A way forward for high throughput fabrication of microcapsules with uniform size and mechanical properties was reported irrespective of the kinetic process of shell assembly. Microcapsules were produced using lab-scale emulsification equipment with a micro-engineered membrane in the size range 10–100 m. The shell of the microcapsules was assembled at the water-oil interface by complexation of polyelectrolytes or cross-linking of proteins providing two different kinetic processes. Elasticity of microcapsules was characterized with an automated extensional flow chamber. Process parameters were optimized to obtain suspensions with size variations of 15%. Some strategies were developed to obtain uniform elastic properties according to the kinetics of shell assembly. If kinetics is limited by diffusion, membrane emulsification and shell assembly have to be split into two steps. If kinetics is limited by the quantity of reactants encapsulated in the droplet, variations of elastic properties result only from size variations.

Whey protein isolates (WPI) can be aggregated upon heating to create new functional properties (e.g. texture), which depend on aggregate size and structural properties. In industrial conditions, aggregates are obtained in continuous processes at high temperature (>= 75 degrees C) in few minutes. When studying the kinetics of WPI aggregation at high temperature and under flow, one major issue is to develop a process in which heat transfer does not limit aggregation. To this end, we used a down-scaling approach in which a WPI solution flows in a heated capillary tube. We show that this process makes it possible to study both the kinetics of aggregation after few seconds and its dependence with the mean shear rate in isothermal conditions. The size and mass of aggregates and protein conformation were characterized by small-angle X-ray scattering and resonant mass measurement for a single physico-chemical condition (pH 7.0, 10 mM NaCl, 92 degrees C, 4% w/w WPI) which led to sub-micrometric aggregates. Firstly, we report that the size of aggregates were three times larger than when produced in a test tube. Secondly, the size and mass of aggregates reached a steady-state value in a few seconds, whereas the kinetics of aggregation and denaturation had a characteristic time of few minutes. Thirdly, the shear rate had no significant effect on the size of the aggregates, or on the aggregation kinetics. We concluded that WPI aggregation at 92 degrees C is limited by a step of nucleation, and that the fact that aggregates produced in test tube were smaller is due to a slower thermalization.

A foam film, free to move and stabilized with tetradecyltrimethylammonium bromide or sodium dodecylsulfate surfactants, is deposited inside of a cylindrical tube. It separates the tube into two distinct gaseous compartments. The first compartment is filled with air, while the second one contains a mixture of air and perfluorohexane vapor (C6F14), which is a barely water-soluble fluorinated compound. This foam film thus acts as a liquid semipermeable membrane for gases equivalent to the solid semipermeable membranes conventionally used in fluid separation processes. To infer the rate of air transfer through the membrane, we measure the displacement of the mobile foam film. From this, we deduce the instantaneous permeability of the membrane. In contrast to the permeability of solid membranes, which inexorably decreases over time because they become clogged, an anticlogging effect is observed with a permeability that systematically increases over time. Because the thickness of the film is constant over time, we attribute this to the possibility of adsorbing or desorbing fluorinated gas molecules on the liquid membrane. Indeed, because the partial pressure of the fluorinated gas is high at the beginning of the experiment, the density of the adsorbed molecules is also high, which leads to a low permeability to air transfer. On the contrary, at the end of the experiment, the partial pressure in fluorinated gas and thus the density of the adsorbed molecules are low. This leads to a higher permeability and a less clogged membrane.

Model proppant transport experiments are conducted at the laboratory scale using a Newtonian carrier fluid in a long tube of rectangular cross section. Under the particular flow conditions studied, we observe the buildup of a dense but flowing sediment, which rapidly reaches a steady-state height. The existence of this steady-state flowing sediment implies that the proppant flux leaving the channel equals that entering the channel; that is, \\\"efficient\\\" proppant transport occurs. As soon as the suspension flow is stopped, the fluidized sediment ceases flowing and quickly becomes more compact. This collapse implies that the particle sediment is maintained in an expanded state while under flow, with an average volume fraction considerably lower than that under static conditions. The relevant mechanism of sediment transport is identified as viscous resuspension because the flow is at a low Reynolds number (Re at approximately 0.1). We estimate the average volume fraction of the resuspended sediment from experimental measurements of the \\\"expanded\\\" flowing sediment height, with the assumption that the corresponding compact sediment volume fraction is phi(0) = 0.61, the volume fraction at which the suspension viscosity diverges. Predictions of the resuspended sediment heights are made with a simple approach based on the diffusive flux model by Leighton and Acrivos (1986) using the average shear stress across the channel width. A good agreement is found between the predicted and experimental values, indicating that 2D effects remain weak. Microscopic observations show that the sediment is fully fluidized while under flow for all the flow rates studied in our channel, and one does not observe the buildup of static sediment banks that are observed in larger-scale tests during the suspension flow (Kern et al. 1959; Babcock et al. 1967; Schols and Visser 1974; Sievert et al. 1981). This apparent difference is explained in the context of the viscous resuspension model.

