School Science Lessons
Physics - Surface tension, surface properties, capillarity, fluid mechanics
Updated: 2008-07-16
Please send comments to: J.Elfick@uq.edu.au
See also: Interesting websites
Table of contents
19.0.0 Fluid mechanics, surface properties, surface tension
19.3.0 Capillary action, capillarity, capillary rise in wicks
19.0.0 Fluid mechanics, surface properties, surface tension
19.0.1 Drops and films
19.0.2 Bubbles in the air
4.212 Soap and surface tension
4.213 Float a needle on water
4.214 Float a razor blade
4.215 Lift the water surface
4.216 Hold water in a sieve
4.217 Heap up water in a glass
4.218 Pinch together water streams
4.219 Drive a boat with surface tension
4.220 Blow soap bubbles
4.221 Soap bubble support
4.222 Soap film and sliding wire
19.1.0 Surface properties
19.1.2 Bubbles in water
19.1.3 Camphor on water
19.1.4 Coalescing oil drops.
19.2.0 Surface tension, surface tension of water and ethanol
19.2.1 Effect of soap on surface tension
19.2.2 Lift the water surface
19.2.3 Hold water in a sieve
19.2.4 Meniscus, heap water up in drinking glass, liquid surface is higher than the brim of glass
19.2.5 Blow soap bubbles
19.2.6 Pinch together water streams
19.2.7 Measure surface tension, surface tension balance
19.2.8 Floating cork in a glass jar
19.2.9 Floating oil sphere
6.12 Floating and sinking (Primary)
19.3.0 Capillary action, capillarity, capillary rise in wicks
19.3.1 String siphon empties dish of water, twisting tea bag string
19.3.2 Capillarity between glass slides
4.56 Capillary action in soil and deposition by groundwater
4.57 Infiltration and capillary action by groundwater
19.3.3.1 Water climbs up soils, capillarity
19.3.3.2 Water climbs up
6.9 Water climbs up (Primary)
19.3.6 Submerged float
19.3.7 Leaky boats
19.3.8 Cohesion plates
19.3.9 Cohesion tube
19.3.10 Drop liquid soap on surface with powder
19.3.11 Inflate and connect two unequal bubbles, balloons
19.3.12 Pressure in a bubble
19.3.13 Sponge action
19.3.14 Water droplets
19.3.15 Tears of wine
19.3.16 Bursting water bubble
19.3.17 Surface tension with electric field
19.3.18 Electrostatic dispersion of water drop
19.3.19 Changing drop size
19.3.20 Thread ring in soap film, minimum energy thread
19.3.22 Soap film minimal surfaces
19.3.23 Catenoid soap film
19.3.24 Castor oil drop
19.3.25 Capillary Action, capillary tubes
19.3.26 Water drops in tapered tubes
19.3.27 Capillary phenomena
19.3.28 Surface tension boat propulsion
19.3.29 Wet mop
19.3.30 Clumping plastic
19.0.0 Surface tension
See diagram 19.0.0: Surface tension, capillarity
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. 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.
19.0.1 Drops and films
Work must be done to extend a liquid surface, so when the liquid surface is increased the potential energy increases. In general potential energy tends to decrease so a liquid surface, tends to decrease. If air,  liquid and solid contact the three surface tensions are S1 between air and liquid,  S2 between liquid and solid, and S3 between air and solid making the air-liquid, liquid-solid, and air-solid surfaces as small as possible.
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 theta, where theta is the " angle of contact," between the liquid and the solid surface.
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 So = surface tension of eater at 0oC,
St = S0 -0.14 t
19.1.0 Surface properties, phenomena at the contact surface between solid and liquid
See also 3.23: Volume of a liquid (Primary)
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. You measure this surface tension in Nm-1. As the surface molecules are under tension the liquid contracts to minimize the surface area. This is why a falling water drop is nearly spherical. 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.
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.
19.1.1 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
1. Shake pure water and allow to stand. A few bubbles form but they soon disappear.
2. Shake solutions of substances in water and allow 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 dissolves substances decrease the surface tension, depending on their concentration. Also the concentration of substance is greater at the surface of a film.
19.1.3 Camphor on water
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.
2. Touch the surface of the water with a rod smeared with castor oil. The oil spreads over the surface of the water forming a thin layer and reducing the surface tension all around the particle of camphor so they stop moving.
3. 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 dissolvingcamphor will very slowly propel the boat.
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.
19.2.0 Surface tension
Surface tension is the property that causes the surface of a liquid to behave as if covered with a weak elastic skin; this is why a needle can float on water. 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 by which water soaks into a sponge. The surface tension of water is the highest of all liquids, 73 mN m-1. his high surface tension controls the shape of a meniscus, raindrops and sea spray.
19.2.1 Effect of soap on surface tension, a short thread difficult to catch
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. Try a similar experiment but substitute 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. 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.
19.2.2 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.
19.2.3 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
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 also: 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.
19.2.5 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.
19.2.6 Pinch together water streams, water twisted together
See diagram 4.218
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.
19.2.7 Measure surface tension, surface tension balance
See diagram 19.2.7
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 Floating cork 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 Floating oil sphere
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
See diagram 19.3.0: Capillarity
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.
When a glass capillary tube is dipped into water,  the water rises in the capillary tube a height h above the level of the water outside. The weight of the water column of water is supported by the surface tension force acting along the line of contact of the water, air and glass.
The upward force due to surface tension = s cos theta X circumference of the capillary tube, 2pi X r (pi = 22/7)
The glass is wet, so theta =0 and cos theta =1.
So S X 2piR = mg
Let height of liquid in capillary tube = h and density of liquid = d.
S X 2piR = mg (weight of liquid) = volume of liquid X density of liquid X g = pir2hdgbeing equal to
S costheta X circumference of the tube, equals S cos theta X 2pi r.
Here theta = 0, cos 0 = 1, the glass being "wet."
S =  2pi r = mg = Vdg = pir2hdg
So s = rhdg/2
However if the liquid does not wet the capillary tube, S =  rhdg/2cos theta, where theta is the angle of contact.
Capillarity occurs in the behaviour of liquids in wicks, 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. Put 1 cm depth of ink or coloured water in a small beaker. Put in a stick of white chalk. The ink rises in the chalk by capillarity.
19.3.1 Empty a dish of water with string siphon, 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 capillarity action
See diagram 19.3.2
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.
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.
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 bottomed 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 bubble
Connect a slant water manometer to a tube supporting a bubble. Vary the size of the bubble and note the change of pressure.
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
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 2.300
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.
19.3.28 Surface tension boat propulsion
Attach a crystal of camphor to the back of a small boat surface. Attach camphor to the edges of a light aluminium propeller cause it to spin on the surface of water. Use alcohol in a surface tension boat. Rub a match stick on a cake of soap or attach a piece of camphor and
place in water. Camphor darts around on the surface of water until soap is introduced. A drop of Duco cement will dart around on the surface of water and two drops will play tag.
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.