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basic procedure
making giant bubbles
what's happening
Did you know that water molecules are actually very sticky? You've probably seen them clinging to lots of surfaces like walls, windows and tables. When one material sticks to another we call it adhesion, and water molecules adhere or stick to many other materials, but they also like to stick to other water molecules. When a material sticks to itself we call it cohesion. If there is no good surface around for water to stick to it's perfectly happy sticking to itself by pulling on several other water molecules nearby. In fact, each water molecule pulls so hard on the others around it that all of the water tries to pull itself into one tight spherical ball if it can't find anything else easier to stick to. This force is sometimes called surface tension. Pour some water onto a freshly waxed car or tabletop and it forms lots of little droplets or "beads" because it can't stick as well to the waxy surface (check the video link at the end to see what water does on the International Space Station). If you pour it on most other surfaces, however, it will spread out if it can stick to those surfaces. Okay, so what's all this have to do with soap bubbles? A bubble is like a balloon, with a thin film or skin on the outside surrounding and trapping air (or some other gas) inside. Water doesn't stick to air though, leaving only other water molecules for it to stick to, but the surface tension of water, or the force with which it pulls on other nearby water molecules, is so strong that it will form small beads full of water (like raindrops) rather than stretching into a thin film. If we want to stretch water into a thin skin to make a bubble we need to add something else that it would rather stick to, and that's exactly what soap does. Soaps and detergents are usually long snake-like molecules with one end (let's call it the head) that likes to grab or pull on water molecules while the other end (tail) likes to get as far away from water as possible (or if possible grab something very different like oil or grease, which, by the way, is what makes them so good at cleaning dirt). The head ends of soap molecules in your bubble solution grab onto water molecules, but since their tail ends want to get out of the water at the same time, they can only grab one side so that their tail ends can remain in the air. Other soap molecules can also grab onto the same water molecules but from the opposite side, while also keeping their tail ends in the air. This leaves the water molecules free to grab some other water molecules, but only in a region where there is no soap, forming a thin three-layer film skin. You can think of these three layers sort of like a peanut butter sandwich, where the pieces of bread on the top and bottom are the soap layers and the peanut butter inside is the water. The surface tension of the water layer inside gives the skin of your bubble its strength, while the soap layers above and below weaken the surface tension just enough to allow the water to stretch out into a thin film so that you can blow a bubble. Materials like soap that reduce the surface tension of water (or other liquids) in this way are called surfactants. The soap-water-soap film may be strong enough to form a skin of a bubble but it's also very, very thin- thinner than the hair on your head- so there is a limit to how big a bubble you can make with just the cohesive force of water holding it together. Making it any bigger and the skin will break, popping the bubble. The guar gum (or cornstarch) you added to the water gives it extra strength to make even bigger bubbles. The baking powder also helps by controlling pH or acidity of the solution. You may have seen some soap bubble recipes that add glycerine to the soap, which works by slowing the evaporation of water (although most bubble experts agree that recipes like ours work much better). Reference links at the end explain in more detail how these ingredients work. variations and related activities
Try blowing a small bubble inside one of your big bubbles. To do this get as close as you can to the big bubble without touching it, then gently blow a puff of air into the wall of the bubble (see the video). Why do bubbles pop? Remember that the skin of a soap bubble is very thin, and it's only the water layer which really holds the bubble together as the soap layers on either side actually weaken the water's cohesive forces or surface tension. As the layer of water evaporates the skin of the bubble gets thinner and thinner until the water molecules can no longer hold onto each other and the bubble pops. Do you think your bubbles would last longer on a hot and dry day, or a cool and humid day? Of course a bubble also pops when you touch it- that is if you touch it with your dry finger or something else that draws the water out of its film just like evaporation does. If you dip your finger or hand into the soap solution you should be able to carefully push it through the skin of the bubble and pull it back out without popping it. You can even catch and hold a bubble in your hand (as long as your hand is completely wet- touch it with any dry skin and it will pop. How can a triangle or square shaped bubble wand still make spherical (ball-shaped) bubbles? Once you release your bubble from the wand you may have noticed that it quickly formed a more spherical shape. This is because a sphere is the most stable shape for a bubble, allowing it to enclose the most air while using the least amount of soap (mathematicians might say enclosing the maximum volume with the minimum surface area). Huge bubbles have a large amount of water in their skins, however, and as gravity pulls on this water (as well as any wind blowing on the bubble) the relatively heavy skin gets tugged quite a bit, which is why the bubble wobbles and changes shape a bit as it moves, but it still tries to remain mostly spherical. Did you also notice how colorful your bubbles are (look at the photo above)? In fact, the different colors form rainbow-like stripes that circle around the bubble (although the colors here are formed in a different way than they are in a rainbow). These colored stripes are called interference fringes and caused by light rays reflecting off the thin soap-water-soap film of the bubble. Some light rays reflect off the outside surface of the film and back to your eyes while others pass through this surface, into the film itself, and then reflect off the inside surface and then back to your eyes. When these various light rays recombine on the way to your eyes we say that they interfere with each other. This also happens when light rays strike a pane of window glass, but the soap film of the bubble is much, much thinner. In fact, the thickness of the soap film is close to the wavelengths of light, so that this interference creates colored bands as the thickness of the film changes, which is why the colors change as the bubble slowly evaporates and its skin gets thinner (check out the references at the end to learn more about the physics of light). Gravity also pulls the water in the bubble downward, so that it's skin is thicker near the bottom. Thus the fringes or bands of color indicate changes in film thickness similar to how the lines on a contour map indicate elevation changes. references and links to more information
Just about everything you ever wanted to know about soap bubbles:
How does soap work, and what is the difference between soap and detergent?:
Experiments and other activities with surface tension and soap:
Surface tension of water in space: Why are bubbles round?:
Colors in soap bubbles (and rainbows, just for fun):
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