Flow of foams is studied in a model porous medium, in a large range of capillary numbers Ca and relative gas flow rates f(g). From pressure measurements, we find that the effective viscosity is a decreasing power-law function of Ca, with the exponent ranging from -1 to -0.75. Direct observation reveals that the flow is heterogeneous. The fraction of preferential paths increases with both f(g) and Ca. In a straight channel of varying cross section, a bubble train behaves as a shear-thinning yield stress fluid. This feature accounts quantitatively for the effective viscosity in the micromodel.

Discrete granular models are a natural choice when simulating dense suspensions, when the small distances between the particles lead to dominant contributions by lubrication and contact forces. In such case one can get rid of the costly resolution of Navier-Stokes equations, using closed form expressions for lubrication terms. However, those terms diverge when two hard spheres approach contact, and there are issues when integrating them directly with the finite precision of floating point calculations. In this paper, we introduce a visco-elasto-plastic interaction model for suspended spheres, which combines lubrication and elastic-frictional contact behaviour depending on surface roughness. An integration scheme is proposed for that model. Unlike earlier methods. the scheme enables an unconditionally stable time-integration of the interactions. The case of perfectly smooth spheres (null roughness), namely, is integrated correctly. The theoretical results are well reproduced in benchmark tests on two-sphere systems: one sphere sedimenting on one other and two spheres in a shear flow. From these benchmark tests, we propose phase diagrams showing the interplay between viscosity, roughness and stiffness. The second test case highlights the origin of non-reversibility particle trajectories. It is controlled by the particle roughness for rigid particles, and by the particle deformation when the capillary number is higher than the relative roughness.

We use X-ray imaging to study viscous resuspension. In a Taylor-Couette geometry, we shear an initially settled layer of spherical glass particles immersed in a Newtonian fluid and measure the local volume fraction profiles. In this configuration, the steady-state profiles are simply related to the normal viscosity defined in the framework of the suspension balance model. These experiments allow us to examine this fundamental quantity over a wide range of volume fractions, in particular, in the semidilute regime where experimental data are sorely lacking. Our measurements strongly suggest that the particle stress is quadratic with respect to the volume fraction in the dilute limit. Strikingly, they also reveal a nonlinear dependence on the Shields number, in contrast with previous theoretical and experimental results. This likely points to shear-thinning particle stresses and to a non-Coulomb or velocity-weakening friction between the particles, as also evidenced from shear reversal experiments.

The rheology of dense suspensions is studied by discrete-element method simulation, focusing on the interplay of the solid fraction, confining pressure, shear rate, and viscosity. Using a minimal model based on lubrication and contact forces, we are able to recover experimental results available in the literature, in a very large range of solid fractions. We show that bulk friction is only weakly dependent on contact friction when a normalized shear rate, the so-called viscous number I-v is kept constant. In contrast, contact friction has a strong influence on the jamming solid fraction (empty set)(m). We provide empirical proof that all the rheology could be accounted for using I-v and phi/phi(m). By separating the contributions of lubrication and contact forces on the total shear stress, it is shown that contacts dominate at a solid fraction above 0.77 of jamming solid fraction. Universal expressions of macroscopic friction and solid fraction as functions of the viscous number are finally offered.

Particles migrate in the transverse direction of the flow due to the existence of normal stress anisotropy in weakly viscoelastic liquids. We test the ability of theoretical predictions to predict the transverse velocity migration of particles in a confined Poiseuille flow according to the viscoelastic constitutive parameters of dilute polymer solutions. First, we carefully characterize the viscoelastic properties of two families of dilute polymer solutions at various concentrations using shear rheometry and capillary breakup experiments. Second, we develop a specific three-dimensional particle tracking velocimetry method to measure with a high accuracy the dynamics of particles focusing in flow for Weissenberg numbers Wi ranging from 10−2 to 10−1 and particle confinement β of 0.1 and 0.2. The results show unambiguously that the migration velocity scales as Wiβ2, as expected theoretically for weakly elastic flows of an Oldroyd-B liquid. We conclude that classic constitutive viscoelastic laws are relevant to predict particle migration in dilute polymer solutions whereas detailed analysis of our results reveals that theoretical models overestimate by a few tenths the efficiency of particle focusing.

