Radiation pressure as a technique for manipulating the particle order in colloidal suspensions

Abstract

Radiation pressure can be used to manipulate the particle organization in collodial suspensions. A single laser beam focussed into a sample will draw in particles and push them in the direction of the light propagation, if the index of refraction of the particles is larger than that of the medium. A single laser beam can be used to trap and move individual particles or to concentrate many particles into a close-packed two-dimensional layer on the cell wall. When two laser beams are crossed in the sample, a set of intensity fringes is produced. Particles become trapped in these fringes and execute essentially one-dimensional Brownian motion along the fringes, being limited by both the cell wall and the radiation field. Highly charged particles will also tend to align along the intensity fringes, producing a particle density wave. However, the strong interaction between the particles can lead to further organization: a sample which exhibits amorphous order in the absence of the fringes will exhibit solid-like order in the presence of the fringes. In this paper results will be presented for the structures observed for these different radiation field geometries.