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Analytical expressions for the thermophoretic forces acting on two typical non-spherical objects, i.e. a cylinder and a flat plate, immersed in a plasma are presented for the extreme case with a thin plasma sheath and at great Knudsen numbers. A combined specular and diffuse reflection at the object's surface is included in the analysis. It is shown that the thermophoretic force is proportional to the surface area of the cylinder or the plate. The thermophoretic force increases with an increasing specular-reflection fraction of gas particles at the cylinder's or plate's surface. This prediction is appreciably different from that obtained previously for the case of sphere (Chen Xi 1999 Plasma Chem. Plasma Process. 19 33), where the thermophoretic force acting on a non-evaporating spherical particle was demonstrated to be independent of the specular-reflection fraction of gas particles at the sphere's surface. Some suggestions are thus proposed concerning how to check the present kinetic-theory analysis and to determine the values of the specular-reflection fraction of gas particles at an object's surface through the thermophoretic force measurements.
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