Trapping and sorting of active matter in a periodic background potential.

We study, numerically, a system of active particles with either a single noise value or a mixture of equal proportions of particles with two noise values under the influence of an attractive periodic background potential, and we observe their diffusion regimes and trapping states. For the single noise system, we show that the slow diffusion is correlated to a significant particle trapping, while normal diffusion is seen for partial or no trapping. Our results indicate that low noise particles are less susceptible to the background, i.e., they have a smaller chance to be trapped as compared to higher noise particles for the same background, and that denser systems achieve a no-trapping state, unless for the largest noise value we studied. For the mixtures, we study the sorting of particles based on their noise value differences and observe that particles with distinct noises are trapped at distinct radii compared to a trap minimum, and, since these radii depend on the density, the latter should be well tuned in order to have an efficient sorting.

Self-propelled particle transport in regular arrays of rigid asymmetric obstacles.

We report numerical results which show the achievement of net transport of self-propelled particles (SPPs) in the presence of a two-dimensional regular array of convex, either symmetric or asymmetric, rigid obstacles. The repulsive interparticle (soft disks) and particle-obstacle interactions present no alignment rule. We find that SPPs present a vortex-type motion around convex symmetric obstacles even in the absence of hydrodynamic effects. Such a motion is not observed for a single SPP, but is a consequence of the collective motion of SPPs around the obstacles. A steady particle current is spontaneously established in an array of nonsymmetric convex obstacles (which presents no cavity in which particles may be trapped), and in the absence of an external field. Our results are mainly a consequence of the tendency of the self-propelled particles to attach to solid surfaces.

Lift and drag in intruders moving through hydrostatic granular media at high speeds.

Recently, experiments showed that forces on intruders dragged horizontally through dense, hydrostatic granular packings mainly depend on the local surface orientation and can be seen as the sum of the forces exerted on small surface elements. In order to understand such forces more deeply, we perform a two-dimensional soft-sphere molecular dynamics simulation, on a similar setup, of an intruder dragged through a 50-50 bi-disperse granular packing, with diameters 0.30 and 0.34 cm. We measure, for both circular and half-circle shapes, the forces parallel (drag) and perpendicular (lift) to the drag direction as functions of the drag speed, with V=10.3-309 cm/s, and intruder depths, with D=3.75-37.5 cm. The drag forces on an intruder monotonically increase with V and D, and are larger for the circle. However, the lift force does not depend monotonically on V and D, and this relationship is affected by the shape of the intruder. The vertical force was negative for the half-circle, but for a small range of V and D, we measure positive lift. We find no sign change for the lift on the circle, which is always positive. The explanation for the nonmonotonic dependence is related to the decrease in contacts on the intruder as V increases. This is qualitatively similar to supersonic flow detachment from an obstacle. The detachment picture is supported by simulation measurements of the velocity field around the intruder and force profiles measured on its surface.

IMDB network revisited: unveiling fractal and modular properties from a typical small-world network.

We study a subset of the movie collaboration network, http://www.imdb.com, where only adult movies are included. We show that there are many benefits in using such a network, which can serve as a prototype for studying social interactions. We find that the strength of links, i.e., how many times two actors have collaborated with each other, is an important factor that can significantly influence the network topology. We see that when we link all actors in the same movie with each other, the network becomes small-world, lacking a proper modular structure. On the other hand, by imposing a threshold on the minimum number of links two actors should have to be in our studied subset, the network topology becomes naturally fractal. This occurs due to a large number of meaningless links, namely, links connecting actors that did not actually interact. We focus our analysis on the fractal and modular properties of this resulting network, and show that the renormalization group analysis can characterize the self-similar structure of these networks.

Lift force on an asymmetrical obstacle immersed in a dilute granular flow.

This paper investigates the lift force exerted on an elliptical obstacle immersed in a granular flow through analytical calculations and computer simulations. The results are shown as a function of the obstacle size, orientation with respect to the flow direction (tilt angle), the restitution coefficient and ellipse eccentricity. The theoretical argument, based on the force exerted on the obstacle due to inelastic, frictionless collisions of a very dilute flow, captures the qualitative features of the lift, but fails to reproduce the data quantitatively. The reason behind this disagreement is that the dilute flow assumption on which this argument is built breaks down as a granular shock wave forms in front of the obstacle. More specifically, the shock wave changes the grains impact velocity at the obstacle, decreasing the overall net lift obtained from a very dilute flow.

Colloids in a periodic potential: driven lattice gas in continuous space.

Motivated by recent studies of colloidal particles in optical-tweezer arrays, we study a two-dimensional model of a colloidal suspension in a periodic potential. The particles tend to stay near potential minima, approximating a lattice gas. The interparticle interaction, a sum of Yukawa terms, features short-range repulsion and attraction at somewhat larger separations, such that two particles cannot occupy the same potential well, but occupation of adjacent cells is energetically favored. Monte Carlo simulation reveals that the equilibrium system exhibits condensation, as in the Ising model or lattice gas with conserved magnetization; the transition appears to be continuous at one-half occupancy. We study the effect of biased hopping, favoring motion along one lattice direction, as might be generated by a steady flow relative to the potential array. This system is found to exhibit features of the driven lattice gas: the interface is oriented along the drive, and appears to be smooth. A weak drive facilitates ordering of the particles into high- and low-density regions, while stronger bias tends to destroy order, and leads to very large energy fluctuations. We also study ordering in a moving periodic potential. Our results suggest possible realizations of equilibrium and driven lattice gases in a colloidal suspension subject to an optical tweezer array.