The instability of an electrolyte surface to a high-frequency, 10 to 200kHz, electric field, normal to the interface is investigated theoretically. From a practical viewpoint, such a high frequency leads to the absence of undesired electrochemical reactions and provides an additional control parameter. The theory of unsteady electric double layer by Barrero and Ramos is exploited. At such a high frequency, which is much larger than the eigenfrequency of the mechanical system, the nonlinear mechanical term does not feel the fast part of the Coulomb force, but it feels its slower component. In fact, the system behaves as if the electric field were a DC field. The observed instability is qualitatively close to the Tonks-Frenkel instability. The problem of the linear stability of the 1D quiescent stationary solution is solved analytically. For the important limiting cases, simple analytical formulas are derived. The linear stability analysis is complemented by the DNS of the full nonlinear system of equations with broadband low-amplitude white-noise initial conditions. After a transition period, the linear instability mechanism filters out the broad spectrum except for a narrow band near the maximum growth rate in rather good agreement with the linear stability analysis. If the external field is large enough, the nonlinear evolution results in coherent structures with sharp tips resembling to a Taylor cone. An evaluation of the cone angle for different conditions gives its value of about 30(degrees) to 60(degrees) , which is smaller than the angle of 98.6(degrees) for DC field and qualitatively corresponds to the experiments (L.Y. Yeo et al., Phys. Rev. Lett. 92, 133902 (2004)) for the high-frequency AC field and to the theoretical evaluation of the AC Taylor cones in E.A. Demekhin et al., Phys. Rev. E 84, 035301(R) (2011).

We extend a previously developed ultrafast ultrasonic technique [T. Gallot et al., Rev. Sci. Instrum. 84, 045107 ( 2013)] to concentration-field measurements in non-Brownian particle suspensions under shear. The technique provides access to time-resolved concentration maps within the gap of a Taylor-Couette cell simultaneously to local velocity measurements and standard rheological characterization. Benchmark experiments in homogeneous particle suspensions are used to calibrate the system. We then image heterogeneous concentration fields that result from centrifugation effects, from the classical Taylor-Couette instability, and from sedimentation or shear-induced resuspension.

We study the repartition of monodisperse bubbles at the inlet node of an asymmetric microfluidic loop for low to high bubble densities. In large loops, we evidence a new regime. Contrary to the classical belief, we point out that bubbles are directed not towards the arm having the higher total flow rate but towards the arm with the higher water flow rate at low and moderate relative gas flow rates. At higher rates, they enter the longer arm when they reach close packing in the shorter arm. In small loops, we evidence a clogging regime at high relative gas flow rates. Collisions between bubbles coming from the two arms at the outlet clog the longer arm. We propose a comprehensive analysis allowing us to explain these results.

We study flows of hydrolized polyacrylamide solutions in two dimensional porous media made using microfluidics, for which elastic effects are dominant. We focus on semi-dilute solutions (0.1%-0.4%) which exhibit a strong shear thinning behavior. We systematically measure the pressure drop and find that the effective permeability is dramatically higher than predicted when the Weissenberg number is greater than about 10. Observations of the streamlines of the flow reveal that this effect coincides with the onset of elastic instabilities. Moreover, and importantly for applications, we show using local measurements that the mean flow is modified: it appears to be more uniform at high Weissenberg number than for Newtonian fluids. These observations are compared and discussed using pore network simulations, which account for the effect of disorder and shear thinning on the flow properties. Published by AIP Publishing.

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.

We explore the flow of highly shear thinning polymer solutions in straight geometry. The strong variations of the normal forces close to the wall give rise to an elastic instability. We evidence a periodic motion close the onset of the instability, which then evolves towards a turbulentlike flow at higher flow rates. Strikingly, we point out that this instability induces genuine drag reduction due to the homogenization of the viscosity profile by the turbulent flow.

We report the development and analysis of a velocimetry technique based on the short time displacement of molecular tracers, tagged thanks to photobleaching. We use confocal microscopy to achieve a good resolution transverse to the observation field in the direction of the velocity gradient. The intensity profiles are fitted by an approximate analytical model which accounts for hydrodynamic dispersion, and allow access to the local velocity. The method is validated using pressure driven flow in microfluidic slits having a thickness of a few tens of micrometers. We discuss the main drawbacks of this technique which is an overestimation of the velocity close to the walls due to the combination of molecular diffusion and shear. We demonstrate that this error, limited to a near wall region of a few micrometers thick, could be controlled by limiting the diffusion of fluorophore molecules or minimizing the bleaching time. The presented technique could be combined with standard particle imaging velocimetry to access velocity differences and allow particle trajectory analysis in microflows of suspensions.

