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CPP: Chemische Physik und Polymerphysik

CPP 14: Poster: Kolloidale Systeme

CPP 14.6: Poster

Mittwoch, 13. März 2002, 16:30–18:00, B

Reversible light induced aggregation of magnetic colloids — •Björn Hoffmann and Werner Köhler — Physikalisches Institut,Universität Bayreuth,D-95440 Bayreuth,Germany

The intensity of a HeNe laser beam transmitted through a 100 µm thick layer of a kerosene-based ferrofluid (particle diameter 10 nm) of different concentrations shows an initial rise on a time scale of a few seconds followed by a decrease on a longer intensity-dependent time scale. The initial photobleaching is caused by the thermophoretic migration of the colloidal particles out of the laser beam. The slower decrease is caused by a reversible photoinduced formation of µm-sized aggregates, which could be observed at laser intensities as low as 100 nW. The aggregation dynamics could be visualized under a microscope. Linear structures with a well defined lateral dimension are formed in the presence of a magnetic field. Different time scales are observed for the loss of orientation and the dissolution of the aggregates after removal of the static magnetic and the light field.