o Silica spheres of 1600-A diameter immersed in the binary fluid water-f 2-6-lutidine have been studied by light-scattering techniques. The scattered light was seen to be strongly dependent on temperature and concentration. This effect is due to the appearance of a lutidine layer on the spheres. The layer thickness dramatically increases near a transition line were the spheres aggregate as a result of attractive interactions. This transition exhibits all the features predicted for the prewetting transition between high and low adsorption of a fluid on a surface.PACS numbers: 68.45. Da, 64.70.Ja, 78.35. + c, 82.70.Kj Wetting phenomena on a wall appear in multicomponent systems when the components exhibit different interactions with the wall. Even in the simplest case of a binary fluid, a rich behavior is expected. 1 " 2 In the region where the system exhibits two phases, i.e., on the coexistence curve ( Fig. 1), a first-order transition between partial wetting (finite contact angle between the wall and the two phases) and complete wetting (one phase wets the wall and surrounds the other phase) has already been observed. 3 In the case of the water-lutidine (W-L) mixture, which presents a lower critical point at the L mass fraction C = 0.286 and at the temperature 7 1 = 34°C, 4 this "wetting transition" takes place for a silica wall at C w -0.07 and T w -50 °C, 5 with the L-rich phase wetting the silica. In the one-phase region of a binary mixture one expects a line of first-order "prewetting" transition which should separate a region of weak chemical adsorption from a region of large adsorption. This line ( Fig. 1) should begin at the wetting transition (C W ,T W ) and end with a prewetting critical point (CA>TA)-which should not be confused with the liquid-liquid critical point (C C ,T C ).Up to now, prewetting has remained undetected. However, the behavior of small silica spheres immersed in the W-L mixture provides, as we shall see, some evidence of the existence of such a prewetting line in the onephase region. (The anomalous adsorption which has already been detected near the bulk critical point 6,7 is related to the bulk critical region and is not connected to the prewetting transition.) Small spheres immersed in binary fluids have already been studied near the liquid-liquid critical point by microscopy 8 or dynamic light scattering. 9,10 In all cases anomalous mobilities have been observed, attributed to correlated layers 8,10 or to surface interactions. 9 Finally, interferometric measurements in the W-L mixture have already proved the existence of a Ladsorption layer on silica spheres. 11Experiments.-Our initial aim was to deduce the thickness of the L layer sticking on silica by measuring the optical cross section of small silica spheres (diameter 2a = 1600 A) immersed in the W-L mixture. For that purpose, we prepared W-L samples with water incorporating a small mass fraction C 0 (0.004 or 0.009) of monodisperse silica spheres prepared with use of the Storber method. 12 No shift of the bulk critical poi...

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Morphogenetic processes, like sorting or spreading of tissues, characterize early embryonic development. An analogy between viscoelastic fluids and certain properties of embryonic tissues helps interpret these phenomena. The values of tissue-specific surface tensions are consistent with the equilibrium configurations that the Differential Adhesion Hypothesis predicts such tissues reach after sorting and spreading. Here we extend the fluid analogy to cellular kinetics. The same formalism applies to recent experiments on the kinetics of phase ordering in two-phase fluids. Our results provide biologically relevant information on the strength of binding between cell adhesion molecules under near-physiological conditions. K nowing the molecular binding strength between cell adhesion molecules is crucial to understanding morphogenesis. Atomic force microscopy can measure the strength of bonds between individual biological receptor molecules and their ligands (1-4). Because the properties of these molecules strongly depend on their extracellular and intracellular environments, interpreting measurements of the binding energy outside of tissues biologically may be difficult. However, the analogy between cell sorting and the separation of immiscible fluids provides important quantitative information on the effective binding energy between cell adhesion molecules in situ under near-physiological conditions.Certain embryonic tissues mimic the behavior of viscous liquids or fluids (5). Often during embryonic morphogenesis one cell population spreads over the surface of another, in the manner of a liquid spreading on a solid or on another liquid. In suspension or on nonadhesive surfaces, various multicellular aggregates round just as liquid droplets do. Cells of two distinct tissues randomly intermixed within such aggregates sort out into separate regions like coalescing droplets of immiscible liquids. Such behavior requires (i) many subunits, which (ii) cohere while (iii) being mobile. In ordinary liquids, the subunits are molecules or droplets; in rearranging cell populations, the subunits are living cells. The interaction potential between both liquid and cellular subunits is of a Lenard-Jones type: hard sphere repulsion and short and longer range attraction. In liquids, van der Waals forces mainly cause the attraction while cells interact via cell adhesion molecules at short distances and via steric interactions caused by long filopodia at a range of a few cell diameters. Filopodia may be significant at low cell density but are absent in the close-packed tissues we study. In liquids, the mobility of the subunits is thermal; in tissues, it is ''amoeboid.'' Despite these differences, shared properties cause tissues whose cells are mobile to behave in many respects like fluids.To account for the fluid-like behavior of cell populations, Steinberg formulated the ''differential adhesion hypothesis,'' or DAH (6, 7), according to which differential expression of adhesion molecules guides certain morphogenetic movements. Recent ex...

The growth dynamics of water drops condensed on a superhydrophobic geometrically patterned surface were studied. Drop size evolution at early and intermediate times is self-similar. Drop growth laws do not differ for a flat surface because of a reduction of both drop and substrate dimensionality. A striking observation is the instantaneous drying of the top surface of grooves at a point in time due to coalescence of the drops with a completely filled channel. At late times, only a few large drops grow connected to the channels, in a mixed Wenzel-penetration regime.

Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and enhancing heat transfer using liquid impregnated surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid-air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth.

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