Wetting dynamics of hydrophobic and structured surfaces. Preface (original) (raw)
Related papers
Wetting on Hydrophobic Rough Surfaces: To Be Heterogeneous or Not To Be?
Langmuir, 2003
Equilibrium wetting on rough surfaces is discussed in terms of the "competition" between complete liquid penetration into the roughness grooves and entrapment of air bubbles inside the grooves underneath the liquid. The former is the homogeneous wetting regime, usually described by the Wenzel equation. The latter is the heterogeneous wetting regime that is described by the Cassie-Baxter equation. Understanding this "competition" is essential for the design of ultrahydrophobic surfaces. The present discussion puts the Wenzel and Cassie-Baxter equations into proper mathematical-thermodynamic perspective and defines the conditions for determining the transition between the homogeneous and heterogeneous wetting regimes. In particular, a new condition that is necessary for the existence of the heterogeneous wetting regime is added. It is demonstrated that when this condition is violated, the homogeneous wetting regime is in effect, even though the Cassie-Baxter equation may be satisfied.
A Theory for the Morphological Dependence of Wetting on a Physically Patterned Solid Surface
We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/ depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using freeenergy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.
Proceedings of the National Academy of Sciences, 2019
Significance Whether a drop of liquid such as water is repelled from a solid substrate yielding a large angle of intersection θ between the drop’s surface and the substrate or is strongly attracted, small θ , is key to the function of a host of physical and biological systems. We elucidate the physics of hydrophobic and hydrophilic substrates in terms of wetting and drying surface phase diagrams. These display a surprising variety of forms that depend upon the nature of the substrate–liquid and liquid–liquid attractive forces. Liquids near weakly attractive substrates exhibit critical drying, a phenomenon occurring as θ → 18 0 ○ that is accompanied by divergent density fluctuations. Our findings provide a conceptual framework for tailoring the properties of superhydrophobic and hydrophilic materials.
Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting
Langmuir, 2015
The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the waterfree contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly-(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a selfassembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS−PDMS contact. The Young−Dupréequation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting.
Wetting characteristics of liquid drops at heterogeneous surfaces
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994
Well-deflned heterogeneous surfaces consisting of hydrophobic and hy'drophilic re-eions were prepared on gold (a 2000A gold film supported on an SiiSiOr/Ti substrate) by patteming self-assenrbled monolayers (SAMs). using an elastomer stamp. One surface was composed of alternating and parallel hr,'drophobic (1.,5 pm) and hydrophilic (3 pm) strips. and the second surface consisted of alternating hydrophilic squares (3 prm x 3 prm) scparated by hydrophobic strips (1.5 frm). The rvetting characteristics of these well-defir-red heterogener)us solid surfaccs \\'ere examined by contact angle measllrements. The contact angles for water drops. which varied in pH from,5.E to 10.0. were measured with the strips both tangential to and normal to the three-phase contact hne. The experirnental contact angles are in good agreement rvith theory as calculated from the Cassie equation when the three-phase contact linc-is non-ct.rtttorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand. rvhen the strips are normal to the drop ed_ee. corrusation of the three-phase contact line affects the contact angle signilicantlr'. Contact angles. measured rvith the strips normiil to the drop edge. were lower bt' 7 16 than those calculated from the Cassie equation. Analysis of these measurements. together with contact angle drop size nrei.rsurernents for fully hydrophobic and hydrophilic surfaces. demonstrate the validity of a modified Cassie equation that inclucles a term describing the line tension contribution.
Water on hydroxylated silica surfaces: Work of adhesion, interfacial entropy, and droplet wetting
The Journal of Chemical Physics, 2021
In the last few years, much attention has been devoted to the control of the wettability properties of surfaces modified with functional groups. Molecular dynamics (MD) simulation is one of the powerful tools for microscopic analysis providing visual images and mean geometrical shapes of the contact line, e.g., of nanoscale droplets on solid surfaces, while profound understanding of wetting demands quantitative evaluation of the solid-liquid (SL) interfacial tension. In the present work, we examined the wetting of water on neutral and regular hydroxylated silica surfaces with five different area densities of OH groups ρ OH A , ranging from a nonhydroxylated surface to a fully hydroxylated one through two theoretical methods: thermodynamic integration (TI) and MD simulations of quasi-two-dimensional equilibrium droplets. For the former, the work of adhesion needed to quasi-statically strip the water film off the solid surface was computed by the phantom wall TI scheme to evaluate the SL interfacial free energy, whereas for the latter, the apparent contact angle θapp was calculated from the droplet density distribution. The theoretical contact angle θ YD and the apparent one θapp, both indicating the enhancement of wettability by an increase in ρ OH A , presented good quantitative agreement, especially for non-hydroxylated and highly hydroxylated surfaces. On partially hydroxylated surfaces, in which θ YD and θapp slightly deviated, the Brownian motion of the droplet was suppressed, possibly due to the pinning of the contact line around the hydroxyl groups. Relations between work of adhesion, interfacial energy, and entropy loss were also analyzed, and their influence on the wettability was discussed.
Current Opinion in Colloid & Interface Science, 1997
Recent advancements in experimental studies of wetting phenomena have helped to bridge the gap between the progress made in theory and simulation over the past decade, and the experimental evidence or verification of the theoretical predictions. These developments include new measurements of the equilibrium thickness of precursor wetting films on solid and liquid substrates and at the liquid/gas interface, experimental studies of critical adsorption, as well as measurements of the dynamics of wetting and spreading and the nucleation of wetting layers in simple and complex systems. There have also been some recent results on dewetting of solid substrates by liquid films. Addresses ·'wan-N.