Physics Department - Living and Soft Matter: From Non-Equilibrium Phase Separation to Anti-Infection Catheters

Abstract
Living matter, such as microbial colonies, animal flocks, or human populations, are intrinsically out of equilibrium. Understanding the fundamental behaviors of these systems not only addresses intriguing theoretical challenges in non-equilibrium statistical physics but also enables impactful applications in biomedical and materials sciences.
In the first part of this talk, I will discuss our recent work resolving a central open question in active matter physics: does motility-induced phase separation (MIPS)—robust in “dry” active models—survive in realistic, fluid (“wet”) environments where long-ranged, many-body hydrodynamic interactions (HI) operate? Using theory and Active Fast Stokesian Dynamics simulations of ‘squirmers’, we reveal that collision-induced pusher dipoles, which is a generic many-body HI effect that arises for any type of microswimmers, destroy MIPS across parameter space. These dipoles destroy MIPS by (1) enhancing the rotational diffusion of the swimmers and (2) generating advective flows that sweeps swimmers away from dense clusters. These results transform how the community interprets dense phases in wet active systems and offer a clear, testable prescription for designing them.
In the second part of this talk, I will present a geometric design for anti-infection catheters. Urinary catheters cause lots of infections in hospitalized patients and cost about 30 million US dollars annually. By understanding microbial transport in channels, specifically how bacteria swim upstream due to flow-induced reorientation, we propose a novel catheter interior design that reduces bacterial contamination by 100-fold, potentially prevent urinary tract infections associated with indwelling catheters.