8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.12Wed 711:10-14:00Effect of radiation pressure on the arrangement ofcolloidsJörg Bewerunge, 1 Matthew C. Jenkins, 1 and Stefan U. Egelhaaf 11 Condensed Matter Physics Laboratory, Heinrich-Heine University, Universitätsstra 1,40225, Düsseldorf, GermanySince the experiments of A. Ashkin in the 1980s the optical manipulation of colloids has attractedincreasing interest in soft matter physics. Most experiments exploit strongly focused Gaussianlaser beams as optical traps, some as modulated light fields [1]; they use the gradient force tohold or guide colloidal particles. Instead, our experiments are based on the optical scatteringforce, i. e. radiation pressure, exerted by a homogeneous beam. This ’top-hat’ beam profile,with a constant intensity over an extended region, is created using refractive optics. This resultsin a space-independent force on the particles whose magnitude depends on the refractive indexdifference between particle and solvent, the particle size and the laser power. We can thus applya controllable force on particle assemblies. We use this set-up to investigate the formation ofdifferent arrangements in response to an external force. The resulting arrangements range fromamorphous packings to crystalline structures, and exhibit instabilities which we observe on anindividual particle level.[1] M. C. Jenkins, S. U. Egelhaaf, J. Phys. : Condens. Matter, 20, 404220 (2008).12