Extreme Science. Surface Tension

Transcription

1 Extreme Science Surface Tension Lesson We've all seen bubbles, or wondered why raindrops look like they do when traveling down a window. Well the answer is a simple one, and a complicated one, all at the same time. It's called surface tension. While there are lots of things to talk about when discussing surface tension, let's start out by defining what it is. Surface tension is basically the property of a liquid that causes its surface to behave like a "stretchy" elastic sheet. It is this property that allows insects, such as the water strider, to walk on the surface of water. In some cases, even small objects can float on the surface of water, even metal ones such as needles, coins, or paper clips, to float on the surface of water. Surface tension is also the cause of capillary action. Pretty much all of the physical and chemical behavior of liquids cannot be explained or understood without taking into account surface tension. Surface tension governs the shape that small bodies of liquid can assume as well as the degree of contact a liquid can make with another substance. When we apply Newtonian physics to the forces that arise due to surface tension, we can observe and even predict many liquid behaviors that are so "ordinary" that most people often take them for granted. If you could actually see molecules of water and watch how they act, you would see that each water molecule electrically attracts its neighbors. Each water molecule contains two hydrogen atoms and one oxygen atom, which we always shorten to the obvious "H20." The amazing "stickiness" of water occurs because the two hydrogen atoms are attracted to the oxygen atoms of other water molecules nearby. This is known as hydrogen bonding.

2 Hydrogen atoms have single electrons, and these electrons typically spend a lot of their time within the water molecule, more towards the oxygen atom. This leaves their outsides positively charged. One the other hand, the oxygen atom has eight electrons, and they are usually found on the side away from the hydrogen atoms, making the opposite side of the whole molecule negatively charged. We all know that opposite charges attract, so it is no surprise that the hydrogen atoms of one water molecule like to point toward the oxygen atoms of other molecules. In their liquid state, water molecules have too much energy to become completely "locked" into a fixed pattern. Even still, all the various temporary "hydrogen bonds" between the molecules make water an incredibly sticky fluid. Keep in mind also that a fluid can be either a gas or a liquid. Within the water itself, there is a battle going on. You see, every molecule is engaged in a "tug of war" with its neighbors on every side. Each molecule is pulling on every other molecule so much, that overall, no given molecule feels any net force at all. However, at the surface of water things are much different. Since there is no liquid above the surface of the water, there are no other "pulls" to equal things out, so surface molecules tend to be pulled back into the liquid itself. It takes a lot of work to pull a molecule up to the surface. When the surface of the liquid is "stretched," it becomes larger in area, meaning more molecules are dragged from within the liquid in order to become part of this increasing area.

3 Bubbles are a great example to observe how surface tension works in everyday life. You never know just how much science you can find while doing the dishes! Bubbles are made of a very thin skin surrounding a volume of air. The principle is very similar to that of a balloon. Balloons have a rubber skin, which is elastic, making it stretch when inflated. If you were to let the mouthpiece of the balloon go free, the rubber skin wants to return to its original shape, so it squeezes the air out of the balloon and it deflates as it flies around the room. The same thing actually happens if you start blowing a bubble and then stop. The liquid skin of the bubble is stretchy due to the surface tension, and like the skin of a balloon it pushes the air out of the bubble, leaving a flat circle of soap in the bubble wand. What's cool about the surface of the bubble is that unlike a sheet of rubber, which when unstretched loses all tension, a bubble always has its "stretch" no matter how small the surface becomes. If you blow a bubble and close the opening by flipping the wand over, the tension in the bubble skin will try to shrink the bubble into a shape with the smallest possible surface area in relation to the volume of air it contains. We know this shape as a sphere.

4 Let's take a look at some common shapes that a bubble could take instead, and see how much surface area they would each require to hold 1 cubic inch of air. Tetrahedron, 4 sides, 7.21 square inches of surface area required Cube 6 sides, 6 square inches of surface area required Octahedron 8 sides, 5.72 square inches of surface area required Dodecahedron 12 sides, 5.32 square inches of surface area required Icosahedron 20 sides, 5.15 square inches of surface area required Sphere infinite sides, 4.84 square inches of surface area required Note how much less surface area the sphere requires.

