UC Santa Barbara

While living systems ubiquitously harness the coupling of active forces to viscoelastic media to robustly generate complex structure and function, translating this paradigm to synthetic materials remains a challenge. In this dissertation, we use two model experimental systems to investigate how active fluid flows can be harnessed to actuate and assemble passive structures. First, we embed rigid inclusions of various shapes in a 2D active nematic film. We investigate the interplay between the inclusion shape, boundary-induced nematic order, and autonomous flows that power the inclusion motion. Specifically, we find that chiral gear-shaped inclusions exhibit long-term rectified rotation, despite the fact that the active nematic flow is chaotic away from the gear. The rectified rotation is correlated with dynamics and polarization of nearby +1/2 topological defects in the vicinity of the chiral gear.Next, we create a system in which a chaotic active fluid assembles and actuates a soft elastic sheet, generating structure and dynamics across multiple length and time scales. Active stresses generated by the microtubule-kinesin fluid act on actin-fascin bundles, driving them to coarsen and percolate into a network. The network contracts vertically in the quasi-2D sample chambers, forming an elastic sheet suspended at the sample midplane that exhibits strain fluctuations driven by the active flows. These local deformations sometimes coarsen into persistent centimeter-scale shear oscillations that span the system size. By tuning the motor concentration of the active fluid or the crosslinker concentration of the passive network, elastic networks with a wide variety of structural and mechanical properties can be assembled. We study the mechanics of actin-fascin networks created with varying amounts of activity, and show that networks created with higher activity appear to be stiffer and more heterogeneous. These experiments, which couple active stresses to rigid and soft structures, demonstrate that chaotic active fluids can transport passive objects in ordered ways and assemble and actuate complex multiscale structures. In each case, these interactions involve a complex feedback between the active and passive components which require further theoretical investigation. These experiments are a preliminary step towards developing microfluidic and material synthesis technologies using active matter.