General thermophoretic force on a small spherical particle in gases

Thermophoresis and diffusiophoresis or the directional motion of particles in liquids and gases driven by a temperature and concentration gradients. These phenomena are of both fundamental interest and practical use. So far, thermophoresis, diffusiophoresis along with their dynamics have been investigated and applied. In this paper, a type of thermodynamic force is studied and applied to explain the thermophoresis processes in gases. This force is known as the general thermophoretic force driven by both the temperature and the concentration gradients. It is caused by elastic collision between gas molecules and particles suspended in the gases. The results show that: (i) Based on the kinetic theory of gases, the equation describes the general thermophoretic force can be derived. (ii) Both the thermophoretic force (which is driven solely by a temperature gradient) and the diffusiophoretic force (which is driven solely by a concentration gradient) are special cases of the general thermophoretic force. (iii) Both the thermophoretic and the diffusiophoretic forces depend on the size of gas molecules and particles. (iv) The general thermophoretic force can be applied to explain the thermophoresis: the negative thermophoresis (negative Soret effect) can occur when the diffusiophoretic force opposes the thermophoretic force. (v) The Soret coefficient can be determined by the balanced state of the general thermophoretic force. (vi) The thermophoretic force and diffusiophoretic force of the gases exerted on gas molecules are not depended on the size of molecules. (vii) Under the normal condition (temperature is 20 degrees C and the pressure is 1 atm), the thermophoretic force and the diffusiophoretic force exerted on gas molecules are approximately 2.45x10(-24) N and 1.74x10(-24) N respectively.