School Science Lessons
19. Fluid mechanics, surface tension, capillarity, forces of cohesion
and adhesion, meniscus, soap films
2012-01-27 SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
19.0.0 Fluid mechanics
19.3.0 Capillary action
19.1.0 Floating and sinking
19.0.0 Surface tension
19.3.0 Capillary action
19.3.0 Capillarity, capillary action, capillary
rise in wicks
19.3.16
Bursting water bubble
4.56 Capillary
action in soil and deposition by groundwater
19.3.2
Capillarity between glass slides, water rise by capillary action
19.3.25
Capillary action, capillary tubes
19.3.27
Capillary phenomena
19.3.24
Castor oil drop
19.3.23
Catenoid soap film
19.3.19
Changing drop size
19.3.30
Clumping plastic
19.3.8
Cohesion plates
19.3.9
Cohesion tube
19.3.31
Damp-course, damp-proof course, in a building
19.3.10
Drop liquid soap on surface with powder
19.3.18
Electrostatic dispersion of water drop
4.57 Infiltration
and capillary action by groundwater
19.3.11
Inflate and connect two unequal bubbles, balloons
19.3.7
Leaky boats
19.3.12
Pressure in a bubble
19.3.22
Soap film minimal surfaces
19.3.13
Sponge action
19.3.1
String siphon empties dish of water, twisting tea bag string
19.3.6
Submerged float
19.3.32
Teapot effect
19.3.15
Tears of wine
19.3.20
Thread ring in soap film, minimum energy thread
19.3.26
Water drops in tapered tubes
19.3.14
Water droplets
4.36 Water
climbs up soil (Primary)
6.9 Water
climbs up (Primary)
19.3.29
Wet mop
19.1.0 Floating and sinking
11.4.0 Buoyancy, flotation, Archimedes'
principle
6.12 Float clay boats (Primary)
1.45 Float different objects (Primary)
9.4 Float eggs
4.204 Float lighted candles
4.211 Float metal boats, Plimsoll line
3.38 Float needles on water (Primary)
4.213 Float needles on water, float metals
4.223 Float razor blades
19.2.8 Float corks in a glass jar
19.2.9 Float oil spheres
19.0.0 Surface
tension
19.0.0 Surface tension
19.0.0 Surfactants
1.40 Blow soap bubbles (Primary)
4.220 Blow soap bubbles
19.0.2 Bubbles in the air
19.1.2 Bubbles in water and aqueous solutions
19.1.2.1 Burn butane bubbles
19.1.4 Coalescing oil drops
19.1.3 Drive a boat with surface
tension, camphor boat, spinning dancers
19.0.1 Drops and films, wetting
4.217 Heap up water in a glass
4.216 Hold water in a sieve
4.215 Lift the water surface
19.2.4 Meniscus, heap water up in drinking glass,
liquid surface is higher than the brim of glass
19.2.7 Measure surface tension, surface tension
balance
4.218 Pinch together water streams
4.212 Soap and surface tension
4.221 Soap bubble support
4.222 Soap film and sliding wire
19.3.17 Surface tension with electric field
4.56 Capillary action in soil and deposition by groundwater
Place a mixture of table salt and fine, dry sand in the bottom of a small
aquarium to a depth of 2 to 5 cm. Cover this layer with about 5 cm of clean
sand, no salt. Insert a glass tube with a funnel, supported by a clamp stand,
into the sand at one corner of the aquarium, see overleaf. Make sure that
the tube reaches the salt layer. Clamp a heat lamp on a stand so that it
can shine down on the other side of the aquarium. Pour water into the funnel.
The tube may have to be shaken slightly to get the water to move down the
tube. Observe the side of the aquarium. The water can be seen to move through
the sand. Put in enough water to wet a layer about 2 cm deep along the bottom
of the aquarium. Light the lamp and let it burn for several hours. In the
vicinity of the lamp, the water will rise through the sand by capillary action,
bringing the salt in solution up with it. The heat will cause the water
to evaporate, and the salt will be deposited near and at the surface. Taste
some of the sand near the heat lamp to see if it is salty. In nature, the
sun has the same effect as the heat lamp in this experiment.
4.57 Infiltration and capillary action by groundwater
Fill two glass tubes, 2 cm in diameter and 30 cm long about half full
of dry, fine sand. Support the tubes vertically by clamp stands, with their
bottoms resting in some type of flat dish or aquarium. Pour water into one
tube. The water will infiltrate down through the pore spaces of the sand,
move into the dish, and partially move up the other tube by capillary action
4.212 Soap and surface tension
1. Fill the large clean plate with cold water and let it stand for a
time on the table until the water is still. Sprinkle some talcum powder
lightly over the surface of the water. Wet a piece of soap in water and
touch it to the water near the edge of the plate. The talcum powder will
be drawn to the opposite side of the plate at once. The soap reduced the
surface tension at one point. The increased surface tension on the other
side contracts the surface and pulls the talcum with it. Try a similar experiment
but substitute flowers of sulfur for the powder and synthetic liquid detergent
instead of the soap. If you use a transparent dish, you can place it on
an overhead projector and display the results on a screen.
4.213 Float needles on water,
float metals
See diagram 19.291: Float a needle on water
1. Use a steel needle and dry it thoroughly. Place it on the tines of
a dinner fork and gently break the surface of some clean water in a dish with
the fork. If you are careful, the needle will float as you take the fork
away. Look at the water surface closely. See how the surface film seems to
bend under the weight of the needle. The weight of the needle is greater than
the weight of the water it displaces, but it does not sink because it is
supported by the elastic "skin" of water molecules in contact with air. Add
a drop of detergent solution. The needle sinks because the water molecules
in the "skin" are now dispersed.
