BUBBLING WITH fUN Archana Ghode, IIT (Madras), Chennai Very often we see people blowing bubbles to sell their bubble solutions on the beach or in markets. It is a lovely sight to see the bubbles floating in air; bubbles make a place beautiful and bright. These bubbles are mostly made up of soap. A soap bubble is a thin film of soap and water stretched by the air inside. The soap film is stretchable. Many of us have made soap bubbles while playing with soap water. Many sellers also have a twisted wire with a loop. The loop is dipped into the soapy solution. When it is removed a soap film clings to the loop. What happens when you blow into a soap film? If you blow quickly and hard, it breaks. However, if you blow slowly and gently, the soap film that is stretched across the surface will start to stretch more until a bubble is formed. This then breaks away from the loop and floats like gossamer. FIGURE ONE If you make bubbles with just soap water, they do not last very long. They eventually burst. This happens when the layer of water evaporates. In fact, it is possible to make bubbles with plain water, but these last for even shorter times. The soap stabilises the bubble by decreasing the surface tension of water. For more details, see the box on surface tension and bubbles. Soap decreases surface tension to approximately one third the surface tension of pure water. Soap does not strengthen bubbles; it stabilizes them. You may have noticed that a stretched balloon usually bursts at its weakest point. The same is true of a bubble. Addition of soap prevents this from happening. Suppose one part of the soap film stretches, so the amount of soap in this part of the surface (its surface concentration) decreases. Since there is less soap, the surface tension increases here and thus strengthens this portion of the bubble surface. In other words, soap prevents a weak portion of the bubble from stretching further. In addition, the soap reduces evaporation so the bubbles last longer. Detergents are better than soaps. This is because detergents don't contain a special chemical compound called carboxylate group. This group reacts with any calcium or magnesium ions in hard water to form a scum. So soap can form scum with hard water and will ruin your bubbles, while detergents don't do this. Of course, if you have clean distilled water, you can use simple soap instead of detergents. Adding glycerine lengthens the life span. Glycerine, with a molecular formula C3H5(OH)3, helps soap bubbles hold water and this helps to keep the bubbles from bursting for a longer time. Both glycerine and water contain hydrogen. (The chemical formula for water is H2O). The hydrogen from different molecules interact to form weak bonds called hydrogen bonds. Hydrogen bonds formed between gycerin and water delay evaporation. Dry air or dry hands can still burst a bubble. Moist air works best of all, which is why bubbles blown on a beach seem to last for ever! You may have noticed that bubbles are always spherical (round in shape) even if the loop is not exactly round. Why do bubbles form a sphere? Well, a sphere is the shape that provides the most space for the air inside with the least stretching of the soap film. So the soap film always contracts to make the smallest possible surface that can contain the air inside it. That means, the sphere provides the minimal surface area needed to enclose a given volume, making it the most efficient shape for a bubble. By doing this, the surface energy of the bubble is minimised. If you are curious about this, read more details in the box on geometry of soap bubbles. What about bubbles in soda or an aerated drink? Are they the same? A typical aerated drink has carbon-dioxide dissolved in it. When the bottle is opened, the carbon-dioxide rises to the surface as little bubbles. The bubble of gas comes out through the liquid surrounding it. So it has only one surface, unlike soap bubbles which have two surfaces (inner layer of soapy water in contact with air inside and outer layer of soapy water in contact with air outside). These bubbles simply break when they reach the surface, thus releasing the carbon-dioxide into the air. Make your own bubble solution You will need clean water, any dish-washing detergent such as vim or pril, and some glycerine. Glycerine can be purchased at most pharmacies. You won't need much, so buy just a little. 1. Popular and simple bubble solution: Here is very common solution for most bubble tricks, experiments, and activities. Take a small amount of any dish-washing detergent and 7-10 times as much water. Mix water, detergent, and 1-2 tablespoons of glycerine in a bowl or plastic container. 