Optically induced nanoscale forces: precipitating the separation of immiscible liquids

Abstract

It is well known that the miscibility or immiscibility of dissimilar liquids can be a sensitive function of external physical conditions, and fractional composition. In regions of thermodynamic instability, phase separation can often be instigated by local nucleation, in regions of the fluid where fluctuations in concentration rise above a threshold value and compositional instabilities arise. Such fluctuations, normally stochastic, are strongly influenced by a dynamic interplay of local intermolecular forces. In fluid regions illuminated by off-resonance laser light, the forces between neighbouring molecules can be significantly modified by an 'optical binding' effect that depends on the polarizabilities of the components. In liquid mixtures, this enables the forces between molecules of the same, or different, chemical composition to be differentially modified. In consequence, a suitably guided laser beam can locally control nucleation events, and so precipitate phase separation.