We investigate pressure-driven motion of liquid-liquid menisci in circular tubes, for systems in pseudopartial wetting conditions. The originality of this type of wetting lies in the coexistence of a macroscopic contact angle with a wetting liquid film covering the solid surface. Focusing on small capillary numbers, we report observations of an apparent contact angle hysteresis at first sight similar to the standard partial wetting case. However, this apparent hysteresis exhibits original features. We observe very long transient regimes before steady state, up to several hundreds of seconds. Furthermore, in steady state, the velocities are nonzero, meaning that the contact line is not strongly pinned to the surface defects, but are very small. The velocity of the contact line tends to vanish near the equilibrium contact angle. These observations are consistent with the thermally activated depinning theory that has been proposed to describe partial wetting systems on disordered substrates and suggest that a single physical mechanism controls both the hysteresis (or the pinning) and the motion of the contact line. The proposed analysis leads to the conclusion that the depinning activated energy is lower with pseudopartial wetting systems than with partial wetting ones, allowing the direct observation of the thermally activated motion of the contact line.

We report experimental and numerical results concerning time-resolved biphasic flows in 2D model porous media involving polymer solutions. We focus on the case where a more viscous but non-Newtonian fluid displaces a wetting fluid. Similar to the Newtonian case, a transition from capillary fingering to a stable invading front is observed when the capillary number is increased. However, our results show that this transition is sharpened because of the shear-thinning behavior of the polymer solutions. At a given capillary number, the width of the invading front and correlatively the residual saturation are greater for a shear-thinning fluid than for a Newtonian one. Furthermore, we also find that the partially hydrolyzed polyacrylamide solutions investigated exhibit a rather strong slippage at low flow rates, which leads to even greater fingering. Experiments conducted in microfluidic micromodels are in quantitative agreement with time dependent non-Newtonian pore-network simulations. All of these effects are well captured by a simple model that leads to quantitative predictions of the drainage by shear-thinning fluids with slip boundary conditions.

Pressure-driven flows of high molecular weight polyacrylamide solutions are examined in nanoslits using fluorescence photobleaching. The effective viscosity of polymer solutions decreases when the channel height decreases below the micron scale. In addition, the apparent slippage of the solutions is characterized macroscopically on similar surfaces. Though slippage can explain qualitatively the effective viscosity reduction, a quantitative comparison shows that the slip length is greatly reduced below the micron scale. This result indicates that chain migration is suppressed in confined geometries.

This work reports on an experimental study of elastic turbulence in a semi-dilute wormlike micelle system made of a highly elastic betaine surfactant solution. The temporal evolution of both rheological quantities and local flow properties is monitored by combining global rheology, optical visualization, and ultrasonic velocimetry. Even at the smallest applied shear rates or shear stresses, we find that the micellar sample develops large Weissenberg (Wi) numbers, leading the flow to undergo a transition to elastic turbulence. Three-dimensional flows are indeed observed all along the flow curve, which therefore cannot be interpreted in the framework of classical shear banding. Strong fluctuations are also recorded in the rheological quantities, in the reflected light intensity, and in velocity profiles. We show that the power spectral densities (PSDs) of these fluctuations display power law behaviours with exponents ranging from −1 to −3 depending on the applied shear stress or shear rate. The exponents inferred from local velocity measurements are found to be spatially dependent, pointing to inhomogeneous turbulence. The nature of the instability and of the transition to elastic turbulence is further discussed in light of recent experimental and theoretical works on wormlike micelles and polymers.

We present studies of axisymmetric drainage in two-dimensional micromodels of porous media using up to date microfabricationand image analysis methods. Drainage of model oil by aqueoussolutions is studied at low capillary numbers (Ca) typically encountered during oil recovery operations. We use two types of oil-wetmicromodels: oneis basedonadepositionmethodforcreatingarandommonolayerofmicronic glass beads, while the other is made using computer generated random networks etched in glass using wet-lithography. Both models have a central injection scheme and a radial geometry, resulting in a continuous variation of the capillary number during the course of drainage. We first carry out an analysis of experiments at global micromodel scale using computer based image analysis coupled with flow rates and pressure drop measurements. Basic relevant parameters such as permeability, porosity of the micromodel and residual oil in place after waterflooding are extracted. We then take advantage of the ease of observation in transparent micromodels to investigate the drainage phenomenon at local scale. Local saturation and front width are measured as a function of the local capillary number. Interestingly, because of the radial flow geometry, our experiments allow a continuous plotting of the micromodels capillary desaturation curve on several decades. As expected but never precisely observed, all points of various experiments collapse on a single capillary desaturation curve for a given micromodel. However, we observe dissimilar behaviors between the two types of micromodels. We discuss this phenomenon in light of the pore scale geometrical differences between the two models.

