Motion of a sphere through an aging system

We have investigated the drag on a sphere falling through a clay suspension that has a yield stress and exhibits rheological aging. The drag force increases with both speed and the rest time between preparation of the system and the start of the experiment, but there exists a nonzero minimum speed below which steady motion is not possible. We find that only a very thin layer of material around the sphere is fluidized when it moves, while the rest of suspension is deformed elastically. This is in marked contrast to what is found for yield-stress fluids that do not age.Comment: latex, 4 figure

Dynamics of progressive pore clogging by colloidal aggregates

International audienceThe flow of a suspension through a bottleneck often leads to its obstruction. Such a continuous flow to clogging transition has been well characterized when the constriction width to particle size ratio, W/D, is smaller than 3-4. In such cases, the constriction is either blocked by a single particle that is larger than the constriction width (W/D < 1), or there is an arch formed by several particles that try to enter it together (2 < W/D < 4). For larger W/D ratios, 4 < W/D < 10, the blockage of the constriction is presumed to be due to the successive accumulations of particles. Such a clogging mechanism may also apply to wider pores. The dynamics of this progressive obstruction remains largely unexplored since it is difficult to see through the forming clog and we still do not know how particles accumulate inside the constriction. In this paper, we use particle tracking and image analysis to study the clogging of a constriction/pore by stable colloidal particles. These techniques allow us to determine the shape and the size of all the objects, be they single particles or aggregates, captured inside the pore. We show that even with the rather monodisperse colloidal suspension we used individual particles cannot clog a pore alone. These individual particles can only partially cover the pore surface whilst it is the very small fraction of aggregates present in the suspension that can pile up and clog the pore. We analyzed the dynamics of aggregate motion up to the point of capture within the pore, which helps us to elucidate why the probability of aggregate capture inside the pore is high

Microbial competition in porous environments can select against rapid biofilm growth

Microbes often live in dense communities called biofilms where competition between strains and species is fundamental to both evolution and community function. While biofilms are commonly found in soil-like porous environments, the study of microbial interactions has largely focused on biofilms growing on flat, planar surfaces. Here we use novel microfluidic experiments, mechanistic models, and game theory to study how porous media hydrodynamics can mediate competition between bacterial genotypes. Our experiments reveal a fundamental challenge faced by microbial strains that live in porous environments: cells that rapidly form biofilms tend to block their access to fluid flow and redirect resources to competitors. To understand how these dynamics influence the evolution of bacterial growth rates we couple a model of flow-biofilm interaction with a game theory analysis. This shows that hydrodynamic interactions between competing genotypes give rise to an evolutionarily stable growth rate that stands in stark contrast with that observed in typical laboratory experiments: cells within a biofilm can outcompete other genotypes by growing more slowly. Our work reveals that hydrodynamics can profoundly affect how bacteria compete and evolve in porous environments, the habitat where most bacteria live

Deformation and flow of a two-dimensional foam under continuous shear

We investigate the flow properties of a two-dimensional aqueous foam submitted to a quasistatic shear in a Couette geometry. A strong localization of the flow (shear banding) at the edge of the moving wall is evidenced, characterized by an exponential decay of the average tangential velocity. Moreover, the analysis of the rapid velocity fluctuations reveals self-similar dynamical structures consisting of clusters of bubbles rolling as rigid bodies. To relate the instantaneous (elastic) and time-averaged (plastic) components of the strain, we develop a stochastic model where irreversible rearrangements are activated by local stress fluctuations originating from the rubbing of the wall. This model gives a complete description of our observations and is also consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure

Three-dimensional jamming and flows of soft glassy materials

Various disordered dense systems such as foams, gels, emulsions and colloidal suspensions, exhibit a jamming transition from a liquid state (they flow) to a solid state below a yield stress. Their structure, thoroughly studied with powerful means of 3D characterization, exhibits some analogy with that of glasses which led to call them soft glassy materials. However, despite its importance for geophysical and industrial applications, their rheological behavior, and its microscopic origin, is still poorly known, in particular because of its nonlinear nature. Here we show from two original experiments that a simple 3D continuum description of the behaviour of soft glassy materials can be built. We first show that when a flow is imposed in some direction there is no yield resistance to a secondary flow: these systems are always unjammed simultaneously in all directions of space. The 3D jamming criterion appears to be the plasticity criterion encountered in most solids. We also find that they behave as simple liquids in the direction orthogonal to that of the main flow; their viscosity is inversely proportional to the main flow shear rate, as a signature of shear-induced structural relaxation, in close similarity with the structural relaxations driven by temperature and density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm

Spin-coating de fluides à seuil

The spin-coating of yield stress fluids is of interest in various fields in which it is necessary to coat a thin film of gel or colloidal suspensions. It is also used with larger volumes in order to disperse at best sewage sludge in fields. Until now, this problem has been mainly dealt with from a theoretical point of view, (Jenekhe and Schuldt, 1985; Burgess and Wilson, 1996 ) and which focused on the stationary flow characteristics. Here, we propose an experimental investigation of the start flow characteristics, which in particular shows that the viscoelastic properties of the material in its solid regime may play an important role. We analysed the deformation of a cylindrical sample of a viscoelastic gel onto a rotating surface submitted to an increasing velocity . We showed that the centrifugal force is the main factor governing the behavior which can be distinguished into three phases. First, at low rotational velocities, smaller than a critical value , the material behaves like a Kelvin-Voigt solid and slightly deforms undergoing an elongational flow: the variations of the radius are weak. Second, when the rotational velocity reaches its critical value, the elastic limit is exceeded. The experiments showed that the deformation is no longer homogenous. The material abruptly behaves locally like a liquid and spreads out quickly over the disc when the shear stress is higher than the yield stress. Two distinct parts can be then defined; a central one which is not affected by the flow and a peripheral one which flows rapidly by forming an external roll. Thirdly, the spreading out is located on the circular edge of the material and digitations occur. Experiments were made by varying the slope of the velocity ramp and the material characteristics. We clearly showed a direct link between the viscoelastic parameters of the material and the beginning of the spreading : the more the yield stress, the more the critical velocity or the time to reach it is high. Moreover, using the lubrication approximation, we calculated the shear stress distribution within the material and we demonstrated that the limit between the two regimes, solid and liquid, is dominated by a ratio between the inertia and the yield stress