School Science Lessons
Physics - Dynamics of fluids, fluid mechanics, hydrodynamics
Updated: 2008-08-13
Please send comments to: J.Elfick@uq.edu.au
See also: Interesting websites
Table of contents
13.0.0 Dynamics of fluids, fluid mechanics, hydrodynamics
13.1.0 Flow rate
13.2.0 Bernoulli force, Bunsen burner
13.3.0 Viscosity, Stokes' law, fluid friction, falling ball in liquid
13.4.0 Turbulent and streamline flow
13.5.0 Vortices
13.6.0 Non-Newtonian fluids
13.1.0 Flow rate, fluid mechanics, hydrodynamics, motion in fluid from applied force
13.1.4 Pressure drop along a line
4.153 Three-holes can
13.2.0 Bernoulli force, Bunsen burner, air streams, Bernoulli theorem, Bernoulli tubes
3.1.0 Bunsen burner
4.186 Reduce friction with an air stream
4.187 Propeller
4.242 Air streams, Bernoulli theorem, funnel and ball, ping-pong ball in a glass funnel
37.4 Make a pressure tube anemometer, Pitot tube, static tube, wind speed indicator, Venturi tube
12.3.27 Put an egg in a bottle, get the egg out of the bottle
15.0.4.1 Axis of rotation
18.3.5.1 Cardboard boomerang
13.2.1 Flowing air can do work
13.2.2 Attracting ping-pong balls, attracting aluminium or paper sheets, attracting drink-cans
13.2.3 Pushed down paper
13.2.4 Card and cotton reel spool
13.2.6 Lift water by blowing
13.2.7 Lift from spin, "swerve ball" curve ball, golf balls, Bjerknes' tube
13.2.8 Aerofoils, parts of an aircraft
13.2.8.01 Comments on diagram 13.2.8
3.40 Paper aircraft (Primary)
4.8 Flying kites (Primary)
13.2.9 Shower curtain and Bernoulli force
13.2.10 Hydrodynamic attraction
13.2.11 Ball in a water stream
13.2.12 Bernoulli loop the loop, Bjerknes' tube
13.2.13 Bernoulli cups
13.2.14 Bernoulli pen barrel
13.2.15 Blow ping-pong ball from cup to cup
13.2.18 Pitot tube
13.2.20 Rayleigh's disc
13.2.21 Spinning ball
13.2.22 Spin out the air
13.2.26 Deep breathing exerciser
13.2.27 Tap and spoon
13.2.28 Flettner rotator
13.4.0 Turbulent and streamline flow
13.4.1 Turbulent and streamline flow, shadows, weather maps
8.42 Weather maps, Buys Ballots law, geostrophic wind and gradient wind
13.4.5 Poiseuille flow
13.4.6 Rayleigh-Taylor instability
13.4.7 Reynold's number
13.5.0 Vortices
13.5.1 Grow a large drop
13.5.2 Liquid vortices
13.5.3 Ring vortices on liquid
13.5.4 Detergent vortex
13.5.5 Tornado vortex
13.5.6 Smoke ring
13.5.7 Tornado tube
13.6.0 Non-Newtonian fluids
13.6.0.1 Viscosity of Newtonian fluids
13.6.0.2 Viscosity of non-Newtonian fluids
13.6.0.3 Shear-thinning, stir-thinning, thixotropy
13.6.0.4 Shear-thickening, stir-thickening, dilatant fluids, rheopectic fluids
13.6.1 Corn starch, cornflour
13.6.2 Density balls in beans
13.6.3 Reynolds' dilatancy
13.6.4 Tomato sauce, ketchup, catsup
13.6.5 Rising stones in soil and particles in cereal box
3.4.11 Slime ball, "Silly putty", silicone polymer to amuse children
13.1.4 Pressure drop along a line
Open tubes along a drain pipe show pressure drop along a line.
13.2.0 Bernoulli force, Bunsen burner
See diagram 4.242.3: Bernoulli's law
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
The four forces are: (a) lift (b) weight (c) drag (d) 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 tube. 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.
13.2.1 Flowing air can do work, flow pipe of uniform cross-section
See diagram 2.0.1: 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 13.2.2.1
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 13.2.3
1. Make cards by cutting out two pieces of light cardboard 6 cm X 10 cm. Fold the cards (a) about the centre line and (b) 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 X 2 cm. Bend it down at one end and hold the bent end in front of you moth. Blow over the paper strip and it rises. Some electric fans have paper strips attached to the safety grill 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 13.2.4
1. Insert a pin through the centre of a 7 X 7 cm piece of cardboard or a playing card. Place the card on the table with the pin pointing vertically up. Hold a 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. 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 13.2.6
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 sandpaper-covered paddle. 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 2. : Aerofoil wing | 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, etc.). 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. Hold one edge of a sheet of paper horizontally, let the rest hang down. Blow across the paper and watch the sheet rise.
2. Cut out a 12 X 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.
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 Shower curtain and Bernoulli force
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
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 Spin out the air
When hanging from a spring, a scale is mounted just above an identical spinning disc, the spring scale will show an increase in force.
13.2.26 Deep breathing exerciser
See diagram 13.2.26
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 Tap and spoon
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.
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.
13.4.1 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 = Pi P R4 / 8 L n. The formula is used to describe the apparent viscosity of non-Newtonian fluids, e.g. fluid polymers. The CGS unit poise comes from the name of the physicist Poiseuille.
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.
13.5.0 Vortices
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 is critical 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. Then surface curves downwards then 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.
13.6.0.1 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 X V / D, where mu is the 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 also 13.3.0: Viscosity, Stokes' Law
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
See also: Cornflour
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.
1. Cornflour slime, "gloop", can be stirred, punched, poured and rolled into a ball. Cornflour slime, "gloop", 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 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 in soil and particles in cereal box
Rising of rocks in the spring is the same as the sifting of fine particles to the bottom of a cereal box.