We study the dynamics of liquid–liquid menisci in a circular tube at small capillary numbers in partial, pseudo-partial and complete wetting conditions. There exists a thin wetting film in the last two situations. By pseudo-partial wetting, we refer to systems having non-monotonic disjoining pressures, as described by Brochard-Wyart et al. (1991) [3]. In this situation, the disjoining pressure allows the coexistence of a microscopic thin film (but not molecular) and a macroscopic contact angle. We measure the meniscus velocity as a function of the pressure drop for different wetting conditions. In pseudo-partial wetting condition, we observe that there is a pressure range where the velocity is extremely low (below 1 μm/s) but non zero, in contrast with partial wetting, where the meniscus is blocked by a pinned contact line corresponding to the standard contact angle hysteresis. The role of quasi-equilibrium and of surface heterogeneity is discussed to explain the observations.

We investigate the drying kinetics of a sessile droplet containing nanoparticles. Using a fast two-color confocal imaging technique, we probe the concentration field of nanoparticles along with the velocity and mobility fields of microparticles that act as flow tracers. The flow patterns evolve according to a complex kinetics which is related to recirculating Marangoni flow coupled to the time-evolving rheology of the suspension. The drop is roughly divided in two parts: a fluid zone in which persistent recirculations flow always in the same direction, and a gelled part at the level of the corner which grows with time and also creeps under the capillary pressure.

Velocity measurement is a key issue when studying flows below the micron scale, due to the lack of sensitivity of conventional detection techniques. We present an approach based on fluorescence photobleaching to evaluate flow velocity at the nanoscale by direct visualization. Solutions containing a fluorescent dye are injected into nanoslits. A photobleached line, created through laser beam illumination, moves through the channel due to the fluid flow. The velocity and effective diffusion coefficient are calculated from the temporal data of the line position and width respectively. The measurable velocity range is only limited by the diffusion rate of the fluorescent dye for low velocities and by the apparition of Taylor dispersion for high velocities. By controlling the pressure drop and measuring the velocity, we determine the fluid viscosity. The photobleached line spreads in time due to molecular diffusion and Taylor hydrodynamic dispersion. By taking into account the finite spatial and temporal extensions of the bleaching under flow, we determine the effective diffusion coefficient, which we find to be in good agreement with the expression of the two dimensional Taylor–Aris dispersion coefficient. Finally we analyze and discuss the role of the finite width of the rectangular slit on hydrodynamic dispersion.

We present an experimental study of drainage in two-dimensional porous media exhibiting bimodal pore size distributions. The role of the pore size heterogeneity is investigated by measuring separately the desaturation curves of the two pore populations. The displaced wetting fluid remains trapped in small pores at low capillary numbers and is swept only above a critical capillary number proportional to the permeability of the big pores network. Based on this observation, we derive a simple criterion for phase trapping based on the balance of viscous to capillary forces. Numerical implementation of this theory in a pore network model quantitatively fits our experimental results. This combination of approaches demonstrates quantitatively the influence of geometrical heterogeneities on drainage in porous media.

This paper presents some experimental results on two-phase flows in model two-dimensional (2D) porous media with different wetting properties. Standard microfluidic techniques are used to fabricate the 2D micromodels that consist of a network of straight microchannels having heterogeneous sizes. The invasion mechanism is analyzed quantitatively for partial and total wetting conditions, and for various stable viscosity ratios and capillary pressure heterogeneity. For capillary numbers ranging from 10−7 to 10−2, we observe a transition between capillary fingering and a stable front. The capillary fingering regime exhibits differences between partial and complete wetting systems: The front width in complete wetting is larger. Simple models are proposed to account for these regimes and indicate that the differences between the systems are likely to be due to the flow of the displaced fluid in the complete wetting situation.

