Direct observation of laser induced freezing

The effect of an externally applied field on the microstructure of aqueous suspensions of charge stabilized polystyrene spheres is studied by direct observation. This work complements the earlier light diffraction work of Chowdhury et al., which demonstrated that radiation pressure forces can induce crystal-like microstructures in suspensions which have equilibrium liquid-like microstructures in the absence of these external fields. Specifically, a spatially periodic radiation pressure field is produced by intersecting two mutually coherent laser beams in the sample. The microstructure is observed as a function of the input intensity and crossing angle of the two laser beams, and the data is presented in terms of one and two dimensional particle distribution functions. We find that both single- and multilayered systems exhibit a transition to crystal-like order. This crystal order is more pronounced at large input power and for a periodic external field commensurate with the lattice spacing for a final undistorted two dimensional hexagonal crystal structure. Monolayer suspensions undergo this recording with no change in density, while multilayer systems restructure to a monolayer with increased layer density.