[show abstract][hide abstract] ABSTRACT: We report evidence for photocontrolled stability and breakage of aqueous foams made from solutions of a cationic azobenzene-containing surfactant over a wide range of concentrations. Exposure to UV or visible lights results in shape and polarity switches in the surfactant molecule, which in turn affects several properties including critical micelle concentration, equilibrium surface tension, and the air-water interfacial composition (cis isomers are displaced by trans ones). We demonstrate that the trans isomer stabilizes foams, whereas the cis isomer forms unstable foams, a property that does not correlate with effects of light on surface tension, nor with total surfactant concentration. Achieving in situ breakage of foam is accordingly ascribed to the remote control of the dynamics of adsorption/desorption of the surfactant, accompanied by gradients of concentrations out of equilibrium. Photomodulation of adsorption kinetics and/or diffusion dynamics on interfaces is reached here by a noninvasive clean trigger, bringing a new tool for the study of foams.
[show abstract][hide abstract] ABSTRACT: We study the adsorption dynamics of an azobenzene-based photo-responsive charged surfactant to investigate how photo-stimulation impacts the dynamics at an air–water interface. The hydrophobic tail of this photo-responsive surfactant photo-converts reversibly from a cis to a trans conformation when the wavelength switches from UV to blue. This change in conformation results in a decrease of the surface tension. Using a kinetically limited model of adsorption, including the electrostatics effects and the competition between the two photo-isomers, we reproduce the dynamics of adsorption of AzoTAB measured experimentally. We find that the cis isomer adsorbs 10 times faster than the trans isomer but the cis conformation also desorbs 300 times faster. As a result, within a few seconds a non-stimulated interface becomes composed of almost 100% trans isomers. We then focus on the competition between the photo-conversion and the adsorption at the interface. Indeed when the interface is stimulated, part of the adsorbed trans isomers rapidly convert to cis. As the latter quickly desorbs, the surface coverage decreases: the light induces a “pumping-out” of the interface. The photo-stimulated interface reaches a stationary state where a vertical gradient of composition is established below the surface. Finally, this study highlights a new way to stimulate a photo-responsive interface: for a solution prepared under blue light, instead of photo-converting the bulk composition by stimulating under UV (which can be quite slow for high absorbance solutions), one can tune and reach a stabilized value of the surface tension in few seconds by stimulating the interface with a blue light with high enough intensity.
[show abstract][hide abstract] ABSTRACT: We study the wetting behaviour of water droplets on three types of thin water-soluble polymeric coatings spin-coated on a silicon wafer—(i) a neutral polymer, (ii) a cationic polyelectrolyte, and (iii) a zwitterionic polymer. We investigate the dynamical spreading of drops as well as their drying on these surfaces. The influence of a salt—added either in the water droplets or inside the coating—is investigated to better differentiate the three types of polymer. We find that the salt tends to promote spreading on charged polymeric coatings when it is added inside the coating, yet not when added in the drops. Salts inside a coating simply increase its surface tension, resulting in lower contact angles during the spreading motion of a drop. On the other hand, when drying starts on the neutral polymer, the drop starts receding (i.e. its radius decreases) immediately after its spreading motion has stopped, whereas for both charged polymers, it remains pinned on the substrate until the very end of drying. When an increasing amount of salt is incorporated inside the two types of charged coatings, the contact line tends to unpin faster. As charges and salts obviously have a strong impact on the pinning/receding of the contact line, we hypothesize that a contribution of osmotic nature drives the motion of the contact line of drying water drops on polymeric coatings. There is a competition between (i) the evaporation from the edge which favours recession and (ii) a flux of water resulting from the osmotic pressure imposed by the higher concentration of species at the edge of the drop. The osmotic pressure being high for charged polymers, water evaporating on a charged coating keeps flowing from the inside of a drop to the outer edge, and compensates for the fast evaporation in the edge: macroscopically, this keeps the apparent contact line at the same position so that it appears pinned as more and more dissolved polymer molecules are dragged toward the edge and accumulate in a ring like deposit. When a salt is added, the osmotic drive weakens and charged polymers eventually dry like neutral ones.
[show abstract][hide abstract] ABSTRACT: Colloids near to the glass concentration are often taken as models for molecular glass formers. Yet, one of the most important aspects of the dynamics of molecular glasses, structural recovery, remains to be examined in colloids. We use DWS to investigate structural recovery in a thermosensitive PNIPAM colloidal suspension in the glass concentration range. The three classical aging signatures observed in molecular glasses: intrinsic isotherms, asymmetry of approach and memory effect, are investigated with this colloid and the results are compared with those typical of molecular glasses. We find: 1 for the intrinsic isotherms, the colloid shows dramatic changes in relaxation time at equilibrium while the times required to reach the equilibrium state are nearly independent of the concentration; 2 for the asymmetry of approach, the observed nonlinearity is similar to that in molecular glasses; 3 for the memory experiment, while the memory effect is seen in the colloid, the response is qualitatively different than in the molecular glass.
