Active depinning of bacterial droplets: the collective surfing of Bacillus subtilis

How systems are endowed with migration capacity is a fascinating question with implications ranging from the design of novel active systems to the control of microbial populations. Bacteria, which can be found in a variety of environments, have developed among the richest set of locomotion mechanisms both at the microscopic and collective levels. Here, we uncover experimentally a new mode of collective bacterial motility in humid environment through the depinning of bacterial droplets. While capillary forces are notoriously enormous at the bacterial scale, even capable of pinning water droplets of millimetric size on inclined surfaces, we show that bacteria are able to harness a variety of mechanisms to unpin contact lines, hence inducing a collective slipping of the colony across the surface. Contrary to flagella-dependent migration modes like swarming we show that this much faster `colony surfing' still occurs in mutant strains of \textit{Bacillus subtilis} lacking flagella. The active unpinning seen in our experiments relies on a variety of microscopic mechanisms which could each play an important role in the migration of microorganisms in humid environment.Comment: 6 pages, 7 figures, SI: 5 movies, 10 figures, 1 tabl

Erosion patterns in a sediment layer

We report here on a laboratory-scale experiment which reproduces a rich variety of natural patterns with few control parameters. In particular, we focus on intriguing rhomboid structures often found on sandy shores and flats. We show that the standard views based on water surface waves come short to explain the phenomenon and we evidence a new mechanism based on a mud avalanche instability.Comment: 4 pages, 4 figures, to appear as Phys. Rev. E rapid com

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass

Dynamics of spreading of liquid on a hydrogel substrate

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Dynamics of the contact line in wetting and diffusing processes of water droplets on hydrogel (PAMPS–PAAM) substrates

International audienceWe studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly(2-acrylamido-2-methyl-propane-sulfonic acid-co-acrylamide) (PAMPS-PAAM)) substrates. The profiles of the droplet and substrate were measured simultaneously using a grid projection method. We observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits successively two different behaviors: pinned and receding, and the transition between these two behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of the local slope of the gel surface near the contact line increases. At the moment where these two angles almost correspond to each other, the contact line starts to recede. Our results indicate that due to the water diffusion, a locally swollen region is formed in the vicinity of the droplet-gel interface, and whether the contact line is pinned or recedes is determined by the surface property of this swollen region

Dynamics of Liquid Contact Line on Visco-Elastic Gels

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