Characterization of fracture aperture field heterogeneity by electrical resistance measurement

We use electrical resistance measurements to characterize the aperture field in a rough fracture. This is done by performing displacement xperiments using two miscible fluids of different electrical resistivity and monitoring the time variation of the overall fracture resistance. Two fractures have been used: their complementary rough walls are identical but have different relative shear displacements which create “channel” or “barrier” structures in the aperture field, respectively parallel or perpendicular to the mean flow velocity →U. In the “channel” geometry, the resistance displays an initial linear variation followed by a tail part which reflects the velocity contrast between slow and fast flow channels. In the “barrier” geometry, a change in the slope between two linear zones suggests the existence of domains of different characteristic aperture along the fracture. These variations are well reproduced analytically and numerically using simple flow models. For each geometry, we present then a data inversion procedure that allows one to extract the key features of the heterogeneity from the resistance measurement.Fil: Boschan, Alejandro. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chertcoff, Ricardo Héctor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Hulin, J. P.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Auradou, H.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; Franci

E coli Accumulation behind an Obstacle

This paper describes our findings regarding the accumulation of motile bacte-ria at the rear of a confined obstacle and the physical description of the me-chanisms at play. We found that the modification of flow due to the presence of the obstacle produces vorticity that favor the diffusion of bacteria towards the downstream stagnation point. By testing different flow rates, we deter-mined the range in which bacteria accumulate. More interestingly, we observe that hydrodynamic interaction between the bacteria and the top and bottom surface of the microfluidic chip maintain the bacteria in the region where the flow velocity is lower than their own velocity. In the case of non-motile bacte-ria, this effect is not observed because bacteria follow the streamlines as pas-sive tracers do.Fil: Miño, Gastón Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Entre Ríos. Universidad Nacional de Entre Ríos. Centro de Investigaciones y Transferencia de Entre Ríos; ArgentinaFil: Baabour, Magali Denise. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Gutkind, Gabriel Osvaldo. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay; ArgentinaFil: Clément, Eric. Centre National de la Recherche Scientifique; FranciaFil: Auradou, Harold. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Experimental study of solute dispersion in macroscopic suspension flow

We experimentally investigate the influence of suspended neutrally-buoyant particles on the dispersion of a passive solute in pressure-driven axial flow in a constant aperture fracture (parallel plates configuration). A dye is employed as solute in order to measure its local concentration by means of a light transmission technique. In the experiments a dyed particle suspension displaces a transparent one at constant flow rate, for volume fractions ϕ ranging from 0 to 0.25 and for solute Péclet numbers (Pes) between 35 and 476 (mean flow velocities U between 0.004 and 0.0544 cm/s). The local time variation of the solute concentration in the measurement zone was well-fitted by the solution of the advection–dispersion equation, and a longitudinal dispersion coefficient D for the solute was measured. For Pes 0) and without particles (ϕ = 0) are equal within the measurement error. For Pes > 300, D decreases for ϕ > 0 compared to ϕ = 0. The magnitude of the reduction increases as ϕ increases, and also as Pes increases. This decrease of D in the presence of suspended macroscopic particles is analyzed in the light of theoretical, numerical and experimental results from other authors that studied suspension flow in similar geometries.Fil: Roht, Yanina Lucrecia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Boschan, Alejandro. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Reversible and Irreversible Tracer Dispersion in an Oscillating Flow Inside a Model Rough Fracture

We study the mixing dynamics of a dyed and a clear miscible fluid by an oscillating flow inside an Hele-Shaw cell with randomly distributed circular obstacles. A transparent setup allows us to analyze the distribution of the two fluids and the reversible and irreversible mixing components. At the lower Péclet numbers Pe (based on the averaged absolute fluid velocity), geometrical dispersion due to the disordered flow field between the obstacles is dominant: the corresponding dispersivity is constant with Pe and, at constant Pe, increases with the amplitude of the oscillations and is negligible at small ones. Compared to echo dispersion with only one injection–suction cycle, oscillating flows are shown to provide additional information when the number of oscillations and, as a result, the distance of transverse mixing are varied. Geometrical dispersion is dominant up to a limiting Pe increasing with the amplitude. At higher Pe′s, the results are similar to those of Taylor dispersion in cells with smooth walls.Fil: Roht, Yanina Lucrecia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Hulin, Jean Pierre. Université Paris Sud; FranciaFil: Auradou, Harold. Université Paris Sud; FranciaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Characterization of wet granular avalanches in controlled relative humidity conditions

This work focuses on the influence of the relative humidity (essentially due to atmospheric conditions) on themany granular media behaviours. To this end, the experimental evolution of the avalanche characteristic anglesof continually tilted granular packing was studied for a wide range of relative humidities in very well controlledconditions (between 5 and 90%). The stability angles were measured for fully developed avalanches. The relationshipbetween the relative humidity (ϕ) and cohesion of granular media (directly related to cohesion forcesbetween grains) was then established to identify the different cohesive states of a wet granular medium usinga reliable and reproducible testing methodology. Finally, a relationship between the hygroscopic equilibriumtime and the stability of the granular packing is discussed.Fil: Gomez Arriarán, I.. Universidad del País Vasco; EspañaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Chertcoff, Ricardo Héctor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Odriozola Maritorena, M.. Universidad del País Vasco; EspañaFil: Schant, R. de. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentin

