Dynamics of Self-Propelled Janus Particles in Viscoelastic Fluids

Abstract

We experimentally investigate active motion of spherical Janus colloidal particles in a viscoelastic fluid. Self-propulsion is achieved by a local concentration gradient of a critical polymer mixture which is imposed by laser illumination. Even in the regime where the fluid's viscosity is independent of the deformation rate induced by the particle, we find a remarkable increase of up to 2 orders of magnitude of the rotational diffusion with increasing particle velocity, which can be phenomenologically described by an effective rotational diffusion coefficient dependent on the Weissenberg number. We show that this effect gives rise to a highly anisotropic response of microswimmers in viscoelastic media to external forces, depending on its orientation. DOI: 10.1103/PhysRevLett.116.138301 Nature offers a plethora of microswimmers moving in complex fluid environments [1], whose properties can deviate from Newtonian behavior due to the presence of suspended macromolecules and colloidal particles [2]. Some examples are bacteria in polymeric solutions [3], spermatozoa in cervical mucus [4], and microbial pathogens in stomach mucus [5]. All of these fluids are viscoelastic; i.e., they may exhibit either liquid-or solidlike behavior, depending on imposed deformation rates. Understanding the dynamics of such kinds of microscopic systems is a topic of fundamental significance in statistical mechanics, as they exhibit new types of nonequilibrium processes [6].Despite their biological and application-related relevance, most experiments with autonomous synthetic microswimmers which are self-propelled, e.g., by diffusiophoresis [7][8][9] and thermophoresis [10], were performed in Newtonian fluids [1]. In contrast, only a few studies have considered non-Newtonian fluids [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] where viscoelasticity [11][12][13][15][16][17], shear thinning [20][21][22][23], and shear thickening [20] strongly impact self-propulsion. Previous studies with biological microswimmers suggest that, under such conditions, the dynamical response of the liquid to configurational body changes (e.g., flagellar and undulatory motion) during self-propulsion must be considered [17,18] and can lead to either an increase [11,13,[16][17][18][19]24], a decrease [12,14,17,18,23], or no change [21] of their swimming speed. To avoid specific effects due to such configurational changes and to focus on how the transient strain of viscoelastic fluids couples to the swimmer's stochastic motion, experiments with rigid microswimmers of simple shape are required.In this Letter, we study the motion of artificial microswimmers in a viscoelastic fluid. Their self-propulsion is achieved by light illumination, which allows us to adjust the propulsion velocity. Contrary to Newtonian liquids, we observe a drastic increase of their rotational diffusion with increasing velocity. Such enhancement is independent of whether the velocity is acquired by self-propulsion or by an external force, and it can be quantitatively described...

Force moments of an active particle in a complex fluid

“…The motion of two active particles in quiescent Newtonian fluids was recently studied by means of the reciprocal theorem, both for diffusiophoretic particles [27], and squirmers [28], likewise hydrodynamic interactions between two passive spheres in weakly nonlinear fluids [24,29], but hydrodynamic interactions amongst two (or many) active bodies in non-Newtonian fluids remain largely unexplored and can lead to quantitatively different dynamics [30]; the same can be said of active particles in non-Newtonian background flows [31]. If the particles are Brownian, fluid rheology can lead to significant differences in observed trajectories [32] and while it is possible to include a stochastic force in the above theory, one requires proper description of that forcing in nonlinear non-Newtonian flows [33,34].…”

Enhanced Rotational Motion of Spherical Squirmer in Polymer Solutions

“…Similarly, we expect polymer adsorption on colloids to be generic, as for polyacrylamide polymers absorbing on both halves of the silica-carbon Janus particles employed in Ref. [24]. By the coarse-grained nature of our polymer model, every monomer bead corresponds to several molecular segments of a real polymer, with a correspondingly enhanced attraction strength.…”

“…In addition, viscoelasticity affects other microswimmer properties, such as their rotational motion. Recent experimental studies of self-propelled Janus colloids in a viscoelastic fluid yield a drastically enhanced rotational diffusion by up to two orders of magnitude [24]. A further increase of activity can even result in persistent rotational motion [25].…”

Diffusion of self-propelled particles in complex media

“…Recent experiments, however, have shown that the diffusion of an active Brownian particle [13] and E. coli [10], even in the dilute regime, is highly influenced by the presence of polymers in the fluid solutions. The underlying mechanism of this behavior is yet to be understood.…”