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.

Stable Freestanding Thin Films of Copolymer Melts Far from the Glass Transition

Thin polymer films have attracted attention because of both their broad range of applications and of the fundamental questions they raise regarding the dynamic response of confined polymers. These films are unstable if the temperature is above their glass transition temperature T_g. Here, we describe freestanding thin films of centimetric dimensions made of a comb copolymer melt far from its glass transition that are stable for more than a day. These long lifetimes allowed us to characterize the drainage dynamics and the thickness profile of the films. Stratified regions appear as the film drains. We have evidence that the stability, thinning dynamics, and thickness profile of the films result from structural forces in the melt. Understanding the key mechanisms behind our observations may lead to new developments in polymeric thin films, foams, and emulsions without the use of stabilizing agents.

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.

The leading edge of evaporating droplets

New experiments on drops evaporating in normal atmosphere from smooth substrates in the situation of complete wetting are reported and compared with the available theoretical model. They are the continuation of previous work with alkane or water sessile drops, which is first briefly summarized. The model accounts very well for the dynamics of the drop radius, but the predictions are only qualitative for the contact angle, especially for small angles. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drops dynamics. Then the main part of the paper concerns new experiments with polydimethylsiloxane oligomers. These silicone oils are similar to alkanes as far as evaporation rate is concerned, but have lower surface tensions, and therefore smaller dynamic contact angles. The purity of the oils appears to be critical for the experiments, and requires a preliminary investigation. Then a systematic study of the drops dynamics is presented, as a basis for forthcoming theoretical work.

Rescaling the dynamics of evaporating drops

The dynamics of evaporation of wetting droplets is investigated experimentally in an extended range of drop sizes to provide trends relevant for a theoretical analysis. A model is proposed, which generalizes Tanner's law in the presence of evaporation. A qualitative agreement is obtained, which represents a first step toward the solution of a very old, complex problem.

Spontaneous spreading of nematic liquid crystals

The spontaneous spreading of macroscopic drops of nematic liquid crystals on hydrophilic substrates has been investigated by interferometric techniques. There is a complex interplay between the elastic energy, due to antagonist anchoring at the interfaces, and the radial flow in the spreading drop. A relevant parameter appears to be the relative humidity of the atmosphere, because it controls the amount of water molecules adsorbed on the substrate and, therefore, the strength of anchoring defects. The spreading laws differ from the ones of simple wetting liquids, and contact line instabilities coupled to short- (anchoring) or large-scale (disclinations) defects of the nematic film are observed.