[show abstract][hide abstract] ABSTRACT: The wettability of coatings is very sensitive to the amount of solvent they may contain. When a droplet of volatile solvent, such as water, is deposited on a substrate, its vapor may quickly condensate just ahead of the contact line. We give an estimation of the extent of solvent uptake by a coating of variable thickness e , in front of an advancing contact line of given velocity U . Depending on the values of e and U , we observe three regimes: at low velocity and for a thin coating, the coating adsorbs a fraction of solvent that can quickly equilibrate across its entire thickness, so that it mainly appears solvophilic, while this is not the case for a thick coating. For high velocities, regardless the coating thickness, the coating ahead of the contact line does not have enough time to adsorb a significant amount of solvent, so that it mainly appears solvophobic. All these phenomena appear to be controlled by a molecular cut-off length.
The European Physical Journal E 10/2010; 33(3):203-10. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have been able to design model filled rubbers with exactly the same chemical structure but different filler arrangements. From these model systems, we show that the particle arrangement in the elastomeric matrix controls the strain softening at small strain amplitude known as the Payne effect, as well as the elastic modulus dependence on the temperature. More precisely, we observed that the Payne effect disappears and the elastic modulus only weakly depends on the temperature when the particles are well separated. On the contrary, samples with the same interfacial physical chemistry but with aggregated particles show large amplitudes of the Payne effect and their elastic modulus decreases significantly with the temperature. We discuss these effects in terms of glassy bridge formation between filler particles. The observed effects provide evidence that glassy bridges play a key role on the mechanical properties of filled rubbers.
The European Physical Journal E 03/2010; 31(3):263-8. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present measurements of the thermal fluctuations of the free surface of an aging colloidal suspension, Laponite. The technique consists in measuring the fluctuations of the position of a laser beam that reflects from the free surface. Analysing the data statistics, we show that, as the fluid ages, the dynamics becomes heterogeneous. The intermittent events correspond to large changes in the local slope of the free surface over a few milliseconds. We show that those quakes are uncorrelated, although they are kept in memory by the surface over short time scales.
[show abstract][hide abstract] ABSTRACT: The present work deals with emulsions of volatile alkanes in an aqueous clay suspension, Laponite, which forms a yield stress fluid. For a large enough yield stress (i.e. Laponite concentration), the oil droplets are prevented from creaming and the emulsions are thus mechanically stabilized. We have studied the evaporation kinetics of the oil phase of those emulsions in contact with the atmosphere. We show that the evaporation process is characterized by the formation of a sharp front separating the emulsion from a droplet-free Laponite phase, and that the displacement of the front vs. time follows a diffusion law. Experimental data are confronted to a diffusion-controlled model, in the case where the limiting step is the diffusion of the dissolved oil through the aqueous phase. The nature of the alkane, as well as its volume fraction in the emulsion, has been varied. Quantitative agreement with the model is achieved without any adjustable parameter and we describe the mechanism leading to the formation of a front.
The European Physical Journal E 04/2009; 28(4):463-8. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we report on the competition between
evaporation and hydrodynamics for advancing drops of polymer solutions. We thus study
advancing drops which are allowed to evaporate. Drying drives the accumulation of polymer at
the contact line, whereas the advancing motion tends to homogenize the drop. At high
velocity, we experimentally verify classical hydrodynamics predictions. At intermediate
velocities, drying dominates and the contact line becomes more viscous than the bulk droplet.
In the limiting case of very low velocities, the contact line can be partially pinned on the
substrate because of the formation of a glassy defect at the contact line.
The European Physical Journal Special Topics 01/2009; 166(1):33-37. · 1.80 Impact Factor
[show abstract][hide abstract] ABSTRACT: Thermal motion gives rise to fluctuations in free surfaces; measurement of the thermally excited waves on such surfaces provides information on the mechanical properties of the medium. We have developed an optical tool to probe the thermally excited waves on free surfaces: surface fluctuation specular reflection (SFSR) spectroscopy. It consists in measuring the fluctuations in the position of a laser beam that is specularly reflected onto the free surface of a medium. The position of the reflected beam is sensitive to the roughness of the probed surface; the thermal waves are detected by subtracting the light intensities collected on the two quadrants of a photodiode, on which the beam is centered. We show how the measured signal is related to the medium properties. We also present measurements performed on Newtonian liquids as well as on a viscoelastic solid; we show that in all cases, there is a very good agreement between experimental and computed spectra. SFSR thus applies to a broad range of materials. It moreover offers a very good temporal resolution and should provide a useful tool for dynamical measurements on complex fluids.
Review of Scientific Instruments 11/2008; · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present experimental results concerning the advancing motion of drops of polymer solutions in the presence of controlled evaporation. We find that at high advancing velocities the classical Cox-Voinov law is verified, i.e. the advancing contact angle varies linearly with the capillary number. Below a critical velocity the contact angle increases as the advancing velocity is reduced. These results can be explained by taking into account the divergence of the rate of evaporation close to the contact line leading to an accumulation of polymer close to the edge of the drop. The induced local increase of the viscosity explains the increase of the contact angle. We show that the accumulation of polymer over a few nanometers is sufficient to slow down the contact line.
