From Coulombic Materials to Fluids: Complex Behaviors in Physical Systems

Abstract

Complex phenomena, ranging from the formation of ordered materials to non-linear fluid flows, are extremely important and challenging to understand. They are challenging to study because they may depend on many-body interactions, they may occur in systems that are out of equilibrium, and/or they may involve energy dissipation. In this thesis, we consider particular complex phenomena and use empirical observations to drive forward our understanding of such phenomena.

First, we report on a novel experimental platform to study the formation and subsequent melting of Coulombic materials in an out-of-equilibrium system. Our system is composed of two types of millimeter-sized beads that tribocharge positively and negatively. The virtue of our system is that the beads are visible without a microscope and, thanks to the development of a sophisticated algorithm, their positions can be identified over time. Upon agitating the dish in which the beads reside, we form a Coulombic material; the material is Coulombic because the constituent elements interact electrostatically. We maintain the total number of beads and alter the relative number of positively and negatively tribocharged species. We note the occurrence of common transient structures across all probed cases. The final structures, however, are unique and depend on the number of positively and negatively charged beads. The final structures minimize the Coulombic energy of the system. When we agitate an ordered Coulombic material at an even higher orbital frequency than that at which we formed the material, we melt it. We note that, in contrast to many examples of melting at equilibrium, the presence of impurities has no significant impact on the melting behavior. Here, impurities refer to beads that do not tribocharge significantly. The melting is shear-induced, and it occurs from the edges of the crystal moving towards its center. The shear is caused by repetitive collisions of the crystal with the walls of the container in which it resides. The shear may simultaneously order and melt, or cause disorder in, the crystal depending on the orientation of the beads relative to the direction of shear and the kinetic energy of the beads relative to their Coulombic binding potential. Our results provide concrete evidence of how a material forms and melts, in a non-equilibrium system, when its elements interact at long-range.

Second, we provide a critical perspective on the future of microfluidics and review non-linear microfluidics. Microfluidics refers to most fluid flows at the micron scale. We encourage researchers to focus their research efforts on applications such as therapeutics, diagnostics, food safety, and ma- terials production. Moreover, we highlight technical challenges that may impede the widespread adoption and simple implementation of microfluidics in non-laboratory settings. These include the identification of materials with broad solvent compatibility for the fabrication of devices, the simplification of sample processing requirements, and the decreased reliance on external equipment to operate microfluidic chips. Next, we report on some important experimental triumphs in non-linear microfluidics, ranging from inertial flows to the flow of non-Newtonian fluids in microchannels. These phenomena are qualitatively unique from diffusion-limited, laminar flow processes typically associated with microfluidics. They open the possibility of expanding the applications of microfluidics, especially in terms of capabilities for particle filtration, manufacturing of metal microstructures, and control of trains of droplets or bubbles. Overall, our work sheds light on a specific class of complex phenomena through experimental-centric approaches.