Diffusive dynamics of elongated particles in active colloidal suspensions of motile algae

Abstract

Swimming microorganisms can influence the diffusion of passive particles. The effect of this swimmer-particle interaction depends on different properties, such as the hydrodynamic field of the swimmer and the relative sizes of microorganisms and particles. We investigated an enhancement of the diffusion of silica doublets in a suspension of microalgae Chlamydomonas reinhardtii in a flat capillary. Depending on the concentration of microswimmers, the translational and rotational diffusion constants increase by several orders of magnitude in the presence of the swimming algae. For low concentrations of algae, the doublets exhibit Brownian motion in a fluctuating flow field generated by multiple swimmers. One can observe strong, diffusive transport caused by occasional large displacements. At high swimmer concentration, the algae form dense clusters, where the rotational motion of the doublets shows a subdiffusive behaviour while the translational motion remains diffusive.

Interplay between Brownian and hydrodynamic tracer diffusion in suspensions of swimming microorganisms

“…2016; Ortlieb et al. 2019; von Rüling, Kolley & Eremin 2021), and theoretically, with microswimmers typically being modelled either as force dipoles acting on the surrounding fluid (Pushkin, Shum & Yeomans 2013; Pushkin & Yeomans 2013; Morozov & Marenduzzo 2014; Nordanger, Morozov & Stenhammar 2022), as spherical ‘squirmers’ with an imposed slip velocity along their body (Thiffeault & Childress 2010; Lin et al. 2011; Thiffeault 2015), or as needle-shaped ‘slender swimmers’ with imposed stresses along their body lengths (Saintillan & Shelley 2012; Krishnamurthy & Subramanian 2015).…”

Active nematic liquid crystals simulated by particle-based mesoscopic methods

“…43,44 All this suggests that active N-MPCD (AN-MPCD) could be very valuable to investigate the dynamics of particles suspended in active media such as colloids and disks in active nematics and particles suspended in swimming bacteria and motile algae. [50][51][52][53] Here we present two MPCD schemes for 3D active nematics. The first one derives from N-MPCD proposed by Shendruk and Yeomans in 2015, in which nematic order arises from an orientation collision operator.…”

“…43,44 All this suggests that active N-MPCD (AN-MPCD) could be very valuable to investigate the dynamics of particles suspended in active media such as colloids and disks in active nematics and particles suspended in swimming bacteria and motile algae. 50–53…”

Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions

“…It also has a more fundamental relevance, as tracers can be used to probe the nature of non-equilibrium fluctuations in active systems, in particular in relation to (equilibrium) Brownian motion [5]. Arguably, the simplest model system for studying tracer diffusion in such "active baths" is a spherical colloidal particle immersed in a dilute suspension of biological microswimmers, such as E. coli [6][7][8][9][10][11][12][13][14][15] or C. reinhardtii [16][17][18][19]. A generic feature of tracer motion in these systems is that the displacement is characterized by ballistic motion at short times (approximately 0.02 − 2 seconds [13]) and diffusive motion at long times, although with a significantly increased translational diffusion coefficient D T compared to the corresponding Brownian one.…”

“…Going beyond the case of spherical tracers, a few recent experimental studies have focused on the dynamics of anisotropic passive particles in the form of ellipsoids [14,18] or dumbbells [19] immersed in active suspensions. In addition to the enhanced overall diffusion seen for spherical tracers, these studies found significantly increased rotational diffusion coefficients D R .…”