Dynamic heterogeneity comes to life

In the physical sciences, glasses are the paradigm of nonequilibrium matter (1–3): When too cold or too dense, fluids cease to flow, forming the amorphous solid-like material we call glass. This solidification occurs in the absence of any apparent structural ordering, in contrast to more conventional condensed matter. Dynamical arrest and jamming like that of glasses are ubiquitous in nature. It occurs in a vast range of systems spanning microscopic to macroscopic scales, including molecular and polymeric liquids, granular media-like sand and powders, colloidal suspensions, foams and pastes, plastics, metallic alloys, and even type II superconductors. A paper in PNAS (4) makes this an even more generic problem by showing that glass transition behavior is relevant in areas way beyond materials science. Angelini et al. (4) report an experimental study of the dynamics of confluent monolayers of cells. Such cell cultures are area-filling, and as cells move they crowd against each other, leading to collective effects. Angelini et al. tracked cell motion over hours under conditions of confluent cell growth and the corresponding increase of cell density. They make two central observations. First, cellular displacement, either via cell migration or via diffusion, decreased markedly with increasing density of cells. Crucially, this occurred without any emergent spatial structural ordering. Second, this slowdown was accompanied by dynamic heterogeneity: When observed over time, spatial correlations emerged in …