Critical phenomena at the single-particle level
Boiling and condensation are among the best recognized phase transitions of condensed matter. Approaching the critical point, a liquid becomes indistinguishable from its vapour, the interfacial thickness diverges and the system is dominated by long-wavelength density fluctuations. Long wavelength usually means hundreds of particle diameters, but here we consider the limits of this assumption, using a mesoscopic analogue of simple liquids, a colloid-polymer mixture.
We simultaneously visualized both the colloidal particles and near-critical density fluctuations, and reveal particle-level images of the critical clusters and liquid-gas interface (see below). Surprisingly, we find that critical scaling does not break down until the correlation length approaches the size of the constituent particles, where there is a smooth transition to non-critical classical behaviour. Our results could provide a framework for unifying the disparate particle and correlation length scales, and bring new insight into the nature of the liquid/gas interface and the limit of the critical regime.
Royall CP, Aarts, DGAL and Tanaka H, 'Bridging lengthscales in colloidal liquid-vapour interfaces: from near-critical divergence to single particles’, Nature Physics, 3 636-640 (2007).