In this paper, we explore experimentally the phase behavior of a dense active suspension of selfpropelled colloids. In addition to a solid-like and a gas-like phase observed for high and low densities, a novel cluster phase is reported at intermediate densities. This takes the form of a stationary assembly of dense aggregates, with an average size which grows with activity as a linear function of the self-propelling velocity. While different possible scenarii can be considered to account for these observations -such as a generic velocity weakening instability recently put forward -, we show that the experimental results are reproduced by a chemotactic aggregation mechanism, originally introduced to account for bacterial aggregation, and accounting here for diffusiophoretic chemical interaction between colloidal swimmers. PACS numbers:Active systems refer generically to collections of particules which consume energy at the individual scale in order to provide self-propelled motion. In assembly these systems usually exhibit a wide variety of collective behaviors, structures and patterns, which depart strongly from the classical equilibrium expectations [1][2][3][4][5][6][7][8][9][10]. In particular, there are many observations in nature of cluster-like phases in very different systems: flocks, schools, swarms of fishes, insects or bacteria [11][12][13][14][15][16]. However this apparent analogy, occuring over a broad range of scales, hides many different mechanisms in the propulsion and interactions. In order to disentangle the universal from the specific behaviors of those complex phases, a systematic experimental exploration of artificial active particles systems at high density is needed. Some experiments have been carried out on self-propelled walkers at high densities [17] but to our knowledge, active particles at the colloidal scale -involving natural brownian noise and solvent induced interactions -were only studied at low densities [18][19][20][21].In this paper we explore experimentally the behavior of a two dimensional dense active suspension of artificial self-propelled colloids (SPC). We first characterize the phase behavior of this active system under an external (gravity) field, from the dilute gas to a dense solidlike phase. A key observation is the emergence of dynamic clustering at intermediate densities. Clusters of SPC form naturally in the system, with an average size which grows with activity, in direct proportionality to the propelling velocity of an individual SPC. Several scenarii are discussed in order to rationalize these experimental results, suggesting in particular a possible chemotactic aggregation mechanism.Experimental description -The active particles are home-made spherical gold colloids of radius a 1µm half covered with platinum [22]. In presence of hydrogen peroxide the particles self-propel consuming H 2 O 2 under a self-phoretic motion (a combination of diffusiophoresis and self-electrophoresis [20,23]). We note that one key aspect here is that the system does self-propel -wit...

In this paper, we investigate experimentally the non-equilibrium steady state of an active colloidal suspension under gravity field. The active particles are made of chemically powered colloids, showing self propulsion in the presence of an added fuel, here hydrogen peroxide. The active suspension is studied in a dedicated microfluidic device, made of permeable gel microstructures. Both the microdynamics of individual colloids and the global stationary state of the suspension under gravity -density profiles, number fluctuations -are measured with optical microscopy. This allows to connect the sedimentation length to the individual self-propelled dynamics, suggesting that in the present dilute regime the active colloids behave as 'hot' particles. Our work is a first step in the experimental exploration of the out-of-equilibrium properties of artificial active systems.

We investigate the hydrodynamic friction properties of superhydrophobic surfaces and quantify their superlubricating potential. On such surfaces, the contact of the liquid with the solid roughness is minimal, while most of the interface is a liquid-gas one, resulting in strongly reduced friction. We obtain scaling laws for the effective slip length at the surface in terms of the generic surface characteristics ͑roughness length scale, depth, solid fraction of the interface, etc.͒. These predictions are successfully compared to numerical results in various geometries ͑grooves, posts or holes͒. This approach provides a versatile framework for the description of slip on these composite surfaces. Slip lengths up to 100 m are predicted for an optimized patterned surface.