The distribution of ions and charge at solid-water interfaces plays an essential role in a wide range of processes in biology, geology and technology. While theoretical models of the solid-electrolyte interface date back to the early 20th century, a detailed picture of the structure of the electric double layer has remained elusive, largely because of experimental techniques have not allowed direct observation of the behaviour of ions, i.e. with subnanometer resolution. We have made use of recent advances in high-resolution Atomic Force Microscopy to reveal, with atomic level precision, the ordered adsorption of the mono- and divalent ions that are common in natural environments to heterogeneous gibbsite/silica surfaces in contact with aqueous electrolytes. Complemented by density functional theory, our experiments produce a detailed picture of the formation of surface phases by templated adsorption of cations, anions and water, stabilized by hydrogen bonding.

The impingement of drops onto solid surfaces 1,2 plays a crucial role in a variety of processes, including inkjet printing, fog harvesting, anti-icing, dropwise condensation and spray coating 3-6. Recent e orts in understanding and controlling drop impact behaviour focused on superhydrophobic surfaces with specific surface structures enabling drop bouncing with reduced contact time 7,8. Here, we report a di erent universal bouncing mechanism that occurs on both wetting and non-wetting flat surfaces for both high and low surface tension liquids. Using high-speed multiple-wavelength interferometry 9 , we show that this bouncing mechanism is based on the continuous presence of an air film for moderate drop impact velocities. This submicrometre 'air cushion' slows down the incoming drop and reverses its momentum. Viscous forces in the air film play a key role in this process: they provide transient stability of the air cushion against squeeze-out, mediate momentum transfer, and contribute a substantial part of the energy dissipation during bouncing. The role of ambient air in drop impact and other dynamic wetting phenomena has long been neglected. Only recently, observations such as the suppression of splashing in drop impact at reduced ambient pressure 10 , the generation of splashes by superhydrophobic spheres falling into a liquid bath 11 , and the entrainment of air by fast-moving contact lines 12 highlighted the relevance of this rather viscous ambient medium. For drop impact, theoretical studies 13-16 suggested that splashing might be related to the presence of a thin lubricating air layer between the impacting drop and the substrate. Subsequent experiments confirmed the transient formation of an air layer with (sub)micrometre thickness 9,17-20. However, it turned out that the air film collapses on a microsecond timescale for typical impact speeds of splashing experiments (of the order of m s −1). However, as we report in this Letter, the air film remains intact if the initial impact speed is reduced to less than ν ∼ 0.5 m s −1. In this case, the drop rebounds without ever directly touching the surface. We release liquid drops of water, glycerol, silicone oil and various other organic liquids (Supplementary Table 1) of millimetric size (R = 0.52. .. 1.03 mm) from a height of several millimetres to a few centimetres to fall onto carefully cleaned and dust-free surfaces of variable wettability (Methods). On their first impact the drops have initial Weber numbers We = ρRν 2 /σ = 0.64. .. 4.3 (ρ: liquid density; R: drop radius; σ : surface tension). For all liquids studied, side-view images taken with a high-speed video camera (Fig. 1a) show that the drops bounce provided that the impact speed is not too high. For water, the maximum impact speed is 0.48 m s −1 , corresponding to We max ≈ 4. Throughout the entire bouncing process, the apparent contact angle observed in side-view images remains close to 180 • , both for clean glass substrates with an equilibrium contact

A microrheological model of aggregating dispersions is proposed in which the shear stress is estimated as the sum of hydrodynamic and structural parts. The former is attributed to the hydrodynamic cores of fractal aggregates, which behave as a suspension of impermeable spheres. The latter accounts for the forces transmitted by chains of particles linking neighboring aggregates into a transient network. To calculate the structural part the concept of fractal aggregation is incorporated into a transient network theory, to account for the creation and breakup of chains of colloidal particles connecting the aggregates. Rigid and soft chains are distinguished. The former have multiply connected backbones which deform as contorted elastic rods, while the latter have at least one soft junction and deform without elastic resistance until fully loaded. The contribution of the soft chains to the stress tensor is neglected. The calculations treat two different mechanisms for the evolution of rigid chains: a purely mechanical one, which corresponds to a shear-controlled structure built up in flow, and a thermal mechanism, which pertains to a quasiequilibrium structure undisturbed by shear. We calculate steady-shear viscosities in the former case and viscoelastic functions in the latter. The model can be fitted satisfactorily to the experimental results for a well-characterized polystyrene latex dispersion with physically acceptable parameters.

