DSMC Analysis of the Gas Mixture Separation Using Optical Lattices

Abstract

Results on separation of gas mixtures due to gradient forces from an interference grating formed by two laser beams of non-resonant frequency (optical lattice) are presented. Selective forces from the lattice to the gas mixture are analyzed by an example of a binary methane-helium mixture with allowance for thermal diffusion and baro-diffusion. The separation effects of these types of diffusion are compared. Analytical approximations for the ponderomotive force are compared with results computed by the direct simulation Monte-Carlo method. Inside the spot of the optical lattice, baro-diffusion and selective forces are demonstrated to separate the mixture more intensively, while thermal diffusion plays a minor role in separation. I. Introduction HE effect of the interference grating on the gas flow (the so-called optical trap of the gas) due to gradient forces from a laser field of non-resonant frequency has been intensively studied for the last decade. It was shown in Ref. 1 that the optical lattice formed by a high-intensity laser can selectively affect gas molecules within a certain range of velocity. In this case, a large force exerted by the lattice on the molecules arises, and the gain in the gas momentum and energy is observed. Possible separation of binary gas mixtures in a capillary was studied in a recent paper 2 . The principal idea here is that the gradient force is selective to a parameter 3 m α ( � is the polarizability and m is the mass of the molecule). The numerical study of the distribution of the mixture species in a closed-end capillary under the optical lattice effect involved analytical estimation of the volume force exerted by the optical lattice, as well as the direct simulation Monte- Carlo (DSMC) method 4 . The presence of the optical lattice leads to strong perturbations of the flow in terms of temperature and pressure. The effect of these gradients (and, thus, thermal and baro-diffusion), however, has not been studied yet. A possible separation effect of temperature and pressure gradients can be of order of or greater than the selective forces from the optical lattice; hence, the question about a comprehensive analysis of the separation process and the contribution of these different phenomena has to be posed. The present paper reports a study of the separation process with thermal and baro-diffusion taken into account for a model test case of a steady flow between two flat plates. The fraction of species was used as an indicator of separation. The pressure and temperature gradients were calculated, and types of separation due to thermal- and baro-diffusion were compared. Approximate analytical estimates of terms in the expression for drift velocity corresponding to the force exerted by the optical lattice, baro-diffusion, and thermal diffusion were used to explain the physical meaning of the simulation results. The goal of our work was to identify the separation properties corresponding to thermal and baro-diffusion. II. Potential of the optical lattice The field of the resulting interference grating formed by two counter-propagating beams along the x axis with