This paper reports some experimental results on two-phase flows in model two-dimensional porous media. Standard microfluidic techniques are used to fabricate networks of straight microchannels and to control the throat size distribution. We analyze both the invasion mechanism of the medium by a nonwetting fluid and the drainage after the percolation for capillary numbers lying between 10−7 and 10−2. We propose a crude model allowing a description of the observed capillary fingering that captures its scaling properties. This model is supported by numerical simulations based on a pore-network model. Numerical simulations and experiments agree quantatively.

We present an experimental investigation of the drying kinetics seen from inside a sessile droplet laden with a colloidal sol of silica nanoparticles. We use fast, two-color confocal microscopy imaging to quantitatively extract on the one hand the concentration field of the rhodamine-tagged nanosol and on the other hand the velocity field and the mobility field of large, fluorescein-tagged tracers. By changing the initial concentration at which the drop dries up, we propose a method that yields a self-consistent way to obtain the rheology of the sol. Based on these results, we analyse the drying kinetics in terms i) of flow patterns that include evaporating and Marangoni flows which compete to determine the final concentration profile and ii) of truncated dynamics that we quantitatively relate to the rheology of the sol.

Pattern formation from a silica colloidal suspension that is evaporating has been studied when a movement is imposed to the contact line. This article focuses on the stick−slip regime observed for very low contact line velocities. A capillary rise experiment has been specially designed for the observation and allows us to measure the pinning force that increases during the pinning of the contact line on the growing deposit. We report systematic measurements of this pinning force and derive scaling laws when the velocity of the contact line, the colloid concentration, and the evaporation rate are varied. Our analysis supports the idea that the pinning of the contact line results from a competition between the geometry of the growing deposit and the force due to gravity.

Drying experiments with a receding contact line have been performed with silica colloidal suspensions and polyacrylamide (PAAm) polymer solutions. The experimental setup allows to control the receding movement of the contact line and the evaporation flux separately. Deposit thickness as a function of these two control parameters has been investigated. The different systems exhibit a similar behavior: in the regime of very low capillary numbers the deposit thickness scaled by the solute volume concentration and the evaporation rate is proportional to the inverse of the contact line velocity. Both the scaling exponent and the constant (which has the dimension of a length) do not depend on the system under study. The observation of this evaporative regime confirms some recent results obtained by Le Berre et al. on a very different system (phospholipidic molecules) and fully supports their interpretation. Following their approach, a simple model based on mass balance accounts for these results. This implies that this regime is dominated by the evaporation and that the deformation of the meniscus induced by viscous forces does not play any significant role. When increasing the velocity, another regime is observed where the thickness does not depend significantly on the velocity.

Receding contact lines of colloidal suspensions are studied in the presence of drying, inside Hele-Shaw cells. At high velocity the contact line movement is continuous and the particle deposition is uniform. At small velocity, a periodic pinning-unpinning of the contact line is observed leading to a patterning of the substrate. We focused on the correlation between the deposition pattern and the pinning force that grows during the pinning. Our results strongly indicate that this pinning force is proportional to the macroscopic slope of the deposit and accounted by a simple capillary balance.

Gravimetry experiments in a well-controlled environment have been performed to investigate aging for a glassy PMMA/toluene film. The temperature is constant and the control parameter is the solvent vapor pressure above the film (i.e. the activity). Several experimental protocols have been used, starting from a high activity where the film is swollen and rubbery and then aging the film at different activities below the glass transition. Desorption and resorption curves have been compared for the different protocols, in particular in terms of the softening time, i.e. the time needed by the sample to recover an equilibrium state at high activity. Non-trivial behaviors have been observed, especially at small activities (deep quench). A model is proposed, extending the Leibler-Sekimoto approach to take into account the structural relaxation in the glassy state, using the Tool formalism. This model well captures some of the observed phenomena, but fails in describing the specific kinetics observed when aging is followed by a short but deep quench.

Within the framework of convection induced by evaporation, an experimental study of the drying of a polymer solution has been performed. Several visualisations of convective pattern development are presented: top view with a video camera and an IR camera, as well as visualisation in a vertical section. The appearance and origin (buoyancy- and/or surface tension-driven instabilities) of the convective structures are analysed as a function of the initial thickness and viscosity. Different regimes are obtained when comparing the lifetime of the convective patterns to the thermal transient regime induced by evaporation and to the formation of a thin viscous skin at the surface.

