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.

Bonn, Wegdam, and Schall Reply:

“…Bonn, Wegdam, and Schall Reply: In our Letter [1], we presented experiments on critical Casimir induced colloidal aggregation in a system with negligibly small van der Waals forces. These forces usually induce aggregation in colloidal systems, but by refractive-index matching, we were able to minimize these forces.…”

Hydrodynamics in bridging and aggregation of two colloidal particles in a near-critical binary mixture

“…As a result, we have the final separation distance is ℓ(∞) = 1.09a in case 2 and 0.69a in case 3. The above potential can arise for charged colloidal particles when the van der Waals interaction is negligible 22 . For weakly charged colloid particles, the screened Coulomb interaction in the Derjaguin, Landau, Verway, and Overbeek (DLVO) theory 31 is of the form,…”

“…For example 22,27 , we may set 2πρ s /e ∼ 1/nm 2 and ℓ B ∼ 1 nm to obtain B ∼ 2a 3 /(aκ) 2 with a in nm, so the adopted values of B in this paper can be realized in experiments.…”

“…Some authors 22,27 used this exponential expression to explain the observed aggregation. In their approach, close contacts of the particles in the range ℓ ξ are needed for aggregation, since the factor exp(−ℓ/ξ) is nearly zero for ℓ ≫ ξ.…”

Drag Coefficient of a Rigid Spherical Particle in a Near-Critical Binary Fluid Mixture

“…Several experimental groups have observed adsorption-induced an aggregation of colloid particles slightly outside the coexistence curve of water-organic solvent mixtures. [7][8][9][10][11][12] The magnitude and range of the heterogeneity in the concentration tend to become larger when the critical demixing point is approached. Furthermore, near the critical point, the concentration profile and inter-particle interaction energy exhibit universal behavior, i.e., it does not depend on the microscopic details of the surface and solvent molecules if the physical quantities are scaled appropriately.…”