2. Put a needle on a small area of absorbent paper tissue, tissue paper.
Float the tissue and needle on a water surface. Use a small stick to sink
the absorbent tissue or leave it to sink when it becomes soaked with water.
The needle remains floating on the water. Add soap solution to the water.
The needle sinks.
3. Float a paper clip on water. Hold a paper clip in a sling made of
a paper towel. Dip the paper-clip on to the water surface. Take away the
paper towel. See the water surface bending under the paper-clip. Add a drop
of detergent solution.
4. Float needles, paper-clips, rings of wire, and a razor blade on water.
Add a drop of detergent solution.
5. Float an aluminium sheet on the surface of deionized water and add
weights until the metal sinks.
6. Tie together two ends of a small piece of cotton to make a cotton
loop. Drop the cotton loop onto clean water so that it makes an irregular
shape on the surface of the water. Dip the end of a needle inside the floating
cotton loop to break the surface film of water. The cotton loop forms a circle
that minimizes the surface area of cotton in contact with the water. Add
a drop of detergent solution.
4.215 Lift the water surface
1. Bend the pointed end of a pin or use a piece of fine wire to make
a hook. File the point of the hook until it is very sharp. Put your eye
on a level with the surface of the water in a drinking glass. Put the hook
under the surface of the water and gently raise the point to the surface.
If you are careful, the point will not penetrate the surface film but will
lift it slightly upwards.
2. To observe surface tension on a liquid surface, bend the sharp end
of a pin into a hook or make a hook with a piece of thin wire. Polish the
hook to make it sharp. Fill a glass cup with water. Immerse the end of the
hook then gently lift it up. The hook does not pierce the water surface but
lifts it slightly. Be careful to keep your line of sight at the same horizontal
plane with the water surface.
4.216 Hold water in a sieve
1. Pour some oil over the wire mesh of a kitchen sieve and shake out
the excess so that the holes are open. Use a pitcher of water and carefully
pour it into the sieve by letting it run down the side of the sieve. When
the sieve is about half full, hold it over a sink or bucket and observe the
bottom. You will see water pushing through the openings but the surface tension
keeps it from running through. Touch the bottom of the sieve with your finger
and the water should run through.
2. To observe the surface tension on a liquid surface, pour some oil
into a sieve with dense net then shake it to get rid of unwanted oil. Add
water along the side of the sieve. Slowly move the sieve above a bucket
when the water reaches half way. Water may come out from the sieve holes
but does not flow down. The water will flow down when you touch the sieve
bottom gently.
3. Make watertight sieves. A mesh boat floats until a drop of water is
placed inside it. Dry cheesecloth holds water in an inverted beaker
4.217 Heap up water up in a
glass
1. Place a drinking glass in a shallow pan or on a saucer. Rub the top
edge of the glass with a dry cloth. Pour water into the glass until it is
full to the brim. You will note that you can fill the glass several millimetres
above the top. Now drop coins or thin metal washers into the water edgewise.
By dropping these in see how far you can heap the water up before it runs
over.
2. The volume of an object with an irregular shape can be measured with
an overflow can. The volume of the object is equal to the volume of water
that overflows. However, small thin objects, e.g. coins, can be added
to a small container of water, e.g. a drinking glass, and instead of an overflow
the surface of the water, the elastic "skin", curves outwards. Fill a drinking
glass until no more water can be added. Draw the curvature of the surface
of the water. Add coins and draw the changes in curvature until the water
spills over the rim of the drinking glass.
4.218 Pinch together water
streams
See diagram 19.296: Pinch together water streams
1. Use a metal can. Punch five holes in the sides about 5 mm apart. Fill
the can with water. Observe that the water comes from the can in five streams.
Pinch the jets of water together with your thumb and forefinger to make one
stream. Brush your hand across the holes in the can and the water again flows
in five separate streams. 2. Use an empty tall tin can or plastic drink
bottle and a nail to drill five holes, very close to the bottom 5 mm apart.
Fill with water. Water flows from the five holes. Move your fingers over
the holes. The five streams of water gather into one stream. If you rub
on any hole on the can, the water changes into five streams again.
4.220 Blow soap bubbles
1. Make a soap bubble solution by putting three level tablespoonfuls
of soap powder or soap flakes into four cups of hot water. Let the solution
stand for three days before using. Try blowing bubbles with a bubble blower
or a drinking straw by slitting the end of the straw with a razor blade into
four parts extending about 1 cm from the end. Bend these pieces outwards.
2. Make the soap solution the day before the lesson. Show how to make
soapy water by putting the pieces of soap or some detergent into the water
in the jar and shake. Put some soapy water in the palms of your hands. Press
your hands together so that a small hole forms. Blow through this hole. Dip
one end of the stem or straw into the soapy water and blow gently through
the stem in the air. Dip a loop of wire into the soap solution. Is there
a thin film of soap across the loop? Blow through the loop slowly. Blow a
big bubble. To make small bubbles, blow quickly. To make big bubbles, blow
slowly. A bubble has a skin of soap. Inside is air. Describe the shape and
colour of a soap bubble. A bubble breaks if the skin is too thin, if it hits
something the skin breaks, if the air inside gets bigger the skin breaks.