Time dependence and local structure of tracer dispersion in oscillating liquid Hele-Shaw flows

Passive tracer dispersion in oscillating Poiseuille liquid flows of zero net velocity is studied experimentally in a Hele-Shaw cell and numerically by 2D simulations: this study is particularly focused on the time dependence and local properties of the dispersion. The dispersion mechanism is found to be controlled by the ratio τm/T of the molecular diffusion time across the gap and the oscillation period (when molecular diffusion parallel to the flow is negligible). The 2D numerical simulations complement the experiments by providing the local concentration c(x,z,t) at a given distance z from the cell walls (instead of only the average over z). Above a time lapse scaling like τm, the variation of c with the distance x along the flow becomes a Gaussian of width constant with z while the mean distance ¯x may depend both on z and t. For τm/T . 2, the front spreads through Taylor-like dispersion and the normalized dispersivity scales as τm/T. The front oscillates parallel to the flow with an amplitude constant across the gap; its width increases monotonically at a rate modulated at twice the flow frequency, due to variations of the instantaneous dispersivity. For τm/T & 20, the molecular diffusion distance during a period of the flow is smaller than the gap and the normalized dispersivity scales as (τm/T)−1 . The oscillations of the different points of the front follow the local fluid velocity: this produces a reversible modulation of the global front width at twice the flow frequency and in quadrature with that in the Taylor-like regime.Fil: Roht, Yanina Lucrecia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Auradou, H.. Université Paris Sud; FranciaFil: Hulin, J. P.. Université Paris Sud; FranciaFil: Salin, D.. Université Paris Sud; FranciaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Transport miscible de solutés dans différentes fractures modèles : influence de la rugosité aléatoire ou multiéchelles

Miscible tracer dispersion measurements in transparent model fractures with different types of wall roughness are reported. The nature (Fickian or not) of dispersion is determined by studying variations of the mixing front as a function of the distance travelled but also as a function of the lateral scale over which the tracer concentration is averaged. The dominant hydrodynamic dispersion mechanisms (velocity profile in the gap, velocity variations in the fracture plane) are established by comparing measurements using Newtonian and shear thinning fluids. For small monodisperse rugosities, front spreading is diffusive with a dominant geometrical dispersion (dispersion coefficient D ∝ Pe or constant dispersivity ld = D/U) at low Péclet numbers Pe; at higher Pe values, one has either ld ∝ Pe (i.e. Taylor dispersion) for obstacles of height smaller than the gap, or ld ∝ Pe0.35 for obstacles bridging the gap. For a self-affine multiscale roughness like in actual rocks and a relative shear displacement over(δ, →) of complementary walls, the aperture field is channelized in the direction perpendicular to over(δ, →). For a mean velocity over(U, →) parallel to the channels, the global front geometry reflects the velocity contrast between them and is predicted from the aperture field. For over(U, →) perpendicular to the channels, global front spreading is much reduced. Local spreading of the front thickness remains mostly controlled by Taylor dispersion except in the case of a very strong channelization parallel to over(U, →).Fil: Auradou, Harold. Université Pierre et Marie Curie; Francia. Université Paris Sud; FranciaFil: Boschan, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Université Pierre et Marie Curie; Francia. Université Paris Sud; Francia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires; ArgentinaFil: D'angelo, María Verónica. Université Pierre et Marie Curie; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Université Paris Sud; Francia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Hulin, Jean-Pierre. Université Pierre et Marie Curie; Francia. Université Paris Sud; FranciaFil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Volume fraction instability in an oscillating non-Brownian iso-dense suspension

The instability of an iso-dense non-Brownian suspension of polystyrene beads of diameter 40 μm dispersed in a water-glycerol mixture submitted to a periodic square wave oscillating flow in a Hele-Shaw cell is studied experimentally. The instability gives rise to stationary bead concentration waves transverse to the flow. It has been observed for average particle volume fractions between 0.25 and 0.4, for periods of the square wave flow variation between 0.4 and 10 s and in finite intervals of the amplitude of the fluid displacement. The study shows that the wavelength λ increases roughly linearly with the amplitude of the oscillatory flow; on the other hand, λ is independent of the particle concentration and of the period of oscillation of the flow although the minimum threshold amplitude for observing the instability increases with the period.Fil: Roht, Yanina Lucrecia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Gauthier, Catherine. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Hulin, J. P.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Salin, D.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Chertcoff, Ricardo Héctor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; ArgentinaFil: Auradou, H.. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; FranciaFil: Ippolito, Irene Paula. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentin