A laser-Doppler velocimeter study of velocity profiles in the laminar boundary layer adjacent to a heated flat plate revealed that the seed particles used for the LDV measurements were driven away from the plate surface by thermophoretic forces. causing a particle-free region within the boundary layer of approximately one half the boundary layer thickness, Measurements of the thickness of this region were compared with particle trajectories calculated according to several theories for the thermophoretic force. It was found that the theory of Brock, with an improved value for the thermal slip coefficient, gave the best a~reement with experiment for low Knudsen numbers, ~/R = O(lo-). where ~ is the mean free path and R the particle radius.Data obtained by other experimenters over a wider ranqe of Knudsen numbers are compared, and a fitting formula for the thermophoretic force useful over the entire range 0 ~ ~/R ~ oo is proposed which agrees within 20-25% with the majority of the available data.

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