3. Rinse a milk carton and fill it with water. Add 1 / 3 cup household
soap and one tablespoon glycerine. Close the carton and turn it over a couple
times to mix, without shaking to avoid suds. Leave to stand for 24 hours.
Use the plastic ring to blow bubbles. Dip it in the bubble solution.
4.221 Soap bubble support
See diagram 19.299: Soap bubble support
1. Make a soap bubble support with a wire loop about 10 cm in diameter.
Dip the loop in soap solution. Blow a large soap bubble and put it in the
loop. Now wet a drinking straw in the soap solution and put it through the
large bubble. Blow a smaller bubble inside the large bubble.
4.222 Soap film and sliding
wire
See diagram 19.300: Soap film and sliding wire
1. Twist wire to make a rectangle with one side missing. Twist each end
of another piece of wire to make a slider. The slider forms the fourth side
of the rectangle. Dip the wire rectangle in the soap solution. Pull the slider
out slightly and watch the film stretch. Release the slider. The contraction
of the film pulls back the slider. In sliding wire experiments, the soap
film provides the force to pull a light wire on a U-shape frame. A sliding
wire frame film with a spring on one end and a string pull on the other
shows that tension does not increase with length.
4.223 Float razor blades
See 3.2: Mosquito life cycle (Primary) See
mosquito larva using surface tension of water
1. Use a razor blade of the double edge type. Try floating it on the
surface of water. Again note the surface and see the surface film.
19.0.0 Surface tension
See diagram 19.19.1: Contact angle | See diagram 19.19.2: Capillarity
| See 3.23: Volume of a liquid (Primary)
Surface tension values of liquids in contact with air at 20oC
Water = 72.8 X 10-3 N / m, i.e. 72.8 mN / m, (millinewton per meter, mN.m-1),
ethanol = 22.3 mN /m, glycerol = 63.1 mN /m, mercury = 465 mN
/m, olive oil = 32.0 mN /m, soap solution = 25.0 mN /m.
Water at 100oC = 58.9 mN / m
Sometimes surface tension values are quoted in the cgs unit the dyne (dyn). However, 1 dyn / cm, dyn.cm-1 = 1 mN /m, mN.m-1
Liquid surfaces behave as if they are enclosed in a stretched elastic
membrane that tends to contract until the surface area is a minimum. Small
quantities of liquids assume the shape of a spherical drop because a sphere
is the solid with the smallest surface area for a given volume. Surface tension
is caused by the forces of molecular attraction, forces of cohesion. A molecule in the liquid
is attracted by all the molecules around it within a sphere of molecular
attraction. For a molecule within the liquid the forces of molecular attraction
act equally in all directions so there is no resultant force in any direction.
However. a molecule near the surface has more molecules below it than above
it within the radius of attraction. So there is a resultant force acting
inwards and work must be done to transfer molecules from the interior to
the surface of a liquid. Surface tension is the work that must be done to
increase a liquid surface by unit area. The work done in increasing a liquid
surface by an area A = S x A, where S = the surface tension, measured in Nm-1.
Surface tension may also be regarded as a force per unit length. If a line
length l is drawn in a liquid surface there is a force equal to S x l acting
in the surface of the liquid and at right angles to the line. If a line length
l is drawn along the line of contact of the liquid and another medium, e.g.
air, there is an unbalanced force due to surface tension, Sl, which tends
to make the liquid contract, and a force equal to S1 that be applied to
prevent the surface contracting.
The surfaces of a liquid act as if a thin elastic membrane covers them.
The surface molecules attract each other by cohesion, force of cohesion, measured in Nm-1. As the surface molecules
are under tension the liquid contracts to minimize the surface area, so a falling water drop is nearly spherical.
Surface tension is the property that causes the surface of a liquid to
behave as if covered with a weak elastic skin. It is caused by the exposed
surface's tendency to contract to the smallest possible area because of
unequal cohesive forces between molecules at the surface, unbalanced molecular
cohesive forces. Surface tension is measured in N m-1. Allied
phenomena include the formation of droplets, the concave profile of a meniscus,
and the capillary action, capillarity, by which water soaks into a sponge
or porous material, or "wets" the surface of a material. The surface tension
of water is the highest of all liquids, 73 mN m-1.
The high surface tension controls the shape of a meniscus, raindrops and
sea spray. Organic impurities lower the surface tension of water. Increase
in temperature lowers the surface tension of a liquid. For water, St =
So - 0.14t, where St and So are the surface tensions at toC
and 0oC respectively.
Surfactants
Detergents or soaps contain
surfactants, surface active agents, which reduce the surface tension of solvents
used for washing. When the distance between solid and liquid is less than
10-8 m, molecular forces between them will apply. The attraction
between molecules of solid and liquid forms a liquid layer attached to the
surface of the solid. When the action of the solid molecules on the liquid
molecules of the liquid layer is stronger than the interaction of the liquid
molecules, or force of cohesion, the interface spreads and molecules stick
to each other between liquid and solid. It is called adhesion, force of
adhesion. surface energy, surface tension: The force of attraction for itself
that gives a liquid, e.g. water, an apparent skin. The "skin" contracts to
form drops rather than sheets on surfaces it does not wet. On some surfaces,
e.g. clean glass, the attraction of the water is greater for the glass than
for itself, so the water "wets" the glass. Water spreads out smoothly on clean
glass but remains in droplets on dirty glass. So wetting refers to the covering of a solid by a liquid with a thin film.
The contact angle the liquid makes on the solid is small.
