Active Matter geometrically manipulated: From bacteria to human spermatozoa

Abstract

This numerical and experimental work is aimed to understand the complex relation between populations of self-propelled micro-swimmers and micro-patterned confinement geometries, in order to optimize the design of sorting microfluidic devices. Both, precise phenomenological models based on experimental motility parameters and simple microswimmer models using Stokesian Dynamics, are developed. It has been shown that the ratchet effect is an effective method to induce inhomogeneous bacterial distributions in nanoliter chambers separated by a wall of asymmetric obstacles. Although the origin of this effect is well established, we show that its efficiency is strongly dependent on the detailed dynamics of the individual microorganism.Simulations indicate that, for run-and-tumble dynamics, the distribution of run lengths and the partial memory of run orientation after a tumble are important factors when computing the rectification efficiency [1]. In addition, we optimize the geometrical dimensions of the asymmetric obstacles in order to maximize the swimmer concentration and we illustrate how it can be used for sorting by swimming strategy using a long array of parallel obstacles [2]. We also investigate human spermatozoa guided by asymmetric obstacles. Using a realistic phenomenological model we optimized the geometrical confinement habitat for accumulating the population and we use it for designing new devices. We conclude that the swimmer strategy, cells size, and swimmer-wall interactions are crucial to design the optimum micropatterned architecture able to achieve efficient physical sperm guidance. Interesting differences between bacteria and sperm micro-geometrical directioning arise from their different interactions with walls and corners (Fig.1). We suggest a new technological application inspired on the observed spermatozoa accumulation near walls.