Abstract
We experimentally study the dynamics of active particles (APs) in a viscoelastic fluid under various geometrical constraints such as flat walls, spherical obstacles and cylindrical cavities. We observe that the main effect of the confined viscoelastic fluid is to induce an effective repulsion on the APs when moving close to a rigid surface, which depends on the incident angle, the surface curvature and the particle activity. Additionally, the geometrical confinement imposes an asymmetry to their movement, which leads to strong hydrodynamic torques, thus resulting in detention times on the wall surface orders of magnitude shorter than suggested by thermal diffusion. We show that such viscoelasticity-mediated interactions have striking consequences on the behavior of multi-AP systems strongly confined in a circular pore. In particular, these systems exhibit a transition from liquid-like behavior to a highly ordered state upon increasing their activity. A further increase in activity melts the order, thus leading to a re-entrant liquid-like behavior.
Highlights
The natural habitat of microorganisms is often rather complex in geometrical aspects [1] and because the surrounding fluid environment, due to presence of colloids and macromolecules, exhibits viscoelastic behavior [2]
We have investigated the motion of active colloidal particles in a viscoelastic fluid under geometrical confinement
The spatial asymmetry imposed by the confinement induces significant hydrodynamic torques on the rotational motion of the particle, which would otherwise be governed by rotational diffusion
Summary
The natural habitat of microorganisms is often rather complex in geometrical aspects [1] and because the surrounding fluid environment, due to presence of colloids and macromolecules, exhibits viscoelastic behavior [2]. Investigating such processes is of major importance for the understanding of intracellular motility, as many organelles have to move through highly confined viscoelastic media within cells [11] These out-of-equilibrium systems have attracted tremendous interest in various scientific communities to design their artificial counterparts which mimic the self-propulsion of natural microswimmers [12]. Instead of being predominantly controlled by thermal diffusion, as observed in Newtonian liquids [37,38,39], in viscoelastic fluids the particle orientation is strongly subjected to the slow response of the surroundings This results in a transition of the orientational particle dynamics from enhanced rotational diffusion to persistent circular motion when increasing their propulsion speed [33]. We repeated the experiments under similar conditions in a Newtonian fluid, where we show in a straightforward manner that the rotational behavior of APs remains largely unaffected by their interaction with solid surfaces, consistent with indirect experimental observations previously reported [40]
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