Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast

Abstract

We numerically and experimentally study the segregation dynamics in a binary mixture of microswimmers which move on a two-dimensional substrate in a static periodic triangular-like light intensity field. The motility of the active particles is proportional to the imposed light intensity and they possess a motility contrast, i.e., the prefactor depends on the species. In addition, the active particles also experience a torque aligning their motion towards the direction of the negative intensity gradient. We find a segregation of active particles near the intensity minima where typically one species is localized close to the minimum and the other one is centered around in an outer shell. For a very strong aligning torque, there is an exact mapping onto an equilibrium system in an effective external potential that is minimal at the intensity minima. This external potential is similar to (height-dependent) gravity, such that one can define effective "heaviness" of the self-propelled particles. In analogy to shaken granular matter in gravity, we define a "colloidal Brazil nut effect" if the heavier particles are floating on top of the lighter ones. Using extensive Brownian dynamics simulations, we identify system parameters for the active colloidal Brazil nut effect to occur and explain it based on a generalized Archimedes' principle within the effective equilibrium model: heavy particles are levitated in a dense fluid of lighter particles if their effective mass density is lower than that of the surrounding fluid. We also perform real-space experiments on light-activated self-propelled colloidal mixtures which confirm the theoretical predictions. arXiv:1901.10201v1 [cond-mat.soft]

Diffusing wave paradox of phototactic particles in traveling light pulses

“…The aligning torque of a positively phototacting AP within a gradient ∇ I is given by 27,40,41 The corresponding reorientation dynamics can be expressed by the angular velocity.where the amplitude ω max grows with increasing ∇ I but eventually saturates 27 (“Methods”). In our experiments, all light gradients were above this saturation threshold, which results in a constant particle reorientation time τ = 3.3 ± 0.75 s being much shorter than 1/ D r (see the section “Methods” for further details).…”

Non-Newtonian rheology in inertial suspensions of inelastic rough hard spheres under simple shear flow

“…A simpler version of the generalized Fokker-Planck model (2.10) has been recently employed to study colloidal Brazil nut effect in microswimmer mixtures. 30 According to Eqs. (2.11) and (2.12), the Boltzmann kinetic equation (2.1) can be written as…”

“…This suspension model has been more recently considered to study a segregation problem of microswimmer mixtures. 30 The suspension model for inelastic rough hard spheres is solved by following two different but complementary theoretical tools. First, Grad's moment method 31 is considered to approximately get the explicit forms of both the (reduced) translational T t /T ex and rotational T r /T ex temperatures and the (reduced) elements P kℓ /(nT ex ) of the pressure tensor in terms of the restitution coefficients α and β and the (reduced) shear rate a * ≡ a/γ t .…”

Heterogeneity-induced lane and band formation in self-driven particle systems

“…Flock and swarm behaviour arises e.g. in bacterial and cell migration 5,6 (see also the pioneering works on morphogenesis by A.M. Turing 7 ), animal aggregation [8][9][10][11] , rods and non-living systems [12][13][14] , microswimmers [15][16][17] , but also in robotic systems 18,19 . This includes a large class of non-equilibrium systems of self-driven particles often called active matter in the literature.…”