Elastically-mediated collective organisation of magnetic microparticles.

Gels are soft elastic materials made of a three-dimensional cross-linked polymer network and featuring both elastic and dissipative responses under external mechanical stimuli. Here we investigate how such gels mediate the organization of embedded magnetic microparticles when driven by an external field. By constructing a continuum theory, we demonstrate that the collective dynamics of the embedded particles result from the delicate balance between magnetic dipole-dipole interactions, thermal fluctuations and elasticity of the polymer network, verified by our experiments.

Stokes layers in oscillatory flows of viscoelastic fluids.

Oscillatory flows of viscoelastic fluids are studied from the perspective of Stokes viscoelastic layers. We identify the governing dimensionless variables, and study the flows in a general way for fluids with linear rheology. Nonlinearities can be treated perturbatively to account for reported flow instabilities.

Spatiotemporal Organization of Correlated Local Activity within Global Avalanches in Slowly Driven Interfaces.

We study the jerky response of slowly driven fronts in disordered media, just above the depinning transition. We focus on how spatially disconnected clusters of internally correlated activity lead to large-scale velocity fluctuations in the form of global avalanches and identify three different ways in which local activity clusters may organize within a global avalanche, depending on the distance to criticality. Our analysis provides new scaling relations between the power-law exponents of the statistical distributions of sizes and durations of local bursts and global avalanches.

Laning, thinning and thickening of sheared colloids in a two-dimensional Taylor-Couette geometry.

We investigate the dynamics and rheological properties of a circular colloidal cluster that is continuously sheared by magnetic and optical torques in a two-dimensional (2D) Taylor-Couette geometry. By varying the two driving fields, we obtain the system flow diagram and report the velocity profiles along the colloidal structure. We then use the inner magnetic trimer as a microrheometer, and observe continuous thinning of all particle layers followed by thickening of the third one above a threshold field.

Experimental study of stable imbibition displacements in a model open fracture. II. Scale-dependent avalanche dynamics.

We report the results of an experimental investigation of the spatiotemporal dynamics of stable imbibition fronts in a disordered medium, in the regime of capillary disorder, for a wide range of experimental conditions. We have used silicone oils of various viscosities μ and nearly identical oil-air surface tension, and forced them to slowly invade a model open fracture at very different flow rates v. In this second part of the study we have carried out a scale-dependent statistical analysis of the front dynamics.

Experimental study of stable imbibition displacements in a model open fracture. I. Local avalanche dynamics.

We report the results of an experimental investigation of the spatiotemporal dynamics of stable imbibition fronts in a disordered medium, in the regime of capillary disorder, for a wide range of experimental conditions. We have used silicone oils of various viscosities μ and nearly identical oil-air surface tension and forced them to slowly invade a model open fracture at different constant flow rates v. In this first part of the study we have focused on the local dynamics at a scale below the size of the quenched disorder.

Disorder-induced capillary bursts control intermittency in slow imbibition.

A multiscale analysis of the spatially averaged velocity of an imbibition front V_{ℓ}(t) measured at scale ℓ reveals that the slow front dynamics is intermittent: the distributions of ΔV_{ℓ}(τ)=V_{ℓ}(t+τ)-V_{ℓ}(t) evolve continuously through time scales τ, from heavy-tailed to Gaussian-reached at a time lag τ_{c} set by the extent of the medium heterogeneities. Intermittency results from capillary bursts triggered from the smallest scale of the disorder up to the scale ℓ_{c} at which viscous dissipation becomes dominant. The effective number of degrees of freedom of the front ℓ/ℓ_{c} controls its intensity.

Capillary rise in Hele-Shaw models of disordered media.

We study the capillary rise of a viscous liquid in large Hele-Shaw models of disordered media, both analytically and experimentally. Compared to the Fries-Dreyer and Lucas-Washburn solutions for capillary rise with and without gravity, our experimental data reveal a systematic deviation at short and intermediate times. The original pressure balance equation leading to Washburn's results is reformulated in order to include an additional resisting term, proportional to the mean velocity of the front h˙, which appears naturally as a result of the geometry of the cell.

Roughness and intermittent dynamics of imbibition fronts due to capillary and permeability disorder.

We follow the propagation of an air-liquid interface during forced-flow imbibition of a viscous wetting liquid by a random medium, using a high resolution fast camera. Our model disordered medium mimics an open fracture by a Hele-Shaw cell with a two-valued gap spacing randomly distributed in the fracture (or Hele-Shaw) plane. By systematically varying the imposed flow rate we achieve average imbibition front velocities in the range 0.

Avalanches and non-Gaussian fluctuations of the global velocity of imbibition fronts.

We present an experimental study of the global velocity V(t) of a viscous fluid interface during forced-flow imbibition in a disordered medium. Our high resolution setup shows that the fronts display an intermittent behavior signature of a burstlike dynamics, with power-law distributed avalanches. When measured at scales comparable to the correlation length, velocity fluctuations follow an asymmetric non-Gaussian distribution, whose skewness increases with decreasing measuring window and/or injection flow rate, offering the effective number of degrees of freedom probed in our experiment.

Anomalous roughening in experiments of interfaces in Hele-Shaw flows with strong quenched disorder.

We report experimental evidence of anomalous kinetic roughening in the stable displacement of an oil-air interface in a Hele-Shaw cell with strong quenched disorder. The disorder consists of a random modulation of the gap spacing that is transverse to the growth direction (tracks). Experiments were performed by varying the average interface velocity and the gap spacing, and measuring the scaling exponents.

Experiments of interfacial roughening in Hele-Shaw flows with weak quenched disorder.

We have studied the kinetic roughening of an oil-air interface in a forced imbibition experiment in a horizontal Hele-Shaw cell with quenched disorder. Different disorder configurations, characterized by their persistence length in the direction of growth, have been explored by varying the average interface velocity v and the gap spacing b. Through the analysis of the rms width as a function of time, we have measured a growth exponent beta approximately 0.