19.0.1 Drops and films, wetting
Work must be done to extend a liquid surface, so when the liquid surface
is increased the potential energy increases. Potential energy tends to decrease
so a liquid surface, tends to decrease, making the air-liquid, liquid-solid,
and air-solid surfaces as small as possible.. The air, liquid and solid contacts
of the three surface tensions are as follows:
S1 between air and liquid,
S2 between liquid and solid, and,
S3 between air
S1 and S2 cause a liquid to form a drop.
S3 and the weight cause a liquid to form a film.
A small volume of mercury on glass forms a drop, but the same small volume
of water on glass forms a film because S3 is greater than S1 and S2. So we
say that water "wets " glass. For a very small drop, ignoring any small gravitational
factors, the form assumed by the liquid is S3 = S2 + S1 cos θ, where
θ is the "angle of contact," between the liquid and the solid surface.
A substance becomes wet if a small volume of a liquid spreads evenly
through it over it instead of forming separate droplets over it.
19.0.2 Bubbles in the air
A bubble of water floating in air will tend to contract, increasing the
pressure air inside the bubble until the tendency to contract due to surface
tension equals the tendency to expand due to the excess pressure. The excess
pressure inside the bubble of radius r = 4S / r. The pressure inside a drop
= 2S / r. The bubble has an inside and an outside skin to contribute to
the pressure due to surface tension. Organic impurities and increase in
temperature lowers the surface tension of water.
If St the surface temperature of water at toC, and S0 = surface
tension of eater at 0oC,
St = S0 - 0.14 t.
19.1.1 Water on different surfaces
Clean the glass with washing powder or soap, rinse with clean water and
dry. Divide the glass into three parts: 1. Wipe butter or lard on the glass.
2. Wipe a thin even layer of wax. 3. Keep clean and dry glass as a control.
Put equal size drops of water on each part. Put equal size drops of water
on the surfaces of newspaper, paint, different types of glass, glass, perspex,
tile, your skin and a leaf of a floating water plant, e.g. Lotus. Observe
and compare the shape of the drops. Put a drop of water on some dirty greasy
clothes. Observe the drop. Add a very small amount of detergent. Describe
the drop again.
19.1.2 Bubbles in water and
aqueous solutions
Order online: Super Bubble Concentrate
See Soap bubbles (Chemistry)
1. Shake pure water and leave to stand. A few bubbles form but they soon
disappear.
2. Shake solutions of substances in water and leave to stand, e.g. soap
and water. Many bubbles appear to form a froth that remain for some time
after the solution becomes motionless. The dissolved substances decrease
the surface tension, depending on their concentration. Also the concentration
of substance is greater at the surface of a film.
3. Make big bubbles with glycerine. Use a 7 : 3: 1 solution of a water
: dish-washing detergent : glycerine solution or 100 mL soap solution, 900
mL water, 50 mL glycerine (glycerin, glycerol). Leave the solution to stand
for half an hour or, better still, overnight, and remove any foam on the
surface of the bubble mix before using it.
4. Make big bubbles with light corn syrup. Use a 6 : 2: 1 solution of
a water : dish-washing detergent : corn syrup solution.
5. Make big bubble wands and bubble stands
5.1 To make big bubble blowers, use a wire coat hanger pulled outwards
in a circle and twist the crook of the coat hanger to make a handle or cut
off the rim of large plastic containers, use a plastic funnel, use
a paper cup with a hole in the bottom, use a plastic drink bottle with the
bottom cut off
5.2 Thread a string through two drinking straws, tie the ends of the
string together but hide the knot in one straw. Grab one straw in each hand,
dip the straws into a strong bubble solution then pull the straws apart.
5.3 To make bubble stands, use an inverted plastic container, use a plastic
funnel in the rim of a bottle, use a piece of wire with a loop at one end
then wind the other end around a pencil fixed in a cotton reel spool.
6. Make bubbles by hand. Dip you hand in a soap solution with the tips of
the first finger and thumb touching to make a ring. Take your hand out of
the soap solution and blow through the the thumb and finger ring.
7. Make two bubbles with one drinking straw. Cut a slit across the middle
of a drinking straw then bend the straw at the slit to make two connected
half straws. Dip the end of one half straw in a bubble solution then blow
at the slit to make a bubble at the end. Dip the end of the other half straw
into the bubble solution then blow at the slit to make a second bubble. When
you blow the second bubble the first bubble gets larger. Make the drinking
straw straight again and close the slit with your finger. Now the first bubble
get even larger and the second bubble get even smaller because the air pressure
exerted on the smaller bubble with smaller surface area is greater than the
air pressure on the larger bubble.
8. Make long-lasting colourful bubbles. Add sugar to a bubble mix and cool
it in the refrigerator. Make a big bubble from the cooled bubble mix and
put it on a bubble stand. At first the bubble has no colour as white light
passes through it but later some air in the bubble evaporates and it becomes
smaller so colours appear because of uneven changes in the width of the soap
solution in the bubble resulting in reflection of some light from the outer
or inner walls of the bubble. The colours formed are caused by interference.
9. Make a bubble inside a bubble. Make a big bubble with a drinking straw
dipped in soap solution and place it on a bubble stand made from a plastic
cup with both ends missing. Invert the bubble stand, suck more soap solution
into the drinking straw and push its end through the big bubble. Blow a smaller
bubble inside the big bubble.
19.1.2.1 Burn butane bubbles
Do this experiment outside but NOT in the laboratory.
1. Dissolve liquid soap or detergent in water. Hold a cigarette lighter
under the water then push the button to release butane, C4H10.
