- Direct observation of colloidal aggregation by critical Casimir forces
- Physical Review Letters
- Volume | Issue number
- 103 | 15
- Number of pages
- Document type
- Faculty of Science (FNWI)
- Van der Waals-Zeeman Institute (WZI)
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.
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