with ketchup then shoot it across the
room and it blobs on the wall. Tomato sauce is a thixotropic liquid that shows a time-dependent response
to shear strain rate over a longer period than that associated with changes in the shear strain rate.
Thixotropic liquids may liquefy on being shaken and then may solidify when the shaking stops. So when
you hit the bottom of a tomato sauce bottle some of it liquefies and spurts out then solidifies again.
Banging a tomato sauce bottle down on the table only projects the tomato sauce by its own inertia deeper
into the bottle.
1. Invert the open bottle over food and hold it tightly forming a fist around the end of the bottle. Hit the fist
with the wrist of the other hand. Some sauce will be ejected from the bottle.
2. Shake the closed bottle to break weak bonds between the starch molecules in the tomato sauce. Open
the bottle and invert it over food. Some tomato sauce drops down onto the food.
3. Hold the closed bottle in one hand and rapidly hit the side of the bottle with the open fist of the other
hand to cause vibration of the tomato sauce inside. Open the bottle and invert it over food. Some tomato
sauce drops down onto the food.
4. Open the bottle and poke the sauce vigorously with a chopstick. Invert the bottle it over food. Some
tomato sauce drops down onto the food.
5. Remove the metal cap of the bottle and place it in a microwave oven for up to 15 seconds to lessen
the viscosity of the contents. Invert the bottle it over food. Some tomato sauce drops down onto the food.
13.6.5 Rising stones, granular mixtures, Brazil nut effect
Rising of rocks in soil is the same mechanism as the sifting down of fine particles to the bottom of a cereal
box. Granular mixtures separate according to particle size. This is called the Brazil nut effect because some
people say the in a can of mixed nuts the largest nut, Brazil nuts, are always on top especially if the can has
been shaken recently. Similarly nuts will rise to the surface of muesli mixtures. This phenomenon may be
caused by greater drag on small particles by friction with air because in a near vacuum all particles,
regardless of size rise at the same rate.