Butane bubbles appear on the surface of the water. Stand well away then
use a long match to light the bubbles.
2C4H10 + 13O2 --> 8CO2
+ 10H2O
C4H10 + 13/2 O2 -> 4CO2
+ 5H2O
19.1.3 Drive a boat with surface
tension, camphor boat, spinning dancers
See diagram 19.297: Surface tension boat, spinning dancers
1. Drop small particles of camphor onto the surface of water. They mover
quickly over the surface of the water because some camphor dissolves in water
especially at sharp points reducing the surface tension of the water where
the concentration of camphor is greatest at the sharp points. The greater
surface tension on the opposite side of the camphor causes it to be pulled
in that direction. Touch the surface of the water with a rod smeared with castor oil or add soap solution.
The oil or soap spreads over the surface of the water forming a thin layer and reducing
the surface tension all around the particle of camphor so it stops moving.
2. Make a camphor boat. Cut out a triangular piece of aluminium foil.
Fold up the side to make a triangular boat. Float the boat on water. Make
a pin hole in one of the corners of the boat and put a piece of camphor in
the boat. Put the boat in water. The dissolving camphor will very slowly propel
the boat as it spreads along the surface of the water.
3. Cut out the shape of a 4.5 cm boat from stiff paper. Cut a notch
in the middle of the stern large enough to hold a small lump of gum camphor
or naphthalene mothball in contact with the water without letting it fall
out. Float the boat in a large round dish of water. Make other boats with
the notch in the stern on the right or on the left of the middle. You can
achieve the same effect with a drop of dishwashing detergent or vegetable
oil that will slowly spread across the surface of the water in a thin film.
This flow is strong enough to act as a jet to propel the boat across the
water until the surface tension of all the water in the round dish is reduced.
4. Attach
camphor to the edges of a light aluminium propeller cause it to spin on
the surface of water.
5. Cut out a picture of two dancers. Use glue to attach a small piece of camphor
to the smaller end of four small corks. Use pliers to force the blunt ends
of four needles into a large flat cork to form a cross. Attach the small corks
to the central large flat cork by sticking the sharp ends of the needles
into the sides of the small corks. Place the apparatus on the table and turn
the small corks to that the small ends with the camphor attached are all
facing in the same direction. e.g. clockwise. Attach the picture of the dancers
to the large flat cork. Place the apparatus on water to observe the perpetually
spinning dancers.
6. A
drop of Duco cement will dart around on the surface of water and two drops
will play tag.
19.1.4 Coalescing oil drops.
Let drops of oil fall into a beaker of water. Stir the water and observe
drops of oil hitting each other and coalescing. Potential energy tends to
decrease. The surface tension potential energy of the drops depends on their
total surface area. When two drops coalesce, the total surface area is reduced
and potential energy is reduced.
19.2.1 Soap and surface tension
1. Select a large plate and rinse it until you are sure that it is very
clean. Fill the plate with cold water and let it stand for a time on the
table until the water is still. Sprinkle some talcum powder lightly over
the surface of the water. Wet a piece of soap in water and touch it to the
water near the edge of the plate. The talcum powder will be drawn to the
opposite side of the plate at once. The soap reduced the surface tension
at one point. The increased surface tension on the other side contracts the
surface and pulls the talcum with it.
Repeat the experiment by substituting flowers of sulfur for the powder
and synthetic liquid detergent instead of the soap. If a transparent dish
is used, it can be placed on an overhead projector and the results displayed
on a screen.
2. A short thread difficult to catch. To observe that different liquids
have different surface tensions, pour clear water into a clean dish. Immerse
a 1 cm thread in hot wax to get a thin coating then dry it. Place it on
a water surface. Use a toothpick with soap or washing power to touch the
thread or the water near the thread. The thread may run away like a frightened
rabbit. Improve the demonstration by using a transparent dish and display
the process with a projector. Surface tension coefficient of soap liquid
is smaller than that of water so the surface tension of water pulls the
thread.
19.2.4 Meniscus, heap water up in a glass, liquid
surface is higher than the brim
1. Place a drinking glass in a shallow pan or on a saucer. Rub the top
edge of the glass with a dry cloth. Pour water into the glass until it is
full to the brim. You will observe that you can fill the glass several millimetres
above the top. Now drop coins or thin metal washers into the water edgewise.
By dropping these in see how far you can heap the water up before it runs
over.
2. To observe that water higher than the rim of a cup does not overflow
due to the surface tension of water, dry a glass cup by wiping it with a
clean dry cloth then place it on a dish with flat bottom. Fill the cup with
water. Put coins or small stones in the glass to make the water heap up.
Shake some salt in the heaped up water. The salt dissolves but he water does
not overflow. You can put in more water without it overflowing because the
convex meniscus keeps the water in place. Gently place a needle on the water
surface; it floats on water; estimate and record the height of the water
above the cup rim. Add a needle on the water surface; the two needles still
float on water; the water surface rises slightly. Again add a needle on the
water surface; then they still float on water; the water surface rises again.
Try to add needles on the water surface until water overflows. Record the
final height of the water and the amount of the needles.
3. Add nails to a full glass of water until it overflows. Objects floating
in a vessel cling to the edge until it is over full when they go to the middle.
4. Make a surface tension hyperbola (See: 3.8.0
Conic sections, hyperbola)
A large meniscus forms between two sheets of glass held at an angle in
a pan of water. Two glass plates are clamped on one edge and separated by
a wire on the other edge.
