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

P9.28Tue 611:23-14:00Transient cage formation around laser-heated goldcolloids in polymer solutionsWerner Köhler 1 and Florian Schwaiger 11 Universität Bayreuth, Universitätsstr. 30, 95440, Bayreuth, GermanyGold colloids dispersed in dilute to concentrated polymer solutions can efficiently be heated bylaser irradiation and act as almost point-like heat sources. The temperature gradient aroundthe colloid couples to the order parameter (the composition) of the polymer solution due tothe Soret effect. In systems with positive Soret coefficients S T of the polymer, such aspolystyrene(PS)/toluene, the polymer is repelled from and the solvent is attracted towards the colloid.Under certain conditions, the polymer can be removed almost completely from the particlesurface. As a consequence, a transient cage of low viscosity and dramatically enhanced mobilityis formed, which follows the motion of the particle with a certain retardation. Based on a completeparameterization of S T (M, c,T), which is so far only available for the PS/toluene system,we analyze in detail the stationary temperature, concentration, and viscosity profiles. Dependingon the polymer molar mass and concentration and on the distance to the glass transition temperature,negative or positive nonlinear feedback-loops are established that lead to either attenuation orself-amplification of the polymer depletion. Particularly strong coupling, corresponding to largeSoret coefficients, is observed in systems with diverging correlation length. Examples are criticalmixtures and, the here discussed, semidilute polymer solutions. The experimental observation bothin the low intensity linear regime and in the strong saturation limit is accomplished by means ofphase contrast microscopy.28