Michael Varga
Published: 2021
Total Pages: 113
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We investigate two soft matter systems that display novel behaviors when driven out of equilibrium by internal stresses, fueled by energy derived from the environment. First, we model shape-morphing dynamics of liquid crystal elastomers. We investigate photoactuation in thin polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination. These polymer materials deform mechanically in response to any stimulus that modifies the strength of their nematic order. Doping the polymer with an azobenzene derivative enables the material to actuate in response to light. The trajectory of mechanical response can be controlled by patterning the orientation of the nematic director field during cross-linking, a process known as "blueprinting", which defines the local axis of induced contraction. We model the mechanics of photo-actuation via a Hamiltonian-based nonlinear finite element elastodynamics simulation. We find that the underlying mechanism enabling continuous wave generation is a feedback loop driven by self-shadowing, along with coupling between the nematic order and illumination. The model further demonstrates the mechanism by which wave direction and propagation speed depend on the blueprinted director pattern. These results explain experimental observations by our collaborators, who exploited this mechanical wave generation effect to produce robotic devices that undergo autonomous light-powered locomotion. Second, we carry out computer simulation studies of pattern formation in an active nematic fluid composed of flexible filaments, modeled as a thin layer of bead-spring polymer chains with active driving forces and thermal noise. We consider filaments that self-propel, representing e.g. gliding motility of filamentous bacteria on a smooth or bumpy surface. We investigate phase behavior and diffusive transport as a function of filament density and bending modulus, and demonstrate that Gaussian curvature of the substrate controls filament density. Next, we consider an active nematic fluid with extensile interactions, representing e.g. microtubules that slide against their neighbors driven by kinesin molecular motors. We analyze emergent non-equilibrium dynamics and microstructural evolution as a function of filament bending modulus and the magnitude of active forces. Analysis of the resulting flow includes tracking nucleation, motion, and annihilation of topological defects. Results from this filament-based model are compared to related experiments and continuum models in the literature. The projects both examine soft matter systems exhibiting emergent phenomena when driven from equilibrium via couplings to their environment. In building simulation codes for both projects, we utilize highly parallelized GPU (Graphics Processing Unit) implementations that decrease the runtime and increase the achievable length scale and duration of simulation studies. Computational models are important tools in understanding and ex- plaining the fundamental mechanisms of complex systems, and have potential for future use in design and optimization of smart material applications.