Motile colloids with tunable random walk: individual dynamics and collective behavior

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Non-technical Abstract:Flocks of birds and schools of fish are familiar examples of emergent collective behavior, where interactions between self-propelled (active) individuals lead to coherent motion on a scale much larger than the isolated unit. Similar phenomena have been observed with active micro-particles such as bacteria and motile colloids. Recently, the Quincke instability (spontaneous spinning of a dielectric particle in an applied uniform DC field) has attracted great interest as a means of propelling colloids, by simply letting the particles roll on a surface. The research team lead by Petia Vlahovska has found that Quincke rollers can be designed to perform Run-and-Tumble-like locomotion mimicking bacteria such as E. coli. Populations of the Quincke random walkers self-organize and exhibit behaviors reminiscent of bacterial suspensions such as dynamic clusters and mesoscale turbulent-like flows. However, the physical mechanisms underlying the self-organization are unknown. To fill this void, the research team will carry out a combined experimental and theoretical study that systematically explores the parameter space of particle density, activity, shape and motility pattern of the individual colloid. In addition to advancing basic knowledge, the research outcomes may lead to the design of novel active materials (e.g., suspensions with microstructure and effective viscosity tunable by electric field). The visually appealing nature of the experiments will excite students and the general public about active matter and electrohydrodynamics. The principal investigator will be involved in the successful outreach programs at Northwestern University to translate the relevance and significance of this work to attract students from underrepresented groups in science and engineering.Technical Abstract:Active particles such as swimming bacteria or self-propelled colloids spontaneously assemble into large-scale dynamic structures. The emergence of the collective states from the motility pattern of the individual particles, typically a random walk, is yet to be probed in a well-defined synthetic system. The Quincke random walker is a promising new experimental platform to explore active locomotion at the microscale and a testbed for the abundant theoretical models of the collective dynamics of active matter. For the first time, the collective dynamics of run-and-tumble microswimmers will be experimentally studied under well defined and controllable conditions e.g., particle density, speed (i.e., activity) and locomotion type (e.g., run-and-tumble and Levy walks), which can yield potentially transformative knowledge about the relation between the macroscale dynamics and the microswimmers motion and interactions. The experimental research will be complemented by theoretical modeling of the Quincke-walker dynamics, using microhydrodynamics approaches, to elucidate the physical mechanisms of the observed phenomena. The project integrates knowledge across the fields of fluid mechanics and soft matter, and thus the principal investigator anticipates both a much deeper understanding of the underlying physics as well as the discovery of new dynamical regimes and engineering opportunities. The research is interdisciplinary which will be very beneficial for the education and development of the students associated with the project.This Division of Materials Research (DMR) grant supports research to develop a novel experimental platform and the planned experiments and simulations for understanding the emergence of self-organization with funding from the Condensed Matter Physics (CMP) Program in DMR of the Mathematical and Physical Sciences Directorate.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.