It has been known for 40 years that the state of a glass cannot be characterized by a single parameter such as its density and depends on its whole thermal history in a complex way. This phenomenon, known as the memory effect, reveals that the spatial distribution of the dynamics in a glass is deeply heterogeneous. Among the various memory effects, we will focus on the following one. After an annealing at a temperature T, a glassy system will exhibit a specific signature of susceptibility—around the temperature T of annealing. This effect shows that relaxations during annealing occur only in some domains of specific dynamics. This has been observed in a variety of glasses, with different techniques. The memory effect, here, is observed for the first time through the dynamical elastic modulus. We show that the dynamical elastic modulus can be described by the simple phenomenological so-called Tool–Narayanaswamy–Moynhian (TNM) model. We evidence the competition between plastic deformation and annealing by applying cyclic strain during the annealing. As a result, we establish that deformation exhibits an effect that is opposite to thermal annealing and less selective in temperature.

The dewetting of ultrathin polystyrene films on a liquid substrate is studied in the vicinity of the glass transition. This technique leads to the measurement of the extensional creep compliance of the film. The results reported in this article show that the rubbery plateau value of the compliance is unchanged for films as thin as 20 nm. Thus, we conclude that the entanglement density is unaffected by the surface at a scale of the coil size. These results are discussed in the context of previous results that report the reduction of the viscosity in the same films. Furthermore, the creep compliance of ultrathin films in the segmental relaxation regime exhibits a small reduction of the characteristic times. For high molecular weight thin films, the time scale reduction in the transition zone is much weaker than the reduction of the terminal relaxation time. This observation shows that the different parts of the time spectrum are not equally sensitive to the confinement. This is consistent with expected effects of the confinement at different length scales.

The dewetting of thin polystyrene films (20–500 nm) on a liquid substrate is studied at time scales that are long compared to the reptation time. It is shown that the kinetics correspond to those of purely viscous flow and that the viscosity measured by this technique is, for the thickest films, consistent with bulk measurements. Films on the order of the coil size are then studied. The effective viscosity of these films displays a large decrease when the film thickness h is below several radius of gyration, Rg. This viscosity reduction is found to depend only on the ratio h/Rg.

The Dewetting of thin polymer films (60–300 nm) on a non-wettable liquid substrate has been studied in the vicinity of their glass transition temperature. In our experiment, we observe a global contraction of the film while its thickness remains uniform. We show that, in this case, the strain corresponds to simple extension, and we verify that it is linear with the stress applied by the surface tension. This allows direct measurement of the stress/strain response as a function of time, and thus permits the measurement of an effective compliance of the thin films. It is, however, difficult to obtain a complete viscoelastic characterization, as the short time response is highly dependant on the physical age of the sample. Experimental results underline the effects of residual stress and friction when dewetting is analyzed on rigid substrates.

Tapping mode atomic force microscopy is frequently used to image the surface of soft materials; it is also a powerful technique for nanomechanical analysis of surfaces. We report here an investigation of the depth sensing of the method on soft polymers. The chosen approach is based on the analysis of phase images of a model filled elastomer material. It leads to the determination of the depths of the hard particles lying under the surface. We found that tapping mode can probe interfaces buried under up to 80 nm of polymer. Under given tapping conditions, the penetration depth of the tip into the polymer is observed to depend on the layer thickness. However we show that, for a given penetration depth, the dissipated energy is independent of the thickness of the polymer layer under the tip. This suggests that the phase signal does not originate in the bulk viscoelasticity of the elastomer. Our observations support the hypothesis that, in tapping mode experiments on elastomers, the phase signal has an adhesive origin. Then, on surfaces with uniform interfacial properties, the phase images may reflect the local elastic properties of the sample, since they modify the tip-surface adhesive interactions. ©2004 American Institute of Physics

e study flow properties of high molecular weight polymer solutions below the micron scale. Fluorescence photobleaching is used as a non-invasive technique to evaluate the velocity of pressure-driven flows in channels from 100 to 4000 nm height. We observe a striking reduction of the effective viscosity of polyacrylamide solutiuons in the semi-dilute regime. This effect increases with molecular weight and concentration. Using a Rabinovitch-like approach, we correlate the data at different thicknesses to obtain the wall slip velocity and the flow curve at sub-microscale. Those properties are also evaluated using particle imaging velocimetry close to similar surfaces and standard rheometry. Comparing the measurements in bulk and in confined geometries, we conclude that the observed viscosity reduction can not be solely explained by slippage. We discuss the possible reasons of this effect that are size-dependant filtration and shear-thinning enhancement due to the confinement.

