Feasibility of using photophoresis to create a concentration gradient of solvated molecules

The objective of this work is to estimate the feasibility of creating a measurable concentration gradient of molecules in a solvent by a laser driven photophoresis process. The molecules are dissolved in a suitable solvent that is not significantly absorbing at the applied radiation frequency. The molecule is anisotropic, or ideally propeller shaped, and has an appropriate transition dipole capable of driving rovibrational motion. The polarization of the laser can be taken as rotating slowly with the molecules. The resulting torque driven hindered rotation imparts a forward thrust, and thus creates a net flow of the molecules which can set up a concentration gradient in a finite cell. The relevant physical parameters are estimated with the aid of instantaneous normal mode and molecular dynamics simulation methods on a prototype system, and the results indicate that a detectable concentration gradient may be established. A practical issue is to treat heating and resultant mixing or turbulence in the medium. Laboratory experiments are needed to further explore the photophoresis process.