title: Direct observation of colloidal aggregation by critical Casimir forces.
abstract: We present a refractive-index-matched colloidal system that allows direct observation of critical Casimir induced aggregation with a confocal microscope. We show that in this system, in which van der Waals forces are negligible, a simple competition between repulsive screened Coulomb and attractive critical Casimir forces can account quantitatively for the reversible aggregation. Above the temperature T(a), the critical Casimir force drives aggregation of the particles into fractal clusters, while below T(a), the electrostatic repulsion between the particles breaks up the clusters, and the particles resuspend by thermal diffusion. The aggregation is observed in a remarkably wide temperature range of as much as 15 degrees. We derive a simple expression for the particle pair potential that accounts quantitatively for the temperature-dependent aggregation and aggregate breakup.
authors.affiliation: van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018XE Amsterdam, The Netherlands.
authors.forename: Daniel
authors.initials: D
authors.lastname: Bonn
authors.forename: Jakub
authors.initials: J
authors.lastname: Otwinowski
authors.forename: Stefano
authors.initials: S
authors.lastname: Sacanna
authors.forename: Hua
authors.initials: H
authors.lastname: Guo
authors.forename: Gerard
authors.initials: G
authors.lastname: Wegdam
authors.forename: Peter
authors.initials: P
authors.lastname: Schall