2. Outdoor adventures thick bubble solution: This is a thick solution that forms bubbles strong enough to withstand a small puff of air. You can blow bubbles inside of bubbles with this mixture, and you don't need a straw. Just make a bubble and blow! Use only 2 parts water to one 1 part dish-washing detergent. Combine in a bowl with 1-2 tablespoons of glycerine. 3. Bouncy bubble solution: This is a fun solution that can give you bubbles that can bounce off your clothes. Dissolve 2 packages of unflavored gelatine in 4 cups hot water. The water must be just boiled; ask an adult's help to do this! Ass 3-5 tablespoons of glycerine and 3 tablespoons of dish-washing detergent. The mixture will gel, so you'll need to reheat it whenever you use it. You can experiment with the proportions so that your bubbles last a long time. Of course commercial bubble solutions contain special ingredients. Some solutions contains a polymer that allows bubbles to resist evaporation. The polymer reacts with air to harden three to four seconds after a bubble is blown. The bubbles can then be caught with dry hands without popping. But the joy of bubbles is being able to make more and more of them! Some tips: 1. Stay clean: Try to keep all bubble-mix equipment free of dust, dirt, fingerprints, bad breath, flying insects, ... If your bubble tools touch anything other than the bubble mix container, you may have a piece of dust in every big bubble. It can burst the bubble very fast. 2. Avoid sun, work in shady areas. Try bubble making right after the sun sets. 3. Make bubbles when the air is still or only slightly breezy. 4. Keep your bubble tools really wet with bubble solution. Play with bubbles on a beach or after a rainstorm. BOX: Soap film and surface tension A soap film consists of a thin sheet of water sandwiched between two layers of soap molecules. One end of each soap molecule is hydrophilic, or attracted to water. The other end consists of a hydrophobic hydrocarbon chain that tends to avoid water but likes to stick to greasy dirt. In fact, that's how soap helps in washing. The hydrophobic end of the soap molecules attaches itself to the dirt in clothes or vessels. The when you vigourously wash it off, the dirt is removed as well. The hydrophobic ends of the soap molecules crowd to the surface, trying to avoid the water, and stick out away from the layer of water molecules. As a result, water molecules separate from each other. The increased distance between the water molecules causes a decrease in surface tension, enabling bubbles to form when air is gently blown into the film. Also, since the hydrophobic ends stick out of the water, the water surface is protected and does not evaporate easily, so the bubble lasts longer. BOX: Geometry and soap bubbles We read in the main text that the surface area of a sphere is least for a given volume. Think of containers of different shapes like the cube, the tetrahedron, sphere, etc., all of which hold 1 litre of water. Then you will need least amount of paint to paint the sphere rather than the cube or any other shape. That's because it has the least surface area. This is hard to see and calculate for three-dimensional shapes. Let us look at a simpler version in two dimensions. Consider different shapes such as triangle, square, hexagon, octagon, and circle. The argument about least area for same volume above means that you have least (smallest) perimeter for the same area in the two dimensional case. The shapes are shown in the figure, along with their perimeter (length of boundary). Each figure has unit area (if area is 1 m2, then perimeter given is in m) so each perimeter or boundary encloses the same area. It is clear that the perimeter decreases as the number of edges of the polygon increases, with the least perimeter being that of the circle (which you can think of as a polygon with infinite sides). A similar argument in three dimensions leads to the fact that the sphere is the smallest surface that can enclose a given volume. There are other nice geometrical facts about soap bubbles. For instance, when there are several bubbles in the air, they can either bounce off each other or come together and merge. When they merge, they acquire a specific shape. They have a common wall. If both bubbles are the same size, the wall is flat. The wall and the outer surface of the two bubbles must always join at 120 degree angle. If the bubbles are not of the same size, the smaller bubble will bulge into the bigger bubble. When three bubbles meet, they will also meet at the centre at an angle of 120 degrees to each other. After some time, the bubbles that are stuck to each other may join to become one bubble.