5. Place two very clean dry glasses on the table and fill one just to
the brim, i.e. no meniscus. Put coins in the glass of water, edge down. Count
the coins are put in before the meniscus breaks. Repeat the experiment with
the second glass in which you have put one drop of detergent. Count the
coins are put in before the meniscus breaks. The water forms a convex meniscus
due to the surface tension caused by the cohesion of the surface molecules.
In the second glass, the surface tension was broken by the detergent so
the cohesion between the surface water molecules less and the water overflows
much sooner.
19.2.7 Measure surface tension, surface tension
balance
See diagram 19.2.7: Measure surface tension
The surface tension of water in contact with air at 20oC = 7.28 X 10-2 N / M
1. If you can measure the force along the edge of a water surface, you
can express surface tension as a force per unit length. Clean a microscope
slide thoroughly with caustic soda solution then deionized water. Attach
the slide to a balance with a light thread as in the diagram. Add masses
to counterpoise the slide so it does not move up or down. Put a clean beaker
of deionized water underneath the slide and to allow the water surface to
attach itself to the slide. Add very small masses, m, to the balance to cause
the slide to just break away from the water surface. Measure the length,
l cm, and the width, d cm of the glass slide.
Perimeter of the bottom edge of the slide = 2(l + d) cm = 2(l + d) /
100 m
Mass added to cause break = m / 1, 000 k
Force to balance surface tension = m / 1 000 newton
Surface tension = (m x 9.8 x 100) / 1 000 x 2 (1 + d) newton per metre.
Repeat the experiment with different solutions and measure their surface
tension.
2. Adhesion balance: Measure surface tension by the direct pull on a
frame touching water surface. A glass plate on one end of a balance beam
is in contact with water. Pull a large ring away from the surface of a liquid
with a spring sale. A flat glass slide on a soft spring is lowered onto the
surface of deionized water and the extension upon pulling the off the water
is noted.
19.2.8 Float corks
in a glass jar
Half fill a glass jar with water and float a flat cork on the water.
Note where the cork floats. It floats near or touching the wall of the glass
jar. Add more water to the glass jar until a meniscus forms. Note where the
cork floats. It floats in the centre of the water surface. If you push the
cork towards the edge of the water surface it returns to the centre. When
the glass jar is half filled with water, the highest level of the water is
the circumference of the meniscus due to the adhesive forces between the
water and the glass molecules. A cork floats at the highest place so it floats
at the circumference. When the glass container is filled with water, the
forces of cohesion between the water molecules allow a meniscus to form across
the whole surface of the water, like a skin. As the highest level of the
meniscus is now in the centre of the water surface, the cork floats at this
highest place.
19.2.9 Float oil
spheres
Pour 50 mL water in a beaker. Hold the beaker in a slant and slowly pour
50 mL ethanol on top. Leave the beaker to stand. Fill a dropper with light
oil, e.g. olive oil. Insert the opening of the dropper to where the two liquids
meet in the beaker and squeeze out drops of oil. Withdraw the dropper out
of the liquid. The oil takes the shape of a sphere and stays between the
two layers of liquid. Ethanol has density 0.794 so it can float on water and
form a layer. It is not totally immiscible with water, but when poured slowly
it can form a layer and stay above the water. Where the two liquids meet,
the water and the oil mix and form a liquid with a density very close to
the density of oil so the oil forms a sphere between the two liquid layers.
A sphere is formed because it has the smallest surface area as compared
to other three dimensional shapes. When the beaker is left standing, the
alcohol evaporates slowly, and the oil sphere moves up slowly until it reaches
the surface and then the sphere slowly becomes a flat circle when all the
alcohol has evaporated.
19.3.0 Capillarity, capillary action, capillary
rise in wicks
See diagram 19.19.1: Contact angle | See diagram 19.19.2: Capillarity
| See 3.23: Volume of a liquid (Primary)
Capillarity
refers to whether a liquid tends to rise above or fall below the hydrostatic
level in a capillary tube, caused by surface tension. The liquid rises in
the tube if it "wets" the solid, i.e. the angle of contact between the liquid
and the solid is less than 90oC, and will fall if the angle of contact is greater than 90oC.
Capillarity occurs in the behaviour of liquids in wicks, paper handkerchiefs,
paper serviettes, paper towels, baby's nappy (diaper), blotting paper ands
soil. Water can rise from the subsoil by capillarity when water in the soil
is lost by evaporation or transpiration by plants.
Capillarity describes the behaviour of water in vertical thin tubes due
to surface tension. The rise of a liquid in a thin tube depends on:
1. the surface tension of the liquid,
2. the angle of contact of the surface of the liquid with the capillary,
i.e. where the meniscus meets the capillary,
3. the density of the liquid and,
4. the radius of the tube.
Height of meniscus = 2 γ cos θ /ρ g r, where γ = surface tension, θ = contact angle, ρ = density of liquid, g = acceleration due to gravity, r = radius of tube
So water will rise higher in a thin tube compared to a thick tube. However,
mercury in a capillary tube is depressed below the level in the reservoir because the forces of
cohesion within the mercury are greater than the forces of cohesion between
the mercury and the glass wall.
Capillarity may occur in bricks and other building materials so moisture
may rise up the walls called rising damp. To prevent this rising moisture
a damp-proof course of lead, bitumen or plastic may be including in the walls.
1. Put 1 cm depth of ink or coloured water in a
beaker. Dip a stick of white chalk in the beaker.. The ink rises in the chalk
by capillarity.
