The rise of bubbles in shear thinning viscoelastic fluids.

Bubbles in a liquid rise under gravity and separate to the top. Bubbly liquids exist commonly in nature and play a significant role in energy-conversion, oil and chemical industries. Therefore, understanding how bubbles rise is of great importance. Rheological properties of the fluid have a strong impact on single bubble rise and have been shown to change collective bubble rise at low gas volume fractions significantly. We expect that a viscoelastic fluid can strongly modify the rise of bubbles in more concentrated suspensions. We generate bubbly liquids up to gas fractions of 30 %. We measure the bubble size and the rise velocity in micellar solutions made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), a common system to create shear-thinning solutions. We show that when the NaSal concentration is small and the solutions are Newtonian, the bubble rise velocity decreases with increasing volume fraction of bubbles and the relationship between the two follows the Richardson-Zaki prediction. For the shear thinning viscoelastic solutions, the Richardson-Zaki relation no longer applies. Bubble clustering leads to faster rise velocities and a weaker dependence on the bubble volume fraction. At the largest concentration two rise regimes are observed. A fast one similar to that in the other shear thinning samples, followed by a very slow bubble rise. The slow rise velocity is attributed to the smallest bubbles rising so slowly, that at the shear rates around them, the fluid behaves as a Newtonian fluid. Therefore, bubble rise becomes again comparable to Stokes expectations. We also show that the peculiar dependence of the rise velocity with volume fraction of bubbles in the shear thinning viscoelastic solutions can have important implications in flotation as the area flux changes strongly with bubble volume fraction.

Effect of the density of pillar-patterned substrates on contact mechanics: Transition from top to mixed contact with a detailed pressure-field description.

Different contact regimes between a spherical lens and a periodically patterned substrate are observed, when they are pressed against each other. Top contact occurs when only the highest substrate sections touch the lens, whereas mixed contact implies that both the highest and the lowest substrate sections come into contact with the lens. In this paper, we study how the pattern density of the substrate, along with its physical properties and those of the lens, determine the transition from top contact to mixed contact. Experiments and numerical simulations had been performed, as complementary approaches to obtain data, and a theoretical analysis has been developed to gain insight on the effect of the physical parameters on the contact transition. As a result, a phase diagram is presented, in terms of the load and the contact radius, that combines the observations of the three approaches (experimental, numerical, and theoretical), unveiling the boundaries of three contact regimes: (1) deterministic-driven contact, (2) top contact, and (3) mixed contact.

Stress concentration in periodically rough Hertzian contact: Hertz to soft-flat-punch transition.

We report on the elastic contact between a spherical lens and a patterned substrate, composed of a hexagonal lattice of cylindrical pillars. The stress field and the size of the contact area are obtained by means of numerical methods: a superposition method of discrete pressure elements and an iterative bisection-like method. For small indentations, a transition from a Hertzian to a soft-flat-punch behaviour is observed when the surface fraction of the substrate that is covered by the pillars is increased. In particular, we present a master curve defined by two dimensionless parameters, which allows one to predict the stress at the centre of the contact region in terms of the surface fraction occupied by pillars. The transition between the limiting contact regimes, Hertzian and soft-flat-punch, is well described by a rational function. Additionally, a simple model to describe the Boussinesq-Cerruti-like contact between the lens and a single elastic pillar, which takes into account the pillar geometry and the elastic properties of the two bodies, is presented.

Role of adhesion between asperities in the formation of elastic solid/solid contacts.

We investigated the formation of a contact between a smooth sphere of elastomer and a micro-patterned elastomer substrate. We focussed our attention on the transition between a contact only established at the top of the pillars, and a mixed contact with a central zone of full contact surrounded by a top contact corona, which was observed when the normal load was increased. The full contact zone always nucleated with a finite radius, and the transition appears to be a first-order transition, with a hysteresis due to the creation of an adhesive zone between the pillars. We propose to include the effect of the new inter-pillar adhesion to produce a realistic treatment of the mechanics of these complex contacts. This new approach quantitatively accounts for the evolution of the observed jump in the radius of the full contact with the geometrical parameters of the pattern.

Landau-Levich menisci.

As shown by Landau, Levich and Derjaguin, a plate withdrawn out of a wetting bath at low capillary numbers deforms the very top of the liquid reservoir. At this place, a dynamic meniscus forms, whose shape and curvature select the thickness of the film entrained by the plate. In this paper, we measure accurately the thickness of the entrained film by reflectometry, and characterize the dynamic meniscus, which is found to decay exponentially towards the film. We show how this shape is modified when reversing the motion: as a plate penetrates the bath, the dynamic meniscus can "buckle" and present a stationary wavy profile, which we discuss.

Wetting and dewetting transition: an efficient toolbox for characterizing low-energy surfaces.

The capillary bridge formed between a solid spherical surface and an infinite liquid bath is an efficient technique for characterizing the adhesion property of a solid surface. When the solid surface is pulled out of the liquid at a sufficiently high velocity, a thin liquid film is deposited on the solid and drains more slowly than the central capillary bridge. The retraction kinetics of this "pancake" and the critical velocity above which it appears are studied as a function of the viscosity of the liquid or the wettability of the solids. The dynamics of the liquid film follows the classical law of dynamic dewetting. This makes the capillary bridge test, used in the dynamical regime, a very efficient tool for discriminating between antiadhesive coatings.

Where does a cohesive granular heap break?

In this paper, we consider the effect of cohesion on the stability of a granular heap and compute the maximum angle of stability of the heap as a function of the cohesion. We show that the stability is strongly affected by the dependence of the cohesion on the local pressure. In particular, this dependence is found to determine the localization of the failure plane. While for a constant adhesion force, slip occurs deep inside the heap, surface failure is obtained for a linear variation of the cohesion on the normal stress. Such a transition allows to interpret some recent experimental results on cohesive materials.

Adhesion between weakly rough beads.

Cohesion effects are of prime importance in powders and granular media, and they are strongly affected by the roughness of the grain surface. We report measurements of the adhesion force between surfaces of Pyrex having a nanometric roughness, with a surface force apparatus. The two surfaces are immersed in liquid n-dodecane. The adhesion force measured is much smaller than expected in the case of smooth surfaces. We find that the adhesion force depends on the maximal load that has been applied on the surfaces, but does not depend on the time during which they have been in contact. We propose a model of plastic deformation of the small asperities in a macroscopic Hertz contact which is in good agreement with the experimental data.

Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces.

It has been shown that the flow of a simple liquid over a solid surface can violate the so-called no-slip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers.

Metastability and nucleation in capillary condensation

This paper is devoted to thermally activated dynamics of capillary condensation. On the basis of a simple model we identify the critical nucleus involved in the transition mechanism and calculate the nucleation barrier from which we obtain information on the nucleation time. Close to the condensation point, the theory predicts extremely large energy barriers leading to strong metastabilities, long time dependencies, and large hysteresis in agreement with experimental observations in mesoporous media. The validity of the model is assessed using a numerical simulation of a time-dependent Ginzburg-Landau model for the confined system.