Laurence Talini

Flow induced by a sphere settling in an aging yield-stress fluid

We have studied the flow induced by a macroscopic spherical particle settling in a Laponite suspension that exhibits a yield stress, thixotropy, and shear thinning. We show that the fluid thixotropy (or aging) induces an increase with time of both the apparent yield stress and shear-thinning properties but also a breaking of the flow fore-aft symmetry predicted in Hershel-Bulkley fluids (yield-stress, shear-thinning fluids with no thixotropy). We have also varied the stress exerted by the particles on the fluid by using particles of different densities. Although the stresses exerted by the particles are of the same order of magnitude, the velocity field presents utterly different features: whereas the flow around the lighter particle shows a confinement similar to the one observed in shear-thinning fluids, the wake of the heavier particle is characterized by an upward motion of the fluid (“negative wake”), whatever the fluid’s age. We compare the features of this negative wake to the one observed in visco...

Dynamic wetting on a thin film of soluble polymer: effects of nonlinearities in the sorption isotherm.

The wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm φ(a(w)), where φ is the volume fraction of water in the polymer and aw is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet φ at distances as small as 5 μm, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length ξ below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Soft Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation-condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation-condensation process at small distances. We demonstrate why it is not modified by the second process.

Surface Fluctuations of an Aging Colloidal Suspension: Evidence for Intermittent Quakes.

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.

High frequency linear rheology of complex fluids measured from their surface thermal fluctuations

We characterize the linear viscoelastic properties of complex fluids using a new technique, based on the measurement of surface fluctuations: surface fluctuation specular reflection (SFSR) spectroscopy. The thermally excited waves propagating on a free surface are measured through the deflection of a laser beam specularly reflected from that surface. Elastic and loss moduli of the complex fluids are inferred from the measured power spectrum density of thermal noise, with the implicit use of Kramers–Kronig relations. The technique, besides being noninvasive, provides rheological data in a large frequency range and at vanishing strains. It is therefore especially well suited for the rheological characterization of complex fluids. We present measurements of the viscoelastic modulus of supramolecular polymer solutions in a frequency range extending up to six decades. We compare the SFSR measurements with rheometric data at low and high frequencies, and show that they are in good agreement. Using supramolecular polymer solutions of different natures, exhibiting or not surface viscoelasticity, we furthermore show that SFSR provides a characterization of the bulk properties of the fluids. In addition, we discuss the accuracy of the measurements.

Axisymmetric two-sphere sedimentation in a shear thinning viscoelastic fluid: Particle interactions and induced fluid velocity fields

We investigate the link between particle interactions and induced flow patterns around two identical spheres sedimenting along their centerline in a polymeric fluid. The fluid is strongly shear thinning and, in agreement with previous results, the spheres are observed to chain even at large initial separation distances. The wake of a single particle displays an upward motion of fluid, i.e., a “negative wake” that is commonly observed in fluids with low extensional viscosities. We show that the features of this negative wake vary only weakly with the Deborah number. In the two-sphere case, the pattern of the induced flow depends on the sphere separation distance. The change in the flow pattern does not, however, induce any significant qualitative change in the sphere interactions. Upstream of the leading sphere and downstream of the trailing one along the sedimentation axis, the variations of the fluid velocity are well described by a single master curve for different values of the sphere separation distan...

High bandwidth linear viscoelastic properties of complex fluids from the measurement of their free surface fluctuations

