Wetting and Spreading with Soft Materials

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Non-technical abstract: The wetting and spreading of fluids appears throughout natural and engineered systems. While the dynamics of droplets resting on rigid substrates are well-understood to be controlled by the contact line separating the three (solid/liquid/gas) phases, soft materials present new challenges. For compliant substrates, the forces at the contact line cause a deformation of the underlying surface, which in turn impacts the wetting and spreading of the droplet. This is a highly nonlinear problem that can result in a large range of behaviors. Predicting and controlling them is relevant to medical technologies, microfluidic flows, tissue growth, and bacterial biofilms. When the materials involved are not just soft, but living, the spreading can even be driven by self-fluidization, increasing the degree of complexity. Understanding this process could provide new means for efficiently fluidizing industrial granular flows, or for introducing circulation into dry fermentation processes. The project not only funds undergraduate and graduate students to perform research, but makes new training in materials characterization available to other students in the department through the rheometer and surface tensiometer to be placed in the physics department's shared Education and Research Lab. The annual "Launch your Excellent Adventures with Physics!" (LEAP!) Workshop encourages high school girls from diverse backgrounds to explore physics through hands-on workshops. The research team strives to recruit and support students who are members of traditionally under-represented groups, both to work on laboratory research and to take part in dissemination/outreach activities. Technical abstract: This research project addresses open questions about how small-scale interactions give rise to macroscopic effects in the wetting and spreading of droplets in soft materials. The research team utilizes two model systems to understand the complex interplay between deformation and flow. The first system addresses the contact line forces at a gel/liquid/gas interface, using systematically-varied material parameters and geometries to resolve the interaction of capillary and elastic forces on soft substrates. The second system uses a new model organism, colonies of Tyrolichus casei (cheese mites), to open a new avenue of study into the onset and dynamics of living hydrodynamic flows. The work starts from detailed characterizations of both the gels and the living fluids to measure such quantities as the complex shear modulus, Poisson ratio, surface tension, and degree of activity. Knowledge of these material parameters will allow the team to both design experiments which probe the necessary range of parameter values, and also allow for comparison and development of models. Together, these two systems allow the team to answer the following questions: Can current controversies surrounding static contact line models be rationalized through experiments on a range of materials? Does the curvature of the contact line play an additional role? What new modes of spreading and transport of droplets can be achieved through gravity and/or gradients in materials properties? Can particle-scale position fluctuations in living fluids be thought of as analogous to the thermal fluctuations in an ordinary fluid?