Thermophoretic force measurements of aggregates of micro-spheres

Direct measurements of the thermophoretic force on spherical and non-spheri cal particles have been made using an electrodynamic balance mounted in a v acuum chamber. The non-spherical particles consisted of aggregates of polys tyrene latex spheres. By varying the system pressure the force was measured over a wide range of Knudsen numbers ranging from the near-continuum regim e to the free-molecule regime. The gases were monoatomic helium and diatomi c nitrogen. The data were analyzed and normalized to determine the thermoph oretic shape factor that relates the force on a non-spherical particle to t hat on a sphere of the same volume. The shape factors and the thermophoreti c forces are compared with available theories. The measured thermophoretic forces are shown to agree with theories for the Knudsen regime and the free -molecule regime, and the thermophoretic shape factors are shown to be cons istent with theories based on the limiting cases of large and small Knudsen numbers. (C) 2001 Elsevier Science Ltd. All rights reserved.