Liquid drops hitting solid surfaces deform substantially under the influence of the ambient air that needs to be squeezed out before the liquid actually touches the solid. Nanometer-and microsecond-resolved dual wavelength interferometry reveals a complex evolution of the interface between the drop and the gas layer underneath. For intermediate impact speeds (We $ 1 . . . 10) the layer thickness can develop one or two local minima-reproduced in numerical calculations-that eventually lead to the nucleation of solidliquid contact at a We-dependent radial position, from a film thickness >200 nm. Solid-liquid contact spreads at a speed involving capillarity, liquid viscosity and inertia. DOI: 10.1103/PhysRevLett.108.074505 PACS numbers: 47.55.DÀ, 47.55.nd, 47.55.np Liquid drops deform substantially upon impact onto a solid surface. Depending on impact speed they rebound, get deposited on the surface, or disintegrate in a splash (for a review, see Ref.[1]). Following experimental reports of tiny air bubbles being incorporated into drops [2][3][4][5] as well as the suppression of splashing upon reducing the ambient air pressure [6] it became clear that the ambient air plays a non-negligible role in the impact process. To describe the expulsion of the air that initially separates the drop from the solid, several models were formulated [7-9] that describe the drop impact primarily in terms of a balance between the inertia of the decelerating liquid and the excess pressure arising from the viscous squeezeout of the thin air layer. A local pressure maximum right under the drop leads to the formation of a ''dimple'', which should eventually evolve into the enclosed air bubble [9]. Using this model and including corrections due to capillary forces, it was shown that a thin air film of almost constant thickness should develop under the drop [10], and the formation of a thin liquid jet was observed using an axisymmetric, curvilinear description [11]. Mandre et al. [10] suggested that this air film plays an important role, e.g., for the splashing process, but recent experiments by Discroll and Nagel [12] question this scenario: while the presence of interference fringes right under the drop indeed confirms the formation of a dimple, their measurements suggest that direct liquid-solid contact forms very quickly around the dimple, separating dimple from splashing dynamics. Whether air films do play an important role in other regimes of drop impact, how they collapse to establish direct liquid-solid contact, and to what extent the proposed visco-inertial models describe these processes remains unexplored at this stage.In this letter, we address these issues by monitoring the evolution and the collapse of the air film for a wide range of liquid properties (interfacial tension , viscosity l , density l ) at moderate impact speeds. To do so, we develop an advanced high-speed dual wavelength interferometry technique that allows us to extract full thickness profiles with an unprecedented resolution of % 10 nm and 50 s. Focusing on...

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The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca2+ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (≈10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K+ and Na+ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca2+. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology.

We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow. The experiments are carried out with a counterrotating Couette apparatus. The deformation depends on the mechanical properties of the lipid bilayer, the vesicle radius, and the viscosity of the surrounding Newtonian liquid. We show that the relevant mechanical parameter is the bending rigidity. A simple model has been developed that describes the deformation of a vesicle. This model takes thermal undulations of the bilayer into account. We have obtained a value for the bending rigidity of dimyristoyl-phosphatidylcholine bilayers and its value has been compared with literature data and with results from micropipette aspiration experiments. From the measurements we are able to discriminate between unilamellar and multilamellar vesicles.

We demonstrate that the equilibrium shape of the composite interface between superhydrophobic surfaces and drops in the superhydrophobic Cassie state under electrowetting is determined by the balance of the Maxwell stress and the Laplace pressure. Energy barriers due to pinning of contact lines at the edges of the hydrophobic pillars control the transition from the Cassie to the Wenzel state. Barriers due to the narrow gap between adjacent pillars control the lateral propagation of the Wenzel state. We demonstrate how reversible switching between the two wetting states can be achieved locally using suitable surface and electrode geometries.