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Abstract
Suspensions of nano- and microparticles are fascinating stress-responsive material systems that, depending on their composition, can display a diverse range of flow properties under shear, such as drastic thinning, thickening, and even jamming (reversible solidification driven by shear). However, investigations to date have almost exclusively focused on nonresponsive particles, which do not allow in situ tuning of the flow properties. Polymeric materials possess rich phase transitions that can be directly tuned by their chemical structures, which has enabled researchers to engineer versatile adaptive materials that can respond to targeted external stimuli. Over the past decades, syntheses of many types of stimuli-responsive colloids have been reported, allowing one to control the shape, stiffness, surface polarity, etc. of particles in situ. However, the majority of these studies are focused on characterizing the properties of individual particles, whereas the collective effects of such transitions in crowded ensembles remain less explored. This thesis work builds on principles of polymer engineering to design stimuli-responsive particles through both experiments and computations to study how particles adapt to mechanical environments and demonstrates how to utilize them to control suspension rheology. To begin with, we have studied suspensions of (readily prepared) micrometer-sized polymeric particles with accessible glass transition temperatures (T_g) designed to thermally control their non-Newtonian rheology. The underlying mechanical stiffness and interparticle friction between particles change dramatically near T_g. Capitalizing on these properties, it is shown that, in contrast to conventional systems, a dramatic and nonmonotonic change in shear thickening occurs as the suspensions transition through the particles’ T_g. In addition to stiffness and adhesion, particle shapes can significantly influence the jamming behaviors of dense suspensions. To achieve an on-demand control of suspension rheology via changing particle shape, we prepare microparticles that display two-way shape memory from liquid crystal elastomers (LCEs). Through thermally regulating the stiffness and shape of the particles, a wide range of suspension rheological properties can be accessed by regulating the temperature relative to T_g and T_NI. In order to better understand how the shapes of LCE microparticles are programmed, simulation studies on the stress-induced alignment and the defect formation process of LCEs under geometric confinements are performed through Ginzburg-Landau relaxation. It is shown that the microstructure in confined LCEs can adapt to various deformation modes, leading to the formation of topological defects and metastable states. In addition, we investigate how collective order arises in jammed particle assemblies with pronounced particle-particle contacts. Interestingly, depending on their interactions, “tissues” consisting of particles with different shapes and orderings can be obtained. Hence it is shown that the microstructural adaptation in LCE microparticles can be leveraged to design hierarchically ordered materials that mimic living organisms. Together, these works provide new strategies that enable \textit{in situ} control over the suspension’s rheological response and lay the groundwork for other types of stimuli-responsive jamming systems through polymer chemistry. This thesis presents an exemplification of the molecular engineering principle, wherein an understanding of how nanoscopic properties can cascade to larger length scales and govern macroscopic phenomena opens new avenues of material design. Here, it is shown that through judicious design of the particle chemistry at the molecular scale, one can tailor the inter-particle interactions at the mesoscale, and consequently control collective properties such as fluid flow at the macroscale.