Investigating non-equilibrium phenomena in active matter systems

Abstract

Active matter systems have very recently received a great deal of interest due to their rich emergent non-equilibrium behavior. Some of the most vital and ubiquitous biological systems and processes are active matter systems including reproduction, wound healing, dynamical adaptation, chemotaxis, and cell differentiation. Active matter systems span multiple length scales from meter to nanometer and can vary depending on the shape of the active agent, mode of motility, and environment. However, active matter systems are unified in that they are all composed of active units or particles that continuously convert ambient, stored, or chemical energy locally into motion and exhibit emergent non-equilibrium collective dynamical or phase behavior. Active matter systems have been studied extensively in the biological context, as well as in simulation and theory. However, there are relatively few artificial or synthetic experimental model soft active matter systems that can effectively mimic the rich emergent behavior exhibited by many active matter systems. Such model experimental systems are crucial not only to confirm the exotic behavior predicted by theoretical and simulation systems, but to study and investigate the underlying physical phenomenon which may contribute to or even drive some emergent phenomenon. These model systems are crucial to help determine what behavior is due to purely physical phenomenon and what behavior requires some type of biological or biochemical stimuli. In this thesis, I will develop several artificial experimental model active matter systems that are able to effectively mimic and reproduce some of the rich emergent non-equilibrium behavior exhibited by several active matter systems or processes, like chemotaxis, in order to uncover the underlying physical phenomenon that govern this emergent behavior. I will start by designing an extremely simple active matter system composed of a single active unit and then build up in complexity by adding many active components, changing the mode of motility, and including passive components which may or may not be fixed. I will show in this thesis that this emergent behavior is guided by fundamental physical phenomenon like friction and the mechanical properties of the environment. The thesis divides this study into two Parts. In Part I, I will develop an artificial soft active matter system that is able to effectively perform chemotaxis in a non-equilibrium manner by leveraging the concept of effective friction. The active component in this system will be magnetic particles that are coated with a biological ligand or receptor and placed on a substrate with the corresponding ligand or receptor. A rotating magnetic field will be applied and the magnetic particle will proceed to rotate with the applied field and convert some of that rotational energy into translational energy due to the effective friction induced by the breaking of reversible bonds between the surface of the particle and the substrate. I will then create gradients in the density of such binding sites and by placing the magnetic particle on a stochastic, random walk the differences in effective friction will lead to directed motion or drift reminiscent of chemotaxis. I will show that this concept of sensing based on effective friction induced by a binding interaction is general and scales with the affinity of the interaction being investigated (i.e. protein-lipid, metal ion, electrostatic, antigen-antibody, or hydrophobic interactions). In Part II, I will build up in complexity and develop an artificial soft active matter system consisting of two active units embedded in a dense passive matrix in order to mimic the emergent behavior of many biological systems composed of both active and passive components. In this system, an ultra-long range attractive interaction emerges due to a combination of activity and the mechanical properties of the dense passive media. The range of the interaction can be tuned by changing the level of activity, the actuation protocol, the mode of motility, the composition of the dense passive monolayer, and the concentration of active units. Alternatively, if the passive components are fixed to the substrate, the active components undergo a disorder induced delocalization and exhibit super-diffusive transport properties. On the basis of these results, I propose several guidelines to developing novel artificial soft active matter systems which bear future investigation. The findings in this thesis represent a comprehensive study of the exotic emergent non-equilibrium behavior exhibited by many active matter systems by developing novel artificial experimental soft model active matter systems. These novel model experimental systems revealed some underlying fundamental physical phenomenon that contribute to some of the non-equilibrium behavior observed in the biological system of interest. These results may generalize not only to other simulation or theoretical active matter systems but potentially to biological systems as well. This work will be essential not only in guiding the design of future artificial experimental soft active matter systems, but can also be extended towards designing hybrid artificial-biological soft active matter systems.