In search of colloidal hard spheres
We recently reviewed the experimental determination of the volume fraction, [curly or open phi], of hard-sphere colloids, and concluded that the absolute value of [curly or open phi] was unlikely to be known to better than +/-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 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.