[Show abstract][Hide abstract] ABSTRACT: In this article, we summarize a series of experimental works and qualitative modelling regarding the dynamics of a liquid contact line on gel substrates. Two different situations were investigated, i.e., water on hydrophilic poly (2-acrylamido-2-methyl-propane-sulfonic acid-co-acrylamide) (PAMPS−PAAM) gels and water on hydrophobic poly (styrene-butadienestyrene)(SBS)−paraffin gels. In both situations, different gel characteristics largely affect the contact line dynamics: liquid diffusion and surface deformation by capillary force. On hydrophilic gels, the contact line of a sessile droplet exhibits successively two different behaviors: pinned and receding, and the transition between the behaviors is closely related to the local deformation of the gel surface due to swelling. On hydrophobic gels, the contact line exhibits several different regimes of motions, i.e., stick-slip, and two continuous motions. These transitions are characterized by a frequency f built upon the apparent contact line velocity v and the droplet radius R as f = v/R, indicating that the gel rheology largely affects the dynamics of liquid contact line. Our results provide a synthetic view of the characteristic features of how the wetting is different on gel surfaces. Finally, we designate unsolved problems and future directions.
[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 dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer—most probably due to a gradient of glass transition temperature around the solid particles—is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.
[Show abstract][Hide abstract] ABSTRACT: The dynamics of a contact line under evaporation and total-wetting conditions is studied taking into account van der Waals interactions and the divergent nature of evaporation near the border of the liquid . The existence of a precursor film at the edge of the liquid is shown analytically and numerically. Its length is controlled by the Hamacker constant and evaporative flux. Past the precursor film, Tanner's law is generalized accounting for evaporative effects.
[Show abstract][Hide abstract] ABSTRACT: We studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly(2-
acrylamido-2-methyl-propane-sulfonic acid -co- acrylamide) (PAMPS–PAAM)) substrates. The
profiles of the droplet and substrate were measured simultaneously using a grid projection method. We
observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits
successively two different behaviors: pinned and receding, and the transition between these two
behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an
early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of
the local slope of the gel surface near the contact line increases.
[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. · 2.18 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. · 2.18 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. · 2.18 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: We propose a simple model of the dynamics of a contact line under evaporation and partial wetting conditions, taking into account the divergent nature of evaporation near the contact line, as evidenced by Deegan et al. (Nature, 389 (1997) 827). We show that evaporation can induce a non-negligible change of the contact angle together with modification of the flow near the contact line. We apply our results to dip-coating of a substrate with non volatile solutes. We show that at small velocities the coating thickness increases and scales like the inverse of the square of the velocity which implies a minimum of the coating thickness at the cross-over with the more familiar Landau-Levich regime.
[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: This article presents the first experimental study of an advancing contact line for a colloidal suspension. A competition between the hydrodynamic flow due to the drop velocity and the drying is exhibited: drying accounts for particle agglomeration that pins the contact line whereas the liquid flow dilutes the agglomerated particles and allows the contact line to advance continuously. The dilution dominates at low concentration and high velocity, but at high concentration and low velocity, the contact line can be pinned by the particle agglomeration, which leads to a stick-slip motion of the contact line. The calculation of the critical speed splitting both regimes gives an order of magnitude comparable to that of experiments. Moreover, a model of agglomeration gives an estimation of both the size of the wrinkles formed during stick-slip and the force exerted by the wrinkle on the contact line.