Colloid Project

Interparticle interactions

Over the past several decades, the experimental literature has consistently reported observations of attraction between like-charged colloidal particles and macromolecules in aqueous solution. Dating to work by Langmuir in the 1940s (which goes even further back than Rosalind Franklin's observations on crystallisation of viruses) and including more recent reports on nucleic acids and colloidal particles in solution, these recurrent reports in the literature are all at odds with the intuitive expectation of an interparticle repulsion between like-charged objects that decays monotonically with distance. Although attraction between like-charged particles can be rationalized theoretically in the strong-coupling regime, e.g., in the presence of multivalent counterions, repeated accounts of long-range attraction in aqueous solution containing monovalent ions at low ionic strength have posed an open conundrum. We recently showed that the behaviour of molecular water at an interface—traditionally disregarded in the continuum electrostatics picture—provides a mechanism to explain the observed attraction in a broad spectrum of experiments.

In this branch of our work we experiment with colloidal microparticles - objects on a scale 1000 times larger than our molecules - and use optical microscopy to examine the intricacies of interparticle interactions in solution and tease out new behaviour. Like in most of the rest of our work, we “just look at the thing”! We are currently engrossed in examining the possibility of a long-ranged solvation force which would carry very broad implications in a host of other experimental situations, such as in tuneable attractions at the molecular scale, frequently observed for species carrying net electrical charge in solution, e.g., in biological liquid–liquid phase separation of proteins and nucleic acids.