In Search of Colloidal Hard Spheres

TL;DR

This paper reviews the challenges in accurately determining the volume fraction of colloidal hard spheres, emphasizing the effects of softness and electrostatic charge on experimental measurements and the approximation of real particles to ideal hard spheres.

Contribution

It surveys current experimental systems and discusses how softness and electrostatic effects influence the effective volume fraction in colloidal hard spheres.

Findings

Polymethylmethacrylate spheres closely approximate hard spheres.

Effective hard sphere diameter is 1-10% larger than the core diameter.

Electrostatic charge effects are significant for larger colloids.

Abstract

We recently reviewed the experimental determination of the volume fraction, $\phi$, of hard-sphere colloids, and concluded that the absolute value of $\phi$ was unlikely to be known to better than $\pm 3$-6%. Here, in a second part to that review, we survey effects due to softness in the interparticle potential, which necessitates the use of an {\em effective} volume fraction. We review current experimental systems, and conclude that the one that most closely approximates hard spheres remains polymethylmethacrylate spheres sterically stabilised by polyhydroxystearic acid `hairs'. For these particles their effective hard sphere diameter is around 1-10% larger than the core diameter, depending on the particle size. We argue that for larger colloids suitable for confocal microscopy, the effect of electrostatic charge cannot be neglected, so that mapping to hard spheres must be treated with caution.