Cluster Phases in Colloids and Proteins

Colloidal suspensions composed of spherical particles suspended in a liquid are excellent model systems for studying fundamental problems such as crystal nucleation, melting and the glass transition. Failure to crystallize can manifest itself in many ways: one such way, observed in both colloids and proteins, is the formation of clusters or aggregates. I will talk about two puzzles we have focused on recently, involving  different length scales and different experimental techniques: confocal microscopy and nuclear magnetic resonance.

First, I will tell you about an ultra-low-density phase of percolating clusters in dipolar colloids, detected via confocal microscopy. I will try to motivate why this discovery came as a surprise, and then outline where experiments and simulations agree (i.e. what we understand), and then what we don't understand.

Second, I will tell you how we detect nanoscale clusters quantitatively via relatively simple liquid-state NMR techniques. We use these methods to shed light (and put numbers) on a contentious problem in dense lysozyme protein solutions: does an equilibrium phase composed of lysozyme clusters exist?