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Understanding flow and transport of bacteria in porous media is crucial to technologies such as bioremediation, biomineralization and enhanced oil recovery. While physicochemical bacteria filtration is well documented, recent studies showed that bacterial motility plays a key role in the transport process. Flow and transport experiments performed in microfluidic chips containing randomly placed obstacles confirmed that the distributions of non-motile bacteria stays compact, whereas for the motile strains, the distributions are characterized by both significant retention as well as fast downstream motion. For motile bacteria, the detailed microscopic study of individual bacteria trajectories reveals two salient features: (i) the emergence of an active retention process triggered by motility, (ii) enhancement of dispersion due to the exchange between fast flow channels and low flow regions in the vicinity of the solid grains. We propose a physical model based on a continuous time random walk approach. This approach accounts for bacteria dispersion via variable pore-scale flow velocities through a Markov model for equidistant particle speeds. Motility of bacteria is modelled by a two-rate trapping process that accounts for the motion towards and active trapping at the obstacles. This approach captures the forward tails observed for the distribution of bacteria displacements, and quantifies an enhanced hydrodynamic dispersion effect that originates in the combined effect of pore-scale flow variability and bacterial motility. The model reproduces the experimental observations, and predicts bacteria dispersion and transport at the macroscale. [ABSTRACT FROM AUTHOR]

Using a 3D Lagrangian tracking technique, we determine experimentally the trajectories of non-tumbling E. coli mutants swimming in a Poiseuille flow. We identify a typology of trajectories in agreement with a kinematic "active Bretherton-Jeffery" model, featuring an axi-symmetric self-propelled ellipsoid. In particular, we recover the "swinging" and "shear tumbling" kinematics predicted theoretically by Zöttl and Stark (Phys. Rev. Lett., 108 (2012) 218104). Moreover using this model, we derive analytically new features such as quasi-planar piecewise trajectories, associated with the high aspect ratio of the bacteria, as well as the existence of a drift angle around which bacteria perform closed cyclic trajectories. However, the agreement between the model predictions and the experimental results remains local in time, due to the presence of Brownian rotational noise. [ABSTRACT FROM AUTHOR]

En 1906, Albert Einstein [1] a montré que l'ajout de particules sphériques solides dans un fluide augmente sa viscosité. En est-il encore ainsi quand les particules sont « actives » (c'est-à-dire motiles) ? Des expériences récentes avec des suspensions de bactéries Escherichia coli, qui sont de type « pousseur », démontrent un effet opposé : aux faibles taux de cisaillement, la viscosité macroscopique du liquide chute jusqu'à devenir nulle. Ce phénomène vient essentiellement du fait que la puissance motrice des bactéries contribue à la puissance mécanique fournie au fluide pour assurer son écoulement, cette contribution ayant une valeur nette non nulle en raison de l'organisation de la nage des bactéries soit individuellement, soit collectivement. [ABSTRACT FROM AUTHOR]

The following is a report on an experimental technique that allows one to quantify and map the velocity field with very high resolution and simple equipment in large 2D devices. Illumination through a grid is proposed to reinforce the contrast in the images and allow one to detect seeded particles that are pixel-sized or even smaller. The velocimetry technique that we have reported is based on the auto-correlation functions of the pixel intensity, which we have shown are directly related to the magnitude of the local average velocity. The characteristic time involved in the decorrelation of the signal is proportional to the tracer size and inversely proportional to the average velocity. We have reported on a detailed discussion about the optimization of relevant involved parameters, the spatial resolution and the accuracy of the method. The technique is then applied to a model porous medium made of a random channel network. We show that it is highly efficient to determine the magnitude of the flow in each of the channels of the network, opening the door to the fundamental study of the flows of complex fluids. The latter is illustrated with a yield stress fluid, in which the flow becomes highly heterogeneous at small flow rates. [ABSTRACT FROM AUTHOR]

