# Direct observation of colloidal aggregation by critical casimir forces

Original entry: Sujit S. Datta, APPHY 225, Fall 2009.

## Reference

D. Bonn, J. Otwinowski, S. Sacanna, H. Guo, G. Wegdam, and P. Schall, PRL 103, 156101 (2009).

## Keywords

critical Casimir force, flocculation, colloidal stability

## Key Points

In analogy to the quantum Casimir effect, in which two uncharged dielectric plates in a vacuum can attract or repel each other due to zero-point fluctuations of the quantized electromagnetic field, the critical Casimir effect describes the possibility of attractive or repulsive forces between two surfaces due to confined density or concentration fluctuations. This effect occurs when the lengthscale $\xi$ of these fluctuations is on the order of the separation between the two surfaces; while this lengthscale is typically very small, it can often be relevant to colloidal systems. This paper discusses the first direct observation of colloidal aggregation and disaggregation by tuning the critical Casimir effect.

Bonn et al. study a refractive index-matched suspension of colloidal latex particles in a binary mixture of water and the organic compound 3-methylpyridine. The index match of the system allows the microstructure of the system to be studied dynamically using confocal microscopy. The particular mixture is used because by tuning the composition and temperature of the system, one can approach a critical point between having a homogeneous mixture and undergoing phase separation. As with critical points in general, this point involves a very large correlation length $\xi$ -- thus making the critical Casimir effect strong enough to observe.

Indeed, Bonn et al. find that, for a suitable composition of the binary mixture, upon increasing the temperature close to the critical point, the colloidal particles -- which previously were stably suspended due to their charge -- aggregated and sedimented out of solution. Importantly, this process was reversible. Confocal microscopy analysis suggests that this aggregation process occurs by diffusion-limited cluster aggregation.

The most straightforward explanation for this effect is due to competition between the attractive critical Casimir effect, and a "typical" screened Coulomb repulsion. Bonn et al. test this hypothesis by altering the salt concentration in their system, thus tuning the Debye length; this leads to a quantitative prediction of the interparticle interaction potential, in agreement with the superposition of an attractive Casimir force and a screened Coulomb repulsion.

While this work is interesting on purely fundamental grounds -- it directly studies the effect of critical Casimir interactions in colloidal systems -- it may also have potential applications in systems of colloidal particles suspended in binary mixtures, in which Casimir interactions may be important, and may be tuned to have a stabilizing or destabilizing effect.