Collective Motion in Soft Materials

When the intermolecular interactions in a condensed phase material become sufficiently strong, atoms and molecules must rearrange collectively. This situation commonly arises due to cooling or compression and is ubiquitous in systems ranging from supercooled liquids and glasses, to biomaterials, colloids, and crystals. We will briefly review the results of molecular simulations that illustrate some of the common phenomena of collective motion and dynamic heterogeneity and how it is manifest in a selection of example systems, including polymers, water and lipid membranes. We consider how the scale of collective motion relates to the characteristic relaxation times in the material and find that the activation barrier for motion at high temperature (where motion is uncorrelated) plays an important role in the dynamics at low temperature. Finally, we discuss unpublished work examining the nature of collective motion in model two-dimensional crystals of colloids or grains in a dusty plasma. These crystals offer simplifying features that make them ideal model systems to quantify the heterogeneity in both space and time without needing to employ ad hoc criteria to distinguish among highly mobile and immobile regions. By successive energy minimizations to map a trajectory of particles onto the ``inherent structure'' (IS) trajectory, we can directly relate the collective motion to discrete states in the potential energy landscape, thereby linking collective motion, energy, and structural defects within a single framework.