We demonstrate and study experimentally two instabilities of a horizontal free cylinder in a vertical viscous Hele-Shaw flow; we show that they depend critically on the confinement of the flow with a different influence of transverse and lateral confinement characterized respectively by the ratios of the diameter (resp. the length) of the cylinder to the gap (resp. the width) of the cell. The onset of the instabilities depends largely on the transverse confinement: for a parameter between 0.4 and 0.6, one observes transverse horizontal oscillations of the cylinder perpendicular to the walls: their frequency is constant with the lateral and transverse confinements at a given cylinder velocity. This instability is shown to be locally two-dimensional and controlled by the local relative velocity between the cylinder and the fluids: it occurs down to Reynolds numbers based on the cell gap ~ 15, far below the corresponding 2D vortex shedding thresholds (150-250) for fixed cylinders between parallel planes. Above transverse confinements of the order of 0.55, we observe a fluttering motion with periodic oscillations of the tilt angle of the cylinder from the horizontal and of its horizontal position: their frequency decreases strongly as the lateral confinement increases but is independent of the transverse confinement and the cylinder velocity. [ABSTRACT FROM AUTHOR]

We investigate the hierarchy of optimal paths in a disordered landscape, based on the best path, the second best path and so on in terms of an energy. By plotting each path at a height according to its energy above some zero level, a landscape appears. This landscape is self-affine and controlled by two Hurst exponents: the one controlling the height fluctuations is 1/3 and the one controlling the fluctuations of the equipotential lines in the landscape is 2/3. These two exponents correspond to the exponents controlling energy and shape fluctations in the directed polymer problem. We furthermore find that the density of spanning optimal paths scale as the length of the paths to --2/3 and the histogram of energy differences between consecutive paths scale as a power law in the difference size with exponent --2.5. [ABSTRACT FROM AUTHOR]

This paper reports experimental measurements performed to test the ability of electrical resistivity tomography (ERT) imaging to provide quantitative information about transport parameters in porous media such as the dispersivity α, the mixing front velocity u, and the retardation factor Rf associated with the sorption or trapping of the tracers in the pore structure. The flow experiments are performed in a homogeneous porous column placed between two vertical set of electrodes. Ionic and dyed tracers are injected from the bottom of the porous media over its full width. Under such condition, the mixing front is homogeneous in the transverse direction and shows an S-shape variation in the flow direction. The transport parameters are inferred from the variation of the concentration curves and are compared with data obtained from video analysis of the dyed tracer front. The variations of the transport parameters obtained from an inversion performed by the Gauss-Newton method applied on smoothness-constrained least-squares are studied in detail. While u and Rf show a relatively small dependence on the inversion procedure, α is strongly dependent on the choice of the inversion parameters. Comparison with the video observations allows for the optimization of the parameters; these parameters are found to be robust with respect to changes in the flow condition and conductivity contrast. [ABSTRACT FROM AUTHOR]

The influence of a small relative density difference (Δρ/ρ≃3×10-4) on the displacement of two miscible Newtonian liquids is studied experimentally in transparent two-dimensional square networks of microchannels held vertically; the channel width distribution is log normal with a mean value of a=0.33 mm. Maps of the local relative concentration are obtained by an optical light absorption technique. Both stable displacements in which the denser fluid enters at the bottom of the cell and displaces the lighter one and unstable displacements in which the lighter fluid is injected at the bottom and displaces the denser one are realized. Except at the lowest mean flow velocity U, the average C(x,t) of the relative concentration satisfies a convection-dispersion equation. The relative magnitude of |U| and of the velocity Ug of buoyancy driven fluid motions is characterized by the gravity number Ng=Ug/|U|. At low gravity numbers |Ng|<0.01 (or equivalently high Péclet numbers Pe=Ua/Dm>500), the dispersivities ld in the stable and unstable configurations are similar to ld∝Pe0.5. At low velocities such that |Ng|>0.01, ld increases like 1/Pe in the unstable configuration (Ng<0), while it becomes constant and close to the length of individual channels in the stable case (Ng>0). Isoconcentration lines c(x,y,t)=0.5 are globally flat in the stable configuration, while in the unstable case, they display spikes and troughs with a rms amplitude σf parallel to the flow. For Ng>-0.2, σf increases initially with the distance and reaches a constant limit, while it keeps increasing for Ng<-0.2. A model taking into account buoyancy forces driving the instability and the transverse exchange of tracer between rising fingers and the surrounding fluid is suggested and its applicability to previous results obtained in three-dimensional media is discussed. [ABSTRACT FROM AUTHOR]

