Active Surface Agents: Enhanced Transport by Active Colloids at Fluid Interfaces

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Active colloids are micron-scale self-propelled objects. They accumulate near interfaces, bringing as yet largely untapped degrees of freedom to interfacial engineering. Here, active colloid motion will be studied to develop these objects as Active Surface Agents. These agents can improve mixing for chemical processes, and can lead to new encapsulation and release methods. Furthermore, they are important in new technological spaces like micro-robotics, where small objects perform tasks like moving cargo or releasing a signal. The field of active colloids is inspired by motile bacteria, which can move at extraordinary rates, typically over distances ten times greater than their size in a second. There are also synthetic active colloids. An important example is a polymer micron bead with a platinum patch which reacts with common chemicals like hydrogen peroxide, which fuels their motion. Plain colloids, absent activity, are used to form organized structures at fluid interfaces between fluids that do not mix to stabilize oil in water emulsions, widely used to encapsulate oily substances in water in personal care products and to formulate pharmaceuticals. Fluid interfaces are special environments for chemical reaction and separation. For example, if catalysts are present at fluid interfaces, reagents in the oil phase can form products that prefer the water phase. This facilitates reaction and separation in green processes. Innovation can have economic and social impact. Furthermore, this research will advance Sciences Technology Engineering and Mathematics education, including doctoral student and undergraduate researcher training, and as course material in the Interfacial Phenomena course. The outcome of the proposed research will be presented in outreach activities to attract students from diverse backgrounds to engineering. Furthermore, research findings will be conveyed in demonstrations for the public.Bacteria will be used as a model system. This research will develop a general framework to identify desired active surface agent properties including trajectory type, propulsion strength, and surface density to generate a desired active interfacial layer. The results will lay the ground work to promote mixing at interfaces of emulsions or of thin liquid films or other multi-phase system using active colloidal layers. All active colloids obey similar hydrodynamic descriptions, which this research will develop. Active colloids accumulate near interfaces and can swim adjacent to them, trapped via hydrodynamic interactions, or can swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatio-temporal implications. This research will develop a library of responses of individual swimmers on and near interfaces to understand their individual behaviors using experiment and theory. This research will probe how motion in the interface, a two dimensional active sheet, propagates into the adjacent fluids. By understanding these implications, new design rules for active interfaces can be established to enhance transport in multiphase systems.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.