2. Dip different absorbent materials, e.g. jute string in the ink and
observe the rate of movement of ink up the material.
3. Dip the end of a glass tube with with narrow bore, capillary tube,
in the ink and observe the rise of the ink. The narrower the bore the greater
the rise. The forces of adhesion between the water and the glass are greater
than the forces of cohesion between the water molecules so the meniscus curves
up and the water rises in the capillary tube.
19.3.1 String siphon empties dish of water, twisting
tea bag string
1. Wash a length of string in hot water and soap. Put a dish filled with
water on the edge of the table. Put one end of the string be in the water
and let the other end hang down over the edge of the table. Water in the
dish rises into the string by capillarity then falls down the string and forms
a siphon. Water drops slowly from the end of the string to empty the dish.
Using wet string to do the experiment needs less time but makes observing
the phenomenon of capillarity more difficult.
2. Repeat the experiment with different lengths of string to test whether
length of string affect the speed of the water lost from the dish.
3. Thread a long thin knitting needle through a plastic drinking straw
and bend them both into a U-shape. Hang the u-shape over the edge of a jar
of water. Water climbs up the drinking straw by capillary action then trickles
down the other side to empty the jar.
4. Examine the twisted cotton fibres in tea bag string. Dip the tea bag
in a cup of hot water. Remove the tea bag with a steady vertical pull and
observe the twisted cotton threads unwind to spin the tea bag. The cotton
threads absorb the tea solution so the threads swell lengthen and unwind.
19.3.2 Capillarity between glass slides, water rise
by capillary action
See diagram 19.3.2: Capillarity between glass
slides
Half fill a drinking glass with water. Add a few drops of ink to colour
the water and stir it. Immerse several papers such as face tissues, toilet
paper and napkin paper that can absorb water into water. Observe and compare
water rises along the different paper. Use a glass tube, its inner diameter
is smaller than 1.2 mm, and put it into coloured water. Observe what happens
in the tube. Use other two glass tubes, one with a smaller inner diameter,
another with a larger diameter. Repeat the experiment above, observe in which
tube water rises to highest height. Use two glass slides with rubber bands
to hold them together. Insert a match stick from top of the glass slides
to separate them. Place the assembly in a pan and pour some coloured water
into the bottom of the pan. Observe the coloured water rises between two
glass plates. Note in which position water rises most and rises least.
19.3.6 Submerged float
When submerged, a wire hoop keeps a float beneath the surface of water
due to surface tension A cork and lead device floats with a wire ring above
the surface. Push the ring below the surface and it remains until soap is
added to reduce the surface tension.
19.3.7 Leaky boats
Float 30 cm long flat bottom boats made of different screen material.
A small metal boat with a large hole floats on water.
19.3.8 Cohesion plates
Two heavy glass plates stick together when a film of water is between
them. Note the difference in cohesion of dry and wet plate glass. However
if they show cohesion why do they fall apart when placed in a bell jar that
is evacuated? Atmospheric pressure holds two plate glass panes together.
19.3.9 Cohesion tube
A 2 m tube full of water and sealed at the top will support the water
column against gravity
19.3.10 Drop liquid soap on surface
with powder
Drop soap on a water surface covered with sawdust or Lycopodium powder.
19.3.11 Inflate and connect two unequal
bubbles, balloons
1. Connect 3 rubber tubes and 3 tube clamps to the ends of a T-tube,
A, B and C. Connect two L-tubes to the rubber tubes at the ends of the cross
arm of the T-tube, B and C. Mix 1 cc of detergent, 2.5 cc of glycerine and
3 cc of water. Dip separately the ends of the L-tubes, B and C, in the solution.
Press open the clamps on the rubber tubes A and B (close clamp C) and blow
into A to make a small bubble at B. Press open the clamps on the rubber tubes
A and C (close clamp B) and blow into A to make a larger bubble at C. Connect
the two bubbles by pressing open clamps B and C (close clamp A). Observe
any change in size of the bubbles. The smaller bubble blows up the larger
bubble because the surface tension in a soap bubble is inversely proportional
to the radius of the bubble. The larger the bubble, the smaller the surface
tension and the lower the pressure inside it.
2. Use a T-tube to blow two soap bubbles of different diameters then
interconnect them. A smaller bubble blows up a larger one when connected
by a tube. Blow and connect two unequal rubber balloons. Apply an equation
relating the internal pressure to the radius to the problem of the two interconnected
unequal rubber balloons.
19.3.12 Pressure in a soap bubble, soap bubble snuffer
1. Connect a slanted water manometer to a tube supporting a bubble. Vary the
size of the bubble and note the change of pressure.
2. Dip the open end of a funnel into detergent solution or soap solution.
Blow into the funnel until a big bubble forms, e.g. 25 cm diameter, then close
the end of the funnel with the tip of a finger. Bring the tip of the funnel
near a lighted candle. Remove you finger to blow out the candle flame. The
surface tension of the soap bubble is great enough to force air escaping
from the bubble to snuff the candle.
3. If the force of escaping air is not enough to blow out the candle
flame, fold a small piece of paper diagonally into four the balance the centre
of the piece of paper on a point. The air escaping from a bursting bubble
should cause some movement in the balancing piece of paper.
19.3.13 Sponge action
Water picked up by a wet sponge is greater than that picked up by a dry
sponge.
19.3.14 Water droplets
Small water droplets form on a surface not wet by water. Droplets bounce
off when sprayed on with an atomizer. Water droplets will roll across the
surface of an over full glass of water when projected out of a pipette at
a small angle.
