Driven, directed or crowded: dynamics of soft matter near and far from equilibrium

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A unifying framework for understanding non-equilibrium physics does not exist. In this research program we will study dynamics in three classes of experimental model systems that are out of equilibrium which address complementary aspects of non-equilibrium behavior: energy injection and dissipation, modification of interparticle interactions and role of environment. The first objective is to create dynamically responsive materials. First, we wish to make micron-scale, monodisperse, asymmetric Brownian designer droplets in large quantities. Making complex colloidal structures for phase behavior studies is challenging. Our hypothesis is that driving systems far from equilibrium, in this case via electrohydrodynamic drop breakup, will enable us to create smaller structures. Second, self-driven motion in living cells (termed as activity) is inherently nanoscale; however, all colloidal analogs of active matter have been micron-scale and 2-dimensional. We will devise a nanoscale light-activated motor, using bacteriorhodopsin (a light-activated proton pump) embedded in a lipid bicelle, and examine, using experiments in a non-living model system, the role of activity in directing organization in living systems. A second objective is to study kinetics of colloidal cluster formation. Competing colloidal interactions can result in structures of intermediate order: e.g., the combination of long-range electrostatic repulsions and short-range depletion forces gives clusters, gels, and glassy states. We will examine, for the first time, the combination of depletion and anisotropic dipolar forces. With this, we hope to address the long-standing question: does the dipolar gas-liquid transition exist? We will then switch dipolar interactions on and off to study kinetics of cluster formation quantitatively. Parallel to this, we will examine peculiar clustering phenomena that are observed but not understood. We will test our hypothesis that this clustering is a consequence of charge regulation, which can give rise to spontaneous charge asymmetries in a homogeneous colloidal system. A third objective is to understand the effect of the crowded environment on molecular dynamics and conformations in living cells: this is referred to as macromolecular crowding. While several qualitative insights have emerged as to its nature, a true quantitative understanding is not achieved even in simple model systems. Using synthetic polymers both as crowder and probe molecule, we will examine the role of charge, polydispersity, and hydrodynamic interactions using a diverse set of experimental tools. We will compare synthetic polymer probe molecules with real proteins, as well as synthetic crowders with real bacterial cell material. Control of colloidal self-assembly has led to fundamental advances and to new materials. The vision is that extending the study of self-assembly to systems far from equilibrium, where intuition often fails, will lead to new fundamental insights.