Microfluidics devices are used to study the drainage of a wetting fluid by a non-wetting one in porous media. Both the geometry and the wetting properties are accurately controlled and allow to obtain quantitative measurements of the features of the capillary fingering occuring during the invasion as a function of the imposed flow rate. In partial wetting systems, a quantitative agreement is found between the experimental mean velocities, a simple model based on scaling arguments, and some simulations based on a pore network model. In total wetting systems, the correlation length is higher than predicted in the capillary regime. Furthermore, a very different behavior is observed after the percolation, the invasion process do not stop in total wetting systems whereas the structure of the flow is frozen in partial wetting ones.

The pinning of the three phase contact line due to particle accumulation in an evaporating droplets plays a crucial role in the final shape of the deposit. Depending on the solute type and size, and on the substrate properties, the contact line may be pinned or not, leading (or not) to the shaping of one or several rings. I will try during this talk to summarize reported observations in an attempt to draw a comprehensive picture of the phenomena that lead to pinning or depinning. I will mainly focus on experimental results that involves moving contact lines,since there is in this case a direct competition between pinning and depinning. The scaling properties of the stick-slip phenoma that are observed for moving contact line involve both the local property of the deposit created (chemical nature, geometry, rheology, size of the particle) and the transport properties due to evaporation.

When considering a biphasic flow (typically water/oil) on heterogeneous surfaces, one expect that the contact line could be pinned by the surface heterogeneities and that one phase could be trapped on these heterogeneities, forming droplets or capillary bridges. In this contribution, we address the question of the morphologies of the phases on a controlled patterned surface, as a function of the geometry of the pattern, and of the velocity of the flow. Indeed, it is expected that capillary trapping and contact line pinning disappear at high capillary numbers, for which viscous forces dominate surfaces forces. Using photolithography combined with controlled grafting of a glass substrate, we are able to make surfaces that are topographically homogeneous, but chemically heterogeneous. In this study we focused on parallel stripes of hydrophobic coating (octadecylsilane) on a hydrophilic surface (clean glass). These stripes are designed in a Hele-Shaw cell on both plates facing each other and are perpendicular to the flow direction. At low capillary numbers (Ca < 10-4), we observe trapping of oil due to capillary bridges between the two plates, on the hydrophobic stripes. These bridges are only observed when the stripe width is greater than the cell thickness, which is in good agreement with some theoretical predictions that can be made in the limit of a vanishing capillary number. At higher capillary numbers (Ca>10-2), the movement of the water/oil interface is not perturbed by the hydrophobic stripes. Between these two extreme regimes, the interface is disturbed by the stripe, without any oil trapping. Even though the interface movement is independent of the stripes at high capillary numbers, we observe in this regime the formation of oil droplets on the hydrophobic stripes, after that the interface has moved through the stripe. This issue might be explained by the destabilization of an oil film above the Landau-Levitch wetting transition.

Experimental results of aging behavior for a polymer solution, when the control parameter is the solvent vapor pressure above the film (i.e. the activity) and the observation is the solvent concentration, are presented. Various aging protocols have been performed and analyzed on the system Polymethylmethacrylate (PMMA)/Toluene.

Dynamic properties of ultrathin polymer films have been the focus of many studies during the last decade, but the mechanical characterization of these films suffers from a lack of experimental results. We studied the dewetting of ultrathin polystyrene films that lie onto a liquid substrate. Above the glass transition temperature, a global contraction of the films is observed, while the thickness remains uniform. We have showed [1] that the strain corresponds to a simple extension and that it is linear with the stress applied by the surface tensions. This allows a direct measurement of an effective creep compliance. In particular, the rubbery plateau and the viscosity of the film could be determined. For films thicker than 200 nm, standard bulk values are obtained for both properties. For thinner films, our results show that the viscosity exhibits a great decrease. By varying the molecular weight, we bring evidence that the relevant length scale for this viscosity reduction is the radius of the gyration. Correlatively, the rubbery plateau is found to remains bulk-like down to 20nm. These results underline the complexity of the dynamics in thin polymer films, since the different parts of the time spectrum deviate differently from the bulk. They may also confirm that the entanglement structure is affected by the surface. [1] H. Bodiguel, C.Fretigny, Eur. Phys. J. E 19, 185-193 ( 2006).

L’AFM en mode tapping est utilisé sur des élastomères chargés par des nanoparticules de silice. Par une étude des images de phase, nous déterminons la profondeur des particules enfouies près de la surface. Nos expériences permettent également une étude de la dissipation de l’interaction pointe-surface. A partir d’expériences d’approche-retrait, nous montrons que la dissipation en mode tapping sur des matériaux mous est d’origine adhésive. Enfin, nous proposons une description du contact intermittent sur ces matériaux déformables, fondé sur ces observations et testé sur un caoutchouc modèle.