We present a new non-invasive optical method to measure the linear viscoelastic properties of materials, ranging from complex fluids to soft solids, within a large frequency range (about 0.1–104 Hz). The surface fluctuation specular reflection (SFSR) technique is based on the measurement of the thermal fluctuations of the free surfaces of materials at which a laser beam is specularly reflected. The propagation of the thermal surface waves depends on the surface tension, density, and complex viscoelastic modulus G*(ω) of the material. The SFSR signal results from the contribution of different surface modes, and we discuss the leading effect measured as a function of |G*| and frequency. For known surface tension and density, we show that the frequency dependent elastic and loss moduli can be deduced from the fluctuation spectrum. Using a viscoelastic solid (a cross-linked PDMS), which linear viscoelastic properties are known in a large frequency range from rheometric measurements and the time–temperature superposition principle, we show that there is a good agreement between the rheological characterization provided by rheometric and fluctuation measurements. We also present measurements conducted with a complex fluid: a supramolecular polymer solution, formed from H-bonded molecules. At low frequencies, the agreement with rheometric measurements is again very good. We further investigate the high frequency rheology of the supramolecular polymer solution, and show that it behaves very similarly to supramolecular polymer solutions formed by wormlike giant micelles.

Wetting of polymers by their solvents

Abstract.We review the studies on the wetting of soluble polymeric substrates by their solvents, both in the literature and conducted in our group in the past decade. When a droplet of solvent spreads on a soluble polymer layer, its wetting angle can strongly vary with the contact line velocity even at capillary numbers smaller than unity, in contrast to non-soluble substrates. The solvent content in the polymer is a key parameter for the spreading dynamics; that content is set by the initial conditions, but also by the transfers occurring from the droplet to the polymer layer during spreading. We focus on hydrophilic amorphous polymers that are glassy at room temperature, and we discuss the consequences on wetting of the very large changes in the polymer physical properties induced by solvent sorption. We finally present new results on polymers of varying molar masses, and show how they open new perspectives for a better understanding of powder dissolution.Graphical abstract

Interplay of entanglement and association effects on the dynamics of semidilute solutions of multisticker polymer chains

We analyze the effects of polymer concentration, Cp, and physical crosslinking ratio, rc, on the linear viscoelasticity of lightly entangled, linear, associating polymers with multiple crosslinkable groups along the chain backbone. To accomplish this goal, we utilize three novel tools: a robust tunable chemistry based on electrophilic methacryl-succinimidyl modified poly(N-isopropylacrylamide); a modified state-of-the-art entanglement theory, called the discrete slip-link molecular model; and a novel experimental technique, surface fluctuation specular reflection. This experimental technique allows the extraction of the complex viscoelastic modulus covering up to six decades of frequency, from the samples surface fluctuations at constant temperature. We demonstrate that our theoretical model with a consistent set of parameters can qualitatively reproduce the observed features of the complex viscoelastic modulus over the entire probed frequency range. Furthermore, the effects of Cp and rc on the viscoelas...

Two beam surface fluctuation specular reflection spectroscopy

In surface fluctuation specular reflection spectroscopy (SFSRS) deflections of a specularly reflected laser beam are used to characterize thermally excited surface waves. Here we report on a new two beam version of SFSRS in which the deflections of two reflected laser beams from separate locations on a surface are correlated. We demonstrate that this new two beam SFSRS technique can be used to determine directly the power spectrum of height fluctuation of thermally excited surface waves over a large range of both frequencies and wavevectors. In addition, we show that the technique is well suited for materials ranging from simple liquids to complex liquids and soft solids, including turbid materials.

Slow dynamics and intermittent quakes in soft glassy systems

We present measurements of the thermal fluctuations of the free surface of oil-in-water emulsions which exhibit a glassy behavior. The Surface Fluctuation Specular Reflection (SFSR) technique was applied to measure the free surface fluctuations. When the volume fraction of the oil droplets is close to or larger than the disordered packing volume fraction, the emulsion ages and its free surface exhibits abnormal fluctuations, consisting of rare but large amplitude quakes. From a statistical analysis of the signal, we observe that the relative importance of such intermittent quakes grows as the system ages. More precisely, we observe for the first time that the kurtosis of the fluctuations increases with aging time with an exponent of about 1.3. These quakes correspond to large changes in the local slope of the free surface over a few tenths of a second. We conjecture that such quakes reflect the dynamics peculiar to glassy systems driven by the relaxations of internal stress.