19.3.15 Tears of wine
As 50 proof alcohol evaporates in a watch glass the remaining liquid
forms drops that run down the sides like tears!
19.3.16 Bursting water bubble
A jet of water directed upward against the apex of a cone will cause
the water to flow around and form a bubble. A drop of ether will decrease
the surface tension and the bubble will collapse.
19.3.17 Surface tension with electric
field
Droplets from an orifice become a steady stream when connected to a Wimshurst
generator surface tension with electric field.
19.3.18 Electrostatic dispersion of
water drop
Water drops from a pipette at high potential are dispersed into droplets.
19.3.19 Changing drop size
As the amount of sodium hydroxide is varied in a dilute solution, the
size of drops formed by an olive oil jet changes with the variation of surface
tension. Olive oil sprayed on hot water forms droplets but on cold water
forms an oil slick.
19.3.20 Thread ring in soap film, minimum
energy thread
A loop of thread in the middle of a soap film forms a circle when the
centre is popped. Make a soap solution with 1 cc detergent, 2.5 cc glycerine,
and 3 cc water. and put it in a shallow container. Make a soap film by dipping
a wire frame in the soap solution. Make a 4 cm diameter loop of thread. Wet
the loop in the soap solution then put it in the soap film. The thread is
probably wrinkled and has no regular shape. Pierce the centre of the loop
with the point of a dry pencil. Move the wire frame and observe the shape
of the loop of thread. It forms a circle and you can shake the frame and
move the thread circle. The thread loop lying in the soap film forms a circle
after you pierce the inside of the loop because the surface tension forces
inside the loop stop and equal forces on the outside of the loop pull on
the thread. The forces of cohesion are of equal magnitude throughout the
soap film and this allows you to tilt the wire frame so that the thread loop
moves without changing shape.
19.3.22 Soap film minimal surfaces
See diagram 19.300: Soap films and sliding wire
1. Wire frames dipped in soap film form minimal surfaces. Twist wire
to make a rectangle with one side missing. Twist each end of another piece
of wire to make a slider. The slider forms the fourth side of the rectangle.
Dip the wire rectangle in the soap solution. Pull the slider out slightly
and watch the film stretch. Release the slider. The contraction of the film
pulls back the slider.
2. Make a T-shape from coat hanger wire with cross arm 10 cm. Make a
soap solution with 1 cc detergent, 2.5 cc glycerine, and 3 cc water. Attach
a 10 cm piece of wire to the cross arm of the T-shape by connecting them
with equal length threads at their ends. Dip the wires into the soap solution.
Pull them out by holding the leg of the T-shape wire. Observe the threads
joining the pieces of wire. The threads are curved. Puncture the soap film
between the wires. the threads are now hanging down straight. The surface
tension in the soap film between the wires and thread consists of forces
of adhesion between the liquid molecules and the solids and forces of cohesion
between the molecules of the soap solution.
3. Light a candle and attach it to the table. Hold a funnel by its long
end and dip it into the soap solution. Blow through the funnel and to form
a bubble that sticks to the funnel. Point the long thin end of the funnel
towards the candle flame and observe the candle flame. Adhesive and cohesive
forces exists between the bubble and the funnel and within the soap solution
in the wall of the bubble. These forces push the air in the funnel out of
the opening to blow on the candle flame.
19.3.23 Catenoid soap film
Dip two concentric circles of wire in soap and separate them to form
a catenoid soap film.
19.3.24 Castor oil drop
Draw a large drop of castor oil under water where it forms a spherical
drop.
19.3.25 Capillary action, capillary
tubes
Sets of capillary tubes of various diameters show capillary rise with
water. Touch the end of a small glass surface with a small glass tube and
the water is drawn into the tube.
19.3.26 Water drops in tapered tubes
A drop on water in a tapered tube moves to the narrow end.
19.3.27 Capillary phenomena
1. Dip your finger in water covered with sawdust or Lycopodium powder.
2. Dip a wet paintbrush in and out of water.
3. Pour water down a wet string.
4. Pour water in a flexible paper box.
19.3.29 Wet mop
Surface tension pulls the strands of a small fluffy mop together when
wet
19.3.30 Clumping plastic
Add water to 2 polystyrene cups 1. 2 cm from top 2. almost overflowing.
Put 1 cm pieces of polystyrene foam on the surfaces. In 1. pieces move to
the edge to reduce plastic foam surface touching the water. In 2. pieces
move away because would need to float downhill but clump to reduce total
amount of hydrophobic plastic surface touching water.
19.3.31 Damp-course, damp-proof
course, in a building
A damp-course is a layer of impervious material, e.g. plastic or bitumen
sheet, built into a wall about 20 cm above ground level to prevent water
from the foundations rising in brick walls by capillarity to cause dampness
in the home or storage area. Also, vertical damp courses may be fitted at
the sides and door and window openings. Damp courses in chimneys are used
to prevent downward passage of water by capillarity.
19.3.32 Teapot effect
The teapot effect refers to the problem of the dribbling teapot when
tea is poured from a teapot runs along the under side
of the spout instead of falling into the teacup. The effect is greater at
low pouring speeds and is not caused by surface tension nor by the force
of adhesion between the tea and the inside of the spout. The explanation
is based on solutions of hydrodynamic equations described as a "hydro-capillary
effect". A common solution to the problem is to use a teapot with a spout
made of thin metal. Another solution is to coat the inside of the spout with
" super hydrophobic materials that repel water, e.g. butter.