Determination of the Strengths of van der Waals' Forces

Also compare with the Casimir Force.

The good thing about this experiment is that it can be done in a couple of days. BUT, it takes a lot of brain-power to think it through and to understand it and its significances. The project is at the boundary of chemistry and physics. Again, it will require a great deal of thought to go into your project report's introduction and discussion (at the report's end).

Fundamental equipment needed:

• a brand new teflon-coated cookie sheet. It MUST BE new - never used for making cookies or pizza.
• a water atomizer
• perhaps some "dry ice."
• some way of determining the mass of droplets of water or other liquids.

Possible Experimental Protocol

1. Spray a little water as small droplets onto the teflon surface, which is laid horizontal on a table.

2. You should see the drops bead up tightly. This is because (you answer this). Since the teflon repells the water, you expect that the water is floating atop the teflon with a very small space between them. If there is a space (meaning that there is repulsion), then it is like "air hockey" where the small game pieces float on a thin layer of air.

3. What would happen were the air hockey game turned upside down? All the pieces fall to the floor. So what should happen were you to turn the cookie sheet upside down? Do all the droplets of water fall off?

4. If they do, you have proven that they are being repelled and that there was a thin layer of space between the water and teflon. But if they drops don't fall off, what is holding them up? What kinds of chemical and physical bonds do you know about?

5. Well, I will tell you that the smaller drops do stick. But why? Which of those bonds would explain why those drops stick? If none explain it, then you are the discoverer of a new type of bond or force. You get to name it and measure it. Those are all questions you can answer for your project.

6. Once you have allowed all the larger drops to fall away, turn the sheet back over so that the drops are on top.

7. Place that portion of the sheet with one of the largest drops on top of the dry ice. The water drop will nearly instantly freeze.
8. Using a pre-chilled tweezers, lift up the frozen droplet and measure the diameter of the flat circle on its bottom. And from that you can determine the "area of contact" between the water and the teflon.

9. You now need to determine the mass of that droplet. Why? Because whatever force was holding up that drop had to at least be strong enough to hold up that mass, which gravity was trying to pull off the teflon surface. There are at least two methods for determining the mass of that droplet:
   1. Direct method: merely set the frozen droplet on a very sensitive scale and weigh it. It doesn't matter if it melts excpet that once it does it will evaporate rather quickly because it is small.

   2. Indirect method: determine its volume (since one milliliter of water weighs one gram). Set a variable pipetter to 20 microliters, and draw up the droplet into the pipetter. If not all the water is drawn up, carefully eject that which was taken up, and set the pipetter to a higher volume and try again. Of course, do the opposite if all of the droplet AND some air are drawn into the pipetter. Remember that one microliter weighs one microgram.

10. Once you have collected your data from perhaps 20 drops (some smaller ones, too!), you must do some math - determine the areas of contact. Of course, you know the formula for the area of a circle!

11. Finally you want to make a graph of mass versus the area of contact. (Is this an infinite line? Why not? Because any drops larger than those you were able to measure have fallen off. So you have a "limit force." Ah, ha!

12. You are now ready to start all over again, but slightly differently:
    1. You might want to try other liquids on the teflon sheet:
       1. salt water
       2. sugar water
       3. vegetable oil
       4. wax - melt first, tweek (splash) some on, etc.

    2. Or you might want to try water on other hydrophobic surfaces - e.g.: wax paper.

Please remember: NO ONE HAS EVER DONE THIS BEFORE. Your report will be THE reading material for ones following you.

Hint: DO NOT WORK ALONE. My website promotes young scientists' working together. While this project is simple to do, it will take a lot of thinking, talking, wondering - do it with a friend, OR I can give you two or three penpals around the country or the world.