Thermophoretic Force Measurements using a nano-Newton Thrust Stand

Thermophoresis is a rarefied gas phenomenon characterized by the migration of particles in a fluid with a macro level temperature gradient. This particle migration occurs in the direction of the heat flow, meaning an object immersed in a gas with a macro-scale temperature gradient experiences a force acting from hot to cold. Applications for this thermophoretic force include commercial precipitators, micro-contamination control, particle extraction from a flow for sampling, and particle manipulation in vapor deposition processes. The thermophoretic force on a sphere was experimentally measured across a wide range of Knudsen numbers with a rotational, nano-Newton thrust stand using an experimental setup similar to Crookes in 1873. Argon was used as the carrier gas for the experiment. Results indicate that the highest force experienced by the sphere occurs at a Knudsen number of 0.3 based on the sphere diameter. Experimental evidence of negative thermophoresis, where the force acts from cold to hot, is also presented.