Microscopic structure of electrowetting-driven transitions on superhydrophobic surfaces (original) (raw)
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Physical Review Letters, 2011
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.
How to Achieve Reversible Electrowetting on Superhydrophobic Surfaces
Langmuir : the ACS journal of surfaces and colloids, 2018
Collapse (Cassie to Wenzel) wetting transitions impede the electrostatically induced reversible modification of wettability on superhydrophobic surfaces, unless a strong external actuation (e.g., substrate heating) is applied. Here we show that collapse transitions can be prevented (the droplet remains suspended on the solid roughness protrusions) when the electrostatic force, responsible for the wetting modification, is smoothly distributed along the droplet surface. The above argument is initially established theoretically and then verified experimentally.
Intermediate States of Wetting on Hierarchical Superhydrophobic Surfaces
Langmuir, 2020
Wetting transition on superhydrophobic surfaces is commonly described as an abrupt jump between two stable states-either from Cassie and Wenzel for non-hierarchical surfaces or from Cassie to nano-Cassie on hierarchical surfaces. We here experimentally study the electrowetting of hierarchical superhydrophobic surfaces composed of multiple length scales, by imaging the light reflections from the gas-liquid interface. We present the existence of a continuous set of intermediate states of wetting through which the gas-liquid interface transitions under a continuously increasing external forcing. This transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nano defects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many heat transfer and self-cleaning applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of meta-stable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces, and provides new considerations for their effective design.
LETTERS Visualizing contact line phenomena on microstructured superhydrophobic surfaces
The authors introduce a direct method for determining droplet solid-liquid-vapor interfacial geometry on microstructured surfaces. A heated liquid metal droplet of size 18 l is deposited onto a microstructured surface and then freezes, preserving the microstructure impressions onto the metal surface. Postsolidification microscopy can measure contact line geometry and identify wetting state. This approach can be used to visualize contact line on curved and opaque microstructured surfaces. The presence of microstructures or nanostructures on a surface can influence how that surface interacts with liquids. 1–4 When simple surface roughening techniques are employed, increased surface area amplifies the natural surface chemistry: hydrophilic surfaces become more hydro-philic upon roughening, and hydrophobic surfaces become more hydrophobic. 5 While the interaction between the drop-let and the surface microstructures is understood to control the macroscopic wetting dynamics, few experimental methods have been reported to study the microscopic interaction between a droplet and a microstructured surface. This article reports metrology of microscopic contact line phenomena via metal droplets frozen onto a microstructured surface. The macroscopic interaction of a liquid droplet with a surface can be understood in terms of the surface chemistry, surface morphology, and contact angle between the droplet and the surface. Figure 1 shows a droplet with contact angle interacting with a microstructured hydrophobic surface. Contact angle is a measure of the static hydrophobicity of a surface and is given by cos = SV − SL / LV , where SV is the interfacial tension between the solid and vapor, SL is the interfacial tension between the solid and liquid, and LV is the interfacial tension between the liquid and vapor. 6 The size, shape, and pitch of microstructures on a surface affect the droplet on the surface in either state. 7,8 Depending on the application, it can be desirable for the droplet to be suspended on top of the microstructures in the Cassie–Baxter state CB because the droplet is significantly more mobile in this state compared to the Wenzel state W , where the water is in intimate contact with both the tops and bottoms of the microstructures. 3 The Wenzel state is described by cos W = r cos , where r is the ratio of the actual contact area versus the projected contact area. 7 If liquid is suspended on the tops of microstructures, will change to CB as cos CB = cos +1 − 1, where is the area fraction of the solid that touches the liquid. 8 This form of the Cassie–Baxter relation requires that the solid-liquid contact should be pla-nar, and the sum of the vapor area fraction and the solid area fraction should be unity. According to one criterion, for the Cassie–Baxter state to exist, the following inequality 3 must be satisfied: cos −1 / r −. Thus the contact angle, area ratio, and area fraction are the main parameters that govern how a droplet wets a solid. The preceding analysis is appropriate for any macroscopic solid-liquid-vapor system where surface forces dominate droplet wetting. Thus it is possible to make analogies between one solid-liquid-vapor system and another if the contact angle, area ratio, and area fraction are similar. The liquid-vapor-solid interface around a droplet is a critical aspect of hydrophobicity 9–13 because it is the portion of a droplet that interacts dynamically with a solid surface. The microscopic physical phenomena of this interface are not fully understood. Some reports claim that a rough and jagged contact line increases hydrophobicity, 14 while others claim that a smooth, continuous contact line is desirable. 15 A clear understanding of this interface is elusive because the contact line is difficult to experimentally investigate without modifying its behavior. 14 At present, the direct diagnostics of droplet wetting on microstructured surfaces are reflection microscopy, interference microscopy, or environmental scanning electron mi-croscopy ESEM. In reflection microscopy, a droplet is placed on a microstructured sample and viewed from the side. Light reflection from the liquid-air interface between the droplet and the solid surface implies the Cassie–Baxter state, while no light reflection implies the Wenzel state. 2,16 This technique is limited to flat samples and solid micro-structures large enough to view using an optical microscope. Laser reflection has the same limitation and cannot measure the actual height of the air gap. 17 In interference microscopy, a
Electric-field–driven instabilities on superhydrophobic surfaces
EPL (Europhysics Letters), 2011
We study possible mechanisms of the transition from the Cassie state to the Wenzel state on superhydrophobic surfaces under the influence of electric fields as a function of the aspect ratio and the wettability of the surface. A simple analytical model for axisymmetric cavities and small deflections of the liquid menisci within the cavities reveals the existence of a novel electricfield-driven instability of the liquid surface. Fully self-consistent calculations of both electric-field distribution and surface profiles show that this instability evolves from a global one towards a local Taylor cone-like instability for increasing aspect ratio of the cavities. A two-dimensional map is derived indicating the prevalence of the interfacial instability as compared to the depinning scenario of the three-phase contact line, which is well known from ordinary superhydrophobic surfaces.
Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting
Applied Physics Letters, 2008
Using electrowetting, we demonstrate reversible spectral tuning of the whispering gallery modes of glycerol/water microdroplets standing on a superhydrophobic surface by up to 4.7 nm at 400 V. Our results can inspire electrically tunable optical switches and filters based on microdroplets on a superhydrophobic surface. The sensitivity of the observed spectral drift to the contact angle can also be used to measure the contact angles of microdroplets on a superhydrophobic surface.
Spontaneous recovery of superhydrophobicity on nanotextured surfaces
Proceedings of the National Academy of Sciences of the United States of America, 2016
Rough or textured hydrophobic surfaces are dubbed "superhydrophobic" due to their numerous desirable properties, such as water repellency and interfacial slip. Superhydrophobicity stems from an aversion of water for the hydrophobic surface texture, so that a water droplet in the superhydrophobic "Cassie state" contacts only the tips of the rough surface. However, superhydrophobicity is remarkably fragile and can break down due to the wetting of the surface texture to yield the "Wenzel state" under various conditions, such as elevated pressures or droplet impact. Moreover, due to large energetic barriers that impede the reverse transition (dewetting), this breakdown in superhydrophobicity is widely believed to be irreversible. Using molecular simulations in conjunction with enhanced sampling techniques, here we show that on surfaces with nanoscale texture, water density fluctuations can lead to a reduction in the free energetic barriers to dewetting by c...
Superhydrophobicity: Localized Parameters And Gradient Surfaces
Contact Angle, Wettability and Adhesion, Volume 6
The use of Cassie and Baxter's equation and that of Wenzel have been subject to some criticism of late. It has been suggested that researchers use these equations without always considering the assumptions that have been made and sometimes apply them to cases that are not suitable. This debate has prompted a reconsideration of the derivation of these equations using the concept of parameters for the Wenzel roughness and Cassie-Baxter solid surface fractions that are local to the three-phase contact lines. In such circumstances, we show the roughness and Cassie-Baxter solid fractions depend not only on the substrate material, but also on which part of the substrate is being sampled by the three-phase contact lines of a given droplet. We show that this is not simply a theoretical debate, but is one which has direct consequences for experiments on surfaces where the roughness or spatial pattern varies across the surface. We use the approach to derive formulae for the contact angle observed on a double length scale surface under the assumption that the smallscale features on the peaks of larger scale features are either wetted or non-wetted. We also discuss the case of curved and re-entrant surface features and how these bring the Young's law contact angle into the formula for roughness and the condition for suspending droplets without penetration into the surface. To illustrate the use of local parameters, we consider the case of a variation in Cassie-Baxter fraction across a surface possessing a homogeneous hydrophobic surface chemistry and discuss the conditions (droplet volume, surface hydrophobicity, gradient in superhydrophobicity and contact angle hysteresis) under which a droplet may be set into motion. We show that different contact angles on each side of a droplet of water placed on such a surface can generate sufficient lateral force for the droplet to move towards the region of the surface with the lowest contact angle. Using an electrodeposited copper surface with a radial gradient in superhydrophobicity we exemplify these ideas by showing experimentally that droplets enter into selfactuated motion and accumulate in the centre of the surface where the wettability is higher. In principle, paths can be defined and water droplets can be collected by creating such gradients in superhydrophobicity through changes in the lateral topography of the surface.