An Experimental Study of Positive and Negative Thermophoresis

A procedure for the measurement of positive and negative thermophoretic force on a macroscopic scale is presented. Force is measured utilizing a nano-Newton Thrust stand and are taken for pressures ranging from 0.015 to 10 Pa with argon as the carrier gas. The pressure range correlates to Knudsen numbers of 10 to 0.01 where the characteristic length in the Knudsen calculation is the diameter of the 5.1 cm spherical test particles. The temperature gradient is +/-35 K/m which is established between thermally driven and ambient monitored walls with a separation distance of 40 cm. Measurements of negative thermophoresis are presented as the first definitive experimental measurements of the effect. Negative thermophoretic force production is confirmed and determined to be on the order of 5% of the maximum positive thermophoretic force for a given thermal gradient. Force measurements are also taken to determine the effect of temperature gradient, wall proximity, location of particle between walls, thermal conductivity of particle, edge perimeter, and extension from spherical to spheroid particles.