What Determines Knudsen Force at the Microscale

Knudsen forces arise in microscale systems when there is a thermal gradient with a characteristic length scale comparable to the molecular mean free path of the ambient gas. These forces are sometimes referred as radiometric or thermo-molecular forces [1] and have been recently measured experimentally in a microscale configuration using heated atomic force microscopy (AFM) probes [2]. The Knudsen force on microstructures with thermal gradients can provide a novel actuation mechanism for mass detection, thermogravimetry, and very high-resolution heat flux measurements. While measuring such forces precisely at microscale can be an arduous task especially since only limited analytical results exist, numerical simulations can provide a basis for understanding the physical mechanisms governing the generation of Knudsen forces. The main goal of this paper is to determine the dependence of the Knudsen force on pressure, geometry and thermal gradients based on rarefied flow simulations and to investigate the effects of the Knudsen force on the dynamics of microbeams. PACS: 47.61.-k;47.61.Fg