Evidence for Failure of Millikan's Law of Particle Fall in Gases

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Abstract
Discovery of a new phenomenon is reported for submicron spheres undergoing a fall in nitrogen in the density range of 1 to 22 atm at 293 K. The measured fall rate of a single test particle increases with density if the particle is sufficiently small that its Brownian diffusion velocity becomes comparable to its velocity of fall, in contradiction to the Millikan law of fall embodying the concept of slip. The trend appears to reverse smoothly to that of Millikan's in the limit of large particle radius.