5 So how does soap affect the surface tension of the water? Well, think about trying to blow a bubble with just plain water. It won't work, will it? So what does soap change about things? Well, first of all, there is a fairly common misconception that water by itself doesn't have the surface tension necessary to maintain a bubble and that by adding soap, you "increase" the surface tension abilities of the water. However, the reality is that soap actually decreases the pull of water's surface tension. Usually to around a third that of plain water. What really happens is that surface tension in plain water is actually too strong for bubbles to really last for any length of time. Another problem with bubbles trying to be made with water alone is evaporation, since the surface becomes thin pretty quickly, causing them to pop. The molecules in soap are made up of long chains of carbon and hydrogen atoms. At one end of the chain is a configuration of atoms which likes to be in water, called hydrophilic. The other end basically repels water, which means that it is hydrophobic. This end also easily attaches to grease (hmm, and what do we use soap a lot for?). When you wash dishes, the "greasy" end of the soap molecule actually attaches itself to the grease on your dirty plate. This lets water seep in underneath. The soap molecule pries the particle of grease loose and surrounds it, so that it can be rinsed away easily. In a soap/water solution the hydrophobic ends of the soap molecule do not want to be in the liquid at all. Those that do find their way to the surface squeeze their way between the surface water molecules, and basically push their hydrophobic ends out of the water, which separates the water molecules from each other. Since the surface tension forces become smaller as the distance between water molecules increases, this means that the soap molecules actually decrease the surface tension. If you take a glass of water and fill it to the top, you can actually "overfill" it by slowly adding more water to the top. be higher than the top edge of the glass. The water level will actually

6 However, if you were to lightly touch the surface of that water with a slightly soapy finger, the pile of water would immediately spill over the edge of the cup because the surface tension "skin" is no longer able to support the weight of the water. Before, the soap molecules had separated the water molecules, which decreased the attractive force between them. Because the greasy end of the soap molecule sticks out from the surface of the bubble, the soap film is more or less protected from evaporation (as grease does not evaporate), which in turn prolongs the life of the bubble substantially. The colors in a bubble actually provide us with an extremely accurate tool for measuring the thickness of the soap film. Light waves, like ocean waves, have peaks and valleys. These are called crests and troughs, respectively. Red light has the longest wavelength and violet the shortest. All waves, not just light, have a curious property; if two waves combine, and the waves meet each other crest-to-crest, they add up and reinforce the effect of each other. However, if they meet crest-to-trough, then they cancel each other out so that they have no effect at all. In this kind of meeting, for every "up" vibration in one wave, there is a corresponding "down" vibration in the other wave. This combination of equal ups and downs causes complete cancellation or interference. Interference is responsible for the pearly luster of an abalone shell, the beautiful colors in some bird feathers and insect wings, and the flowing patches of color in an oil slick on the street after a rain shower - and for the color of bubbles. Learn something new every day, don't ya?

7 We know that white light is made up of all colors; all wavelengths. If one of these colors is subtracted from white light, say for example, by interference, then we see the complementary color. For example, if we subtract blue light from white light, we end up seeing yellow. The skin of a bubble shows us the complementary colors produced by interference. Depending on the thickness of the bubble wall, a certain wavelength will be cancelled and its complementary color will be seen. For example, long wavelengths such as red light need a thicker bubble wall to get out of step than short wavelengths, such as violet or purple. When red is cancelled, it leaves a blue-green reflection. As the bubble skin gets thinner, the yellow is cancelled out, leaving blue; then green is cancelled, leaving magenta; and finally blue is cancelled, leaving yellow. Eventually the bubble will become so thin that cancellation occurs for all wavelengths and the bubble appears black against a black background. What's cool, is that if you let a bubble hang from a bubble wand for a little while, the interference colors begin forming horizontal stripes, because gravity is forcing the bubble film to become thicker at the bottom than at the top, forming a sort of wedge shape. As the bubble drains, the wedge of bubble solution gets thinner and thinner. We will look more at the effect of capillary action caused by surface tension in an upcoming lab assignment.