Experimental study of the conformation and dynamics of active colloidal polymers

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An active fluid is a dispersion of self-driven particles in a liquid medium. This type of fluid is relevant to a broad class of biological and physical systems including suspensions of swimming microorganisms, synthetic colloidal swimmers and vibrated granular rods. Recent theories and simulations have greatly extended the study of simple active fluids made of compact solid particles to elongated flexible objects such as soft chains and filaments. Such research has transformed our understanding of the dynamics of flexible objects in nonequilibrium conditions (for example, biopolymers within living cells) and revealed unexpected collective dynamics of active fluids. Nevertheless, benchmark experiments that can quantitatively verify the wide range of theoretical and numerical predictions are still missing. This research project aims to address this gap and to perform experiments that study the shape and dynamics of active DNA-linked long chains of colloidal particles, the so-called colloidal polymers in analogy of linear polymer molecules. Two specific systems included in these studies are passive colloidal polymers immersed in light-powered bacterial suspensions and active Janus colloidal polymers whose activity arises internally from individual Janus particles via chemical reactions. The ultimate objective is to explore the possibility of using self-driven active colloidal polymers as artificial flagella. The project includes outreach efforts on designing demos for undergraduate fluid classes and for summer short courses attended by industrial practitioners. The project also aims to forge close collaborations between academic and industrial researchers.DNA-linked colloidal chains provide a concrete example of the classic bead-spring model of polymer molecules. By conveying activity to DNA-linked colloidal polymers, the research team will investigate the conformation and dynamics of active colloidal polymers and understand the fundamental polymer scaling relations out of equilibrium. The researchers will verify important theoretical and numerical predictions on the unusual behaviors of active polymers such as the swelling of polymer chains with activity, the self-assembly of hairpin structures, and the activity-induced softening and coil-to-globule transition. Moreover, by exploiting the collective dynamics of linked active Janus particles, the project participants will study a new approach for creating self-driven artificial flagella with periodic non-reciprocal beating motions. Such a flagellar structure would then be tested for driving the locomotion of synthetic microswimmers. Taken together, the project provides not only benchmark experiments corroborating existing theories and simulations but also a new way to engineer synthetic microswimmers for cargo delivery at microscopic scales.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.