title: Particle diffusion in active fluids is non-monotonic in size.
abstract: We experimentally investigate the effect of particle size on the motion of passive polystyrene spheres in suspensions of Escherichia coli. Using particles covering a range of sizes from 0.6 to 39 microns, we probe particle dynamics at both short and long time scales. In all cases, the particles exhibit super-diffusive ballistic behavior at short times before eventually transitioning to diffusive behavior. Surprisingly, we find a regime in which larger particles can diffuse faster than smaller particles: the particle long-time effective diffusivity exhibits a peak in particle size, which is a deviation from classical thermal diffusion. We also find that the active contribution to particle diffusion is controlled by a dimensionless parameter, the Péclet number. A minimal model qualitatively explains the existence of the effective diffusivity peak and its dependence on bacterial concentration. Our results have broad implications on characterizing active fluids using concepts drawn from classical thermodynamics.
authors.affiliation: Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA. parratia@seas.upenn.edu.
authors.forename: Alison E
authors.initials: AE
authors.lastname: Patteson
authors.affiliation: Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA. parratia@seas.upenn.edu and School of Engineering, University of California Merced, Merced, CA 95343, USA.
authors.forename: Arvind
authors.initials: A
authors.lastname: Gopinath
authors.affiliation: Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA. parratia@seas.upenn.edu.
authors.forename: Prashant K
authors.initials: PK
authors.lastname: Purohit
authors.affiliation: Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA. parratia@seas.upenn.edu.
authors.forename: Paulo E
authors.initials: PE
authors.lastname: Arratia
mesh_terms: D004058:Diffusion; D004926:Escherichia coli / Q000737:chemistry*; D008954:Models, Biological; D010316:Particle Size; D013816:Thermodynamics