From collections of independent, mindless robots to flexible, mobile, and directional superstructures

A swarm of simple active particles confined in a flexible scaffold is a promising system to make mobile and deformable superstructures. These soft structures can perform tasks that are difficult to carry out for monolithic robots because they can infiltrate narrow spaces, smaller than their size, and move around obstacles. To achieve such tasks, the origin of the forces the superstructures develop, how they can be guided, and the effects of external environment, especially geometry and the presence of obstacles, need to be understood. Here, we report measurements of the forces developed by such superstructures, enclosing a number of mindless active rod-like robots, as well as the forces exerted by these structures to achieve a simple function, crossing a constriction. We relate these forces to the self-organization of the individual entities. Furthermore, and based on a physical understanding of what controls the mobility of these superstructures and the role of geometry in such a process, we devise a simple strategy where the environment can be designed to bias the mobility of the superstructure, giving rise to directional motion. Simple tasks-such as pulling a load, moving through an obstacle course, or cleaning up an arena-are demonstrated. Rudimentary control of the superstructures using light is also proposed. The results are of relevance to the making of robust flexible superstructures with nontrivial space exploration properties out of a swarm of simpler and cheaper robots.

Large variability in the motility of spiroplasmas in media of different viscosities

Spiroplasmas are bacteria that do not possess flagella and their motility is linked to kink propagation coupled to changes in the cell body helicity. While the motility of bacteria with flagellar motion has been studied extensively, less work has been devoted to the motility of spiroplasmas. We first show that the motility of such bacteria has large variability from individual to individual as well as large fluctuations in time. The Brownian motion of such bacteria both in orientation and translation is also highlighted. We propose a simple model to disentangle the different components of this motility by examining trajectories of single bacteria in different viscosity solvents. The mean velocity of the bacteria turns out to depend on the viscosity of the medium as it increases with viscosity. Further, the temporal fluctuations of the bacteria motility turn out to be very strong with a direct link to tumbling events particular to this bacteria.

Boundaries Control Collective Dynamics of Inertial Self-Propelled Robots

Simple ingredients, such as well-defined interactions and couplings for the velocity and orientation of self-propelled objects, are sufficient to produce complex collective behavior in assemblies of such entities. Here, we use assemblies of rodlike robots made motile through self-vibration. When confined in circular arenas, dilute assemblies of these rods act as a gas. Increasing the surface fraction leads to a collective behavior near the boundaries: polar clusters emerge while, in the bulk, gaslike behavior is retained. The coexistence between a gas and surface clusters is a direct consequence of inertial effects as shown by our simulations. A theoretical model, based on surface mediated transport accounts for this coexistence and illustrates the exact role of the boundaries. Our study paves the way towards the control of collective behavior: By using deformable but free to move arenas, we demonstrate that the surface induced clusters can lead to directed motion, while the topology of the surface states can be controlled by biasing the motility of the particles.

Structure of velocity distributions in shock waves in granular gases with extension to molecular gases

Velocity distributions in normal shock waves obtained in dilute granular flows are studied. These distributions cannot be described by a simple functional shape and are believed to be bimodal. Our results show that these distributions are not strictly bimodal but a trimodal distribution is shown to be sufficient. The usual Mott-Smith bimodal description of these distributions, developed for molecular gases, and based on the coexistence of two subpopulations (a supersonic and a subsonic population) in the shock front, can be modified by adding a third subpopulation. Our experiments show that this additional population results from collisions between the supersonic and subsonic subpopulations. We propose a simple approach incorporating the role of this third intermediate population to model the measured probability distributions and apply it to granular shocks as well as shocks in molecular gases.

Unstable blast shocks in dilute granular flows

Shocks and blasts can be readily obtained in granular flows be they dense or dilute. Here, by examining the propagation of a blast shock in a dilute granular flow, we show that such a front is unstable with respect to transverse variations of the density of grains. This instability has a well-defined wavelength which depends on the density of the medium and has an amplitude which grows as an exponential of the distance traveled. These features can be understood using a simple model for the shock front, including dissipation which is inherent to granular flows. While this instability bears much resemblance to that anticipated in gases, it is distinct and has special features we discuss here.

Drag coefficient for a circular obstacle in a quasi-two-dimensional dilute supersonic granular flow

The measurement of the drag coefficient of a dilute granular flow around a cylinder is carried out over a wide range of Knudsen numbers. The variation of this coefficient shows a smooth transition from a freely falling grains regime to a continuous flow regime. This is reminiscent of the behavior of gases in the supersonic regime. This transition is accompanied by remarkable changes of the density and velocity profiles near the cylinder. A simple model is proposed for the transition regime which is in agreement with the experimental measurements.

Blast shocks in quasi-two-dimensional supersonic granular flows

In a thin, dilute, and fast flowing granular layer, the impact of a small sphere generates a fast growing hole devoid of matter. The growth of this hole is studied in detail, and its dynamics is found to mimic that of blast shocks in gases. This dynamics can be decomposed into two stages: a fast initial stage (the blast) and a slower growth regime whose growth velocity is given by the speed of sound in the medium used. A simple model using ingredients already invoked for the case of blast shocks in gases but including the inelastic nature of collisions between grains accounts accurately for our results. The system studied here allows for a detailed study of the full dynamics of a blast as it relaxes from a strong to a weak shock and later to an acoustic disturbance.

Shock front width and structure in supersonic granular flows

The full structure of a shock front around a blunt body in a quasi-two-dimensional granular flow is studied. Two features, a large density gradient and a very small thickness of the front, characterize this shock and make it different from shocks in molecular gases. Both of these features can be understood using a modified version of the granular kinetic theory. Our model separates the particles into two subpopulations: fast particles having experienced no collisions and randomly moving particles. This separation is motivated by direct measurements of the particle velocities which show a bimodal distribution. Our results not only shed new light on the use of the granular kinetic theory under extreme conditions (shock formation) but bring new insight into the physics of shocks in general.

Dynamics of impact cratering in shallow sand layers

When a solid sphere impacts a shallow layer of sand deposited on a solid surface, a crater can be obtained. The dynamics of the opening of the crater can be followed accurately. During this opening, the radius of the crater can be conveniently modeled by an exponential saturation with a well-defined time constant. The crater then closes up partially once the opening phase is over as the sand avalanches down the slope of the crater. We here present a detailed study of the full dynamics of the crater formation as well as the dynamics of the corrola formed during this process. A simple model accounts for most of our observations.

Role of fluctuation-induced interactions in the axial segregation of granular materials

The movement of a few large diameter spheres immersed in a granular medium composed of smaller beads in a rotating cylinder is studied. We evidence attractions and repulsions between the large spheres depending on the rotation frequency. The large spheres also show relative position fluctuations which are Gaussian. A complete study of this problem sheds new light on the problem of size segregation in granular materials and points to the importance of fluctuation-induced interactions.

Dynamic sand dunes

When sand falling in the spacing between two plates goes past an obstacle, a dynamic dune with a parabolic shape and an inner triangular region of nonflowing or slowly creeping sand forms. The angle of the triangular zone increases with the height of the dune and saturates at a value determined by the geometry of the cell. The width of the dune, related to the radius of curvature at the tip, shows universal features versus its height rescaled by geometrical parameters. The velocity profile in the flowing part is determined and found to be nonlinear. The parabolic shape can be accounted for using a simple driven convection-diffusion equation for the interface.