A change of solute dispersion regime with the flow velocity has been studied both at the macroscopic and pore scales in a transparent array of capillary channels, using an optical technique allowing for simultaneous local and global concentration mappings. Two solutions of different polymer concentrations (500 and 1000 ppm) have been used at different Péclet numbers. At the macroscopic scale, the displacement front displays a diffusive spreading: for Pe≤10, the dispersivity ld is constant with Pe and increases with the polymer concentration; for Pe>10, ld increases as Pe1.35 and is similar for the two concentrations. At the local scale, a time lag between the saturations of channels parallel and perpendicular to the mean flow has been observed and studied as a function of the flow rate. These local measurements suggest that the change of dispersion regime is related to variations of the degree of mixing at the junctions. For Pe≤10, complete mixing leads to pure geometrical dispersion enhanced for shear thinning fluids; for Pe>10, weaker mixing results in higher correlation lengths along flow paths parallel to the mean flow and in a combination of geometrical and Taylor dispersion. [ABSTRACT FROM AUTHOR]

Abstract: The effect on the transport properties of a fracture of a shear displacement between its complementary surfaces is investigated experimentally and numerically. The shear displacement induces an anisotropy of the fracture aperture field with a correlation length scaling of , which is significantly larger in the direction perpendicular to . This reflects the presence of long fluid flow channels perpendicular to the shear displacement, resulting in a higher effective permeability in that direction. Such channels will have a strong influence on the transport characteristics of a fracture, such as, for instance, its thermal exchange area, crucial for geothermal applications. Miscible displacement fronts in shear-displaced fractures obtained experimentally display a self-affine geometry with a characteristic exponent directly related to that of the fracture surfaces. We present a simple model, based on the channeling of the aperture field, which reproduces the front geometry when the mean flow is parallel to the channels created by the shear displacement. [Copyright &y& Elsevier]

An oscillatory instability has been observed experimentally on an horizontal cylinder free to move and rotate between two parallel vertical walls of distance H. The vertical motion of the cylinder, its rotation about its axis, and its transverse motion across the gap have been investigated as a function of its diameter D, its density ρs, of the mean vertical velocity U of the fluid, and of its viscosity ν. The relevant Reynolds number Re is shown to be based on the cell aperture H and on the relative velocity Vr between the fluid and the cylinder. For a blockage ratio D/H above 0.5 and Re above 20, oscillations of the rolling angle of the cylinder about its axis and of its transverse coordinate in the gap were observed together with periodic variations of the vertical velocity. For a given fluid-cylinder pair, the relative velocity Vr as well as the frequency f and the amplitude of the transverse velocity for these oscillations are nearly independent of the flow velocity U. For given cylinder density and fluid characteristics, f is also nearly independent of the ratio D/H in the range investigated. The oscillations could be observed down to values of Re as low as 30: this is lower than usual values for vortex shedding in confined geometries, which suggests that one might deal with a different instability mechanism. [ABSTRACT FROM AUTHOR]

We analyze experimentally chemical wave propagation in the disordered flow field of a porous medium. The reaction fronts travel at a constant velocity that drastically depends on the mean flow direction and rate. The fronts may propagate either downstream and upstream but, surprisingly, they remain static over a range of flow rate values. Resulting from the competition between the chemical reaction and the disordered flow field, these frozen fronts display a particular sawtooth shape. The frozen regime is likely to be associated with front pinning in low velocity zones, the number of which varies with the ratio of the mean flow and the chemical front velocities. [ABSTRACT FROM AUTHOR]