Enabling new microactuation materials through understanding the influence of shear-dependent viscosities on acoustic field-driven assembly of particles

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Polymer composites may consist of mixture of a polymer (think of a liquid plastic) and very small rigid particles. The addition of such particles can enhance polymer properties and could change their functionality. For example, such composites can become electrically conductive, or change their shape (e.g., bend) in response to external stimuli such as light or heat. This could provide a powerful approach to achieve three-dimensional shapes from one or two-dimensional structures. To achieve such functionalities, an accurate design of the location of such particles in the polymer is required. This project will focus on using sound waves (i.e., pressure vibrations) to program the location and time-dependent behavior of small particles to tune the physical response of composite polymeric systems. This project will create new opportunities leading to the advancement of next-generation materials that can respond to changes in the environment. These materials can be used in designing small-scale soft robots for biomedical applications. This award will engage and train graduate and undergraduate students in subjects related to fluid mechanics, mass transfer, and inorganic and organic materials. Elements of this research will be used to create hands-on activities for K-12 students and to educate the public in collaboration with the Fleet Science Center. Existing summer research programs at the University of California San Diego will be leveraged to engage women and underrepresented minorities to promote diversity and inclusion in STEM.Manipulating the spatial distribution and assembled structure of functional nanoparticles at the micro- or nanoscale is recognized as a critical barrier to the fabrication and design of miniaturized stimuli-responsive polymer-based actuators and shape reconfigurable matter. To overcome such challenges, this project will explore the fundamental mechanism of surface acoustic wave-driven spatiotemporal distributions of nanoparticles in shear-thinning polymer solutions capable of three-dimensional shape transformation after crosslinking by understanding associated principles in physics, mechanics, and dynamics. This project will test the hypothesis that shear-thinning behaviors of polymeric fluids affect the degree of localization, the spatial distribution, and assembled structures of nanoparticles within stimuli-responsive polymer matrices under surface acoustic waves. It will further assess the time-dependent disassembly of nanoparticles after removal of surface acoustic waves by quantifying the effective viscosity of polymeric solutions in terms of polymer concentrations and surface acoustic wave-induced shear rates. This fundamental understanding will allow precise particle manipulation in more complex patterns, thereby enabling the formation of unconventional assembled features like helical structures. These outcomes will contribute to understanding the fundamental mechanism of surface acoustic wave-driven spatiotemporal assembly of nanoparticles in stimuli-responsive polymer matrices, enabling programmable shape reconfiguration and motion at the sub-mm scale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.