Lane formation in driven mixtures of oppositely charged colloids

Abstract

We present quantitative experimental data on colloidal laning at the single-particle level. Our results demonstrate a continuous increase in the fraction of particles in a lane for the case where oppositely charged particles are driven by an electric field. This behavior is accurately captured by Brownian dynamics simulations. By studying the fluctuations parallel and perpendicular to the field we identify the mechanism that underlies the formation of lanes.Far from thermodynamic equilibrium, a wealth of fascinating selforganization processes can emerge along with unusual pattern formation and novel transport properties. 1,2 One of the simplest prototypes of non-equilibrium pattern formation is lane formation, exhibited by dusty plasmas, 3,4 granular matter, 5 pedestrian dynamics, 6,7 and army ants. 8 In this paper, we characterize the patterning and dynamical signatures of lane formation in a colloidal system experimentally and with computer simulations. Our results may find use in electronic ink, which also contains oppositely charged colloids that are driven by electric fields. 9,10 A fundamental and microscopic understanding of non-equilibrium phenomena requires resolving the underlying dynamical processes on the scale of the individual particles. For this, colloidal dispersions are excellent model systems since they can be brought out of equilibrium in a controlled way by external fields and the trajectories of the individual particles can be tracked in real space using confocal microscopy, which allows unparalleled comparison with computer simulation and particle-level theory. 11-13 Here, we first study the formation of lanes in driven colloidal mixtures as a function of the driving strength using both experiments and Brownian dynamics computer simulations. Lane formation in this 3D system is found to be a continuous process as a function of driving field. Starting from an initial mixed state, the dynamical mechanism behind the formation of lanes is identified: there is an enhanced lateral mobility of particles induced by collisions with particles driven in the opposite direction, which sharply decreases once lanes are formed. Therefore, particles in a lane can be regarded as being in a dynamically 'locked-in' state.In our experiments, we used a binary dispersion of sterically stabilized, nearly equal sized, but oppositely charged polymethylmethacrylate (PMMA) spheres inside a rectangular capillary. The particles were synthesized by dispersion polymerization, 14 and fluorescently labeled with either 7-nitrobenzo-2-oxa-1,3-diazole (NBD) or rhodamine isothiocyanate (RITC). The two species are color-coded as 'green' (s green ¼ 1.06 mm, polydispersity 6%, NBDlabeled) and 'red' (s red ¼ 0.91 mm, polydispersity 7%, RITC-labeled). The overall volume fraction of the suspension f green (0.090) + f red (0.090) was 0.18.To match the density and refractive index of the particles with the solvent, the particles were dispersed in a mixture of 27.2 w% cisdecahydronaphthalene and cyclohexylbromide containing 75 mM tetrabut...

Effective temperature concept evaluated in an active colloid mixture

“…The effect of the rotating magnetic field is modeled as an external torque driving the circular motion of each particle. Without this rotation, oppositely swimming particles would separate into lanes to reduce collisions, a known phenomenon (19). However, at finite R, lanes grow only to a finite length (Fig.…”

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

“…Novel effects are expected for a strongly attractive cross-interactions leading to mutual mixing of A-and Bparticles. These interactions are for example realized in oppositely charged suspensions 75,76 . It would be interesting to check whether a colloidal brazil-nut effect can still be observed in this case.…”

Instabilities and turbulence-like dynamics in an oppositely driven binary particle mixture

“…When two kinds of particles are driven in opposite directions, the system exhibits a self-organization from a uniformly mixed state into strongly ordered anisotropic patterns. This driven segregation phenomenon, first found in the computer simulations [2][3][4][5][6][7][8], has been observed in laboratory experiments such as mixtures of oppositely charged colloids [9,10] or dusty plasmas in presence of an external electric field [11]. Another important class of examples is pedestrian and traffic flow dynamics [12][13][14][15].…”

“…Another important class of examples is pedestrian and traffic flow dynamics [12][13][14][15]. By tracing single-particle motions in colloidal dispersions, a recent experiment has proposed the underlying mechanism of the lane formation as a dynamical "lock-in" state [10]. A lateral mobility of particles is initially enhanced by frequent collisions, which however decreases considerably once lane is formed.…”