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...

We study catalytic Janus particles and Escherichia coli bacteria swimming in a two-dimensional colloidal crystal. The Janus particles orbit individual colloids and hop between colloids stochastically, with a hopping rate that varies inversely with fuel (hydrogen peroxide) concentration. At high fuel concentration, these orbits are stable for 100s of revolutions, and the orbital speed oscillates periodically as a result of hydrodynamic, and possibly also phoretic, interactions between the swimmer and the six neighbouring colloids. Motile E. coli bacteria behave very differently in the same colloidal crystal: their circular orbits on plain glass are rectified into long, straight runs, because the bacteria are unable to turn corners inside the crystal.

The flagellated bacterium Escherichia coli is increasingly used experimentally as a self-propelled swimmer. To obtain meaningful, quantitative results that are comparable between different laboratories, reproducible protocols are needed to control, 'tune' and monitor the swimming behaviour of these motile cells. We critically review the knowledge needed to do so, explain methods for characterising the colloidal and motile properties of E. coli cells, and propose a protocol for keeping them swimming at constant speed at finite bulk concentrations. In the process of establishing this protocol, we use motility as a high-throughput probe of aspects of cellular physiology via the coupling between swimming speed and the proton motive force. Keywords:Escherichia coli, active colloids, motility, differential dynamic microscopy, metabolism, bioenergetics, proton motive force Some time ago, our lab wanted to culture motile bacteria as 'model active colloids'. We obtained a strain of Escherichia coli with the full complement of motility genes and a culturing protocol from a local microbiologist. For some time, we thought we were experimenting with motile E. coli, until one day we checked in the microscope. Few, if any, of the cells were swimming! So we set out to learn how to modify the standard protocol to optimise motility by collating literature, talking to other researchers and trial and error; we also implemented differential dynamic microscopy (DDM) to quantify motility.This article reviews what we have learnt. Some of the material is previously known, but seldom critically discussed in one place. We have explained some basic bacterial bioenergetics and genetics, because physical scientists can use E. coli and collaborate with biologists more effectively if these topics are understood. Much of the materials is new, arising from using DDM to quantify motility. While we aim primarily at researchers working on active colloids [1], this article should also be useful to others studying motility biophysics [2].From the outset, we refer to various culture media (BMB, TB, LB) and protocols, and freely use terminology related to molecular biology (plasmid, gene names, etc.) and measurement techniques (OD, DDM, etc.). Readers should refer to Section 5 on matters of cell culture, Sections 3 and 4 for methodology, and Section 10 and Appendix C for biological jargon. We also provide a table of symbols in Appendix F.

We present a numerical study on the phase diagram for a simple model of Janus colloids, including ordered and disordered structures. Using a range of techniques, we generate a set of crystal structures and investigate their relative stability field in the pressure-temperature and temperature-density planes by means of free-energy calculations and thermodynamic integration schemes. We find that despite the Janus colloids' simple architecture, they form stable crystal structures with complicated bond-topologies on an underlying face-centered-cubic or hexagonal-close-packed lattice. In addition, we find a phase consisting of wrinkled bilayer sheets, competing with both the fluid and the crystal phases. We detect a metastable gas-liquid coexistence which displays a micellization-driven re-entrant behavior.