[show abstract][hide abstract] ABSTRACT: 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.
Journal of Statistical Mechanics Theory and Experiment 01/2008; 2008(01):P01020. · 1.87 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use optical techniques to study the aging behavior of glassy colloidal suspensions of Laponite, a synthetic Hectorite clay. Multi-speckle diffusing wave spectroscopy measurements of tracer particle motion in Laponite suspensions show that the slow collective relaxation time τ of the glassy medium scales linearly with the age tw of the sample, a universal feature of many glassy systems. Moreover, the mean-square displacement of the tracer particles in the terminal regime scales as Δr2(tw,t) ~ (t/tw)γ, where γ = 1.5 ± 0.1. Light scattering studies of pure glassy Laponite suspensions show that the scattered light intensity is q-independent and decreases slowly with age, indicating that homogenization of the sample occurs during the aging process. We propose that the novel slow dynamics in this system is governed by random, localized stress relaxation events in the glassy phase.
[show abstract][hide abstract] ABSTRACT: Glasses are known to age continuously, thus exhibiting self-similar temporal evolution of their relaxation spectrum. We investigated the effect of a temperature step on physical ageing at a given temperature, on an amorphous polymer—polybutylacrylate. Measuring the dynamics of the system through creep measurements, it appeared that a negative temperature step induces both a slowing down of the physical ageing and a thinning of the relaxation spectrum. The situation was more complex for a positive temperature jump. Shortly after its completion, the system appeared younger—there was an apparent speeding up of the relaxation dynamics, in other words, a rejuvenation. Later, the system appeared older—it behaved as if an apparent acceleration of the physical ageing occurred through 'overageing'. In addition there was in each case a widening of the relaxation spectrum. In conclusion, the effect of a temperature jump could not be described by a simple shift of the time-relaxation distribution of the system. An evolution of the relaxation spectra following thermally activated processes is at first sight compatible with the observed phenomena, even if some results—the effective time as a function of the total time—are not compatible with this latest approach. This suggests a complex spatial arrangement of the dynamics.
Journal of Statistical Mechanics Theory and Experiment 03/2006; 2006(03):P03003. · 1.87 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have studied the dynamics of the flocculation of poly(styrene-butadiene-acrylic acid) latex suspensions. These suspensions were flocculated by the addition of Ca2+ ions at high concentrations of latex particles. Using diffusing wave spectroscopy and dynamic single light scattering after dilution, we have observed--depending on the pH and on the Ca2+ concentration--several scenarios for flocculation including successive flocculation and deflocculation. This complex behavior reveals that the Ca2+ migration within the shell of the latex is slow in acidic solvent but fast in basic solvent.
Journal of Colloid and Interface Science 07/2005; 286(2):564-72. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration phi(c) of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l(0) are high. We found that a Peclet number Pe, defined as Pe = El(0)/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above phi(c) water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe' by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe' > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe' > 1.
The European Physical Journal E 05/2005; 17(1):69-76. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: With the eventual aim of describing flowing elasto-plastic materials, we focus here on the elementary process of such a flow, a plastic event, and compute the long-range perturbation it elastically induces in a medium submitted to a global shear strain. We characterize the effect of a nearby wall on this perturbation, and quantify the importance of finite-size effects. Although most of our explicit formulae refer to 2D situations, our statements hold for 3D situations as well.
The European Physical Journal E 01/2005; 15(4):371-81. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the slow dynamics of concentrated colloidal suspensions during drying and rewetting under conditions of reversible concentration changes without coalescence or aggregation. We used multispeckle diffusing-wave spectroscopy to monitor the slow dynamics of particles. We found that the alpha relaxation of the suspensions exhibits successively slowing-down, acceleration and a stationary regime during drying at constant rates. Under rewetting conditions, we observed slowing-down and a stationary regime. The characteristic time of the stationary regime is inversely proportional to the rate of concentration change and identical for both drying and rewetting. We explain these regimes as aging (overaging), rejuvenation and plastic flow of the suspensions induced by a deviatoric stress (a combination of compressive and elongational stresses) which is induced by the uniaxial compressive strain generated by evaporation.
The European Physical Journal E 08/2004; 14(3):287-92. · 1.82 Impact Factor
[show abstract][hide abstract] ABSTRACT: We analyze the influence of a glass transition gradient near the particle surfaces on the nonlinear mechanical behavior of reinforced elastomers. We studied systems consisting of grafted silica particles dispersed in a cross-linked poly(ethyl acrylate) matrix. Both particle/matrix interfaces and dispersion states of the particles have been precisely characterized. On the basis of previous studies on the same systems evidencing a glass transition gradient around the particles, we show that the precocious nonlinear mechanical behavior of these filled systems is related to the strain-softening of the glassy polymer shell surrounding the particles surfaces. Comparing strain-softening and melting of the glassy polymer shell, we observe that the strain-softening is enhanced by strong local stress amplifications.