Thermophoretic force on micron-sized particles in rarefied gas conditions*

The thermophoretic force may be an effective mechanism of preventing deposition of micron-sized particles in high-tech micromechanical systems under low pressure conditions. This force occurs when a particle is placed within a temperature gradient and its direction is opposite to this temperature gradient. For small particles at low pressure this force can be an active form of transport. We used the Direct Simulation Monte Carlo (DSMC) method to obtain the thermophoretic force on a micron-sized particle. Varying the size of the particle and the gas pressure we evaluated the force in the whole transition regime (Kn based on the particle radius=0.1-20). The particle has a finite size and is thus physically present inside the simulation domain. Gas molecules collide with the particle. The gas molecules impact location on the particle surface is determined by means of a raytracing method in combination with an analytical representation of the particle shape. The force exerted by the gas on the particle surface is evaluated via the momentum exchange between the gas molecules and the particle. We compare the results from simulations to the thermophoretic force values in the free molecular and continuum regime for which analytical expressions are available.