wendt@arizona.UUCP (09/01/83)
Several years ago John Galloway, Dave Pearson, and I ran some simulations of particles approaching and bouncing off simple funnel shapes (trapezoids, actually). Assuming perfect reflection, the surprising result was that it can be up to 3 times easier to throw a ball through the narrow end of a funnel than through the wide end!! For these simulations the funnels were actually trapezoidal channels cut through walls. I can see one of two possibilities here: first, molecules don't reflect perfectly. Undoubtedly. Second, if we set up a wall full of little holes in a good enough vacuum (and with small enough holes), so that the inter-molecular collisions can be neglected, shortly the pressure on the wide side of the funnels would be three times that on the narrow side!! It seems strange to me that the second law of thermodynamics should depend so critically on the characteristics of molecular impact with surfaces. If we use big enough molecules we ought to start approaching perfect reflection. Can somebody explain this please? Can somebody make a such a wall which is thin enough and with lots of sufficiently small holes (45% angles are fine)? Perhaps by dropping very fine sand on a very thin sheet of glass in a vacuum? Alan Wendt