How to Have an Awesome Design Project: Light-Mills!!

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Step One: Recruit some amazing partners, like my fellow VSA, Caitlin Sarian

Step Two: Choose a cool project that you have a personal interest in, but isn’t too big to accomplish in a semester. I’ve always been fascinated with space and energy technologies, so one night this summer I was lying in bed thinking about solar sails and wondering if it would be possible to use that energy in a mill to generate renewable energy mechanically in space. So I googled “radiometers” and started reading. Apparently, back in the 1800’s some guy much smarter than I will ever be thought the same thing. He designed this experiment to prove it, and VOILA the light mill!!

Turns out, the “light mill” doesn’t actually turn due to solar pressure, it works because the mills “panes” have 1 side painted white and the other side black. The light heats the black side up and the rarified molecules in partial vacuum get pushed to that side of the pane. The resulting pressure differential causes the panes to turn.

Unfortunately, the force that our mill will generate is maybe 1mN (or 0.01 lbs)

So far we’ve cut panes with holes in them, because the increased surface area increases the rate that molecules “creep” over to the black side of they pane. We make those using thin acrylic sheet and a laser cutter.

Then we use a 3D printer to make the center structures, which we will glue the panes to. The middle, part, which we call the thimble, has a hole in the center which rests on a pointy stick and turns with minimal friction.

Lastly, we’ll put the apparatus in a small vacuum chamber, shown the bottom, take the vacuum down to 20 mTorr, turn on a flashlight for the light energy, and count how fast the mill turns!


 

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