Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties (original) (raw)
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Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures
Physical Review Letters, 2010
When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.
Numerical study of energetics and wetting stability of liquid droplets on microtextured surfaces
The suspended wetting state (Cassie-Baxter state) on a microstructured surface tends to collapse to a wetted state (Wenzel state) if the liquid-air interface is perturbed. Multiple metastable Cassie-Baxter (CB) wetting states, separated by an energy barrier from Wenzel state, may also exist. In this study, numerical method is applied to study the wetting properties of liquid droplets on a variety of microtextured surfaces with a particular focus on the stability of the CB wetting state. A dimen-sionless form of droplet energy is used to compare the relative stabilities of multiple metastable states. The sequence of stable drop configurations with increasing droplet volume on a particular substrate is analyzed for both isotropic and anisotropic cases. Applying dimensional variation, characterized by the pillar spacing and pillar width, on surface microtexture, the key parameter which plays dominant role in the stability of droplet is explored. The solid-fraction that the droplet avails at the drop-base is observed to be the most vital parameter for the droplet stability. Spreading of droplet from one isotropic wetting configuration to an aniso-tropic configuration is not favorable unless the spreading of the droplet is restricted to be unidirectional.
Wetting characteristics of liquid drops at heterogeneous surfaces
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994
We!l-defined heterogeneous surfaces consisting of hydrophobic and hydrophilic regions were prepared on gold (a 2000 A gold film supported on an Si/SiO2/Ti substrate) by patterning self-assembled monolayers (SAMs), using an elastomer stamp. One surface was composed of alternating and parallel hydrophobic (2.5 ~) and hydrophilic (3 ~) strips, and the second surface consisted of alternating hydrophilic squares (3 ~ x 3 ~) separated by hydrophobic strips (2.5 ~). The wetting characteristics of these well-defined heterogeneous solid surfaces were examined by contact angle measurements. The contact angles for water drops, which varied in pH from 5.8 to 10.0, were measured with the strips both tangential to and normal to the three-phase contact line. The experimental contact angles are in good agreement with theory as calculated from the Cassie equation when the three-phase contact line is non-contorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand, when the strips are normal to the drop edge, corrugation of the three-phase contact line affects the contact angle significantly. Contact angles, measured with the strips normal to the drop edge, were lower by 7-160 than those calculated from the Cassie equation. Analysis of these measurements, together with contact angle/drop size measurements for fully hydrophobic and hydrophilic surfaces, demonstrate the validity of a modified Cassie equation that includes a term describing the line tension contribution.
The influence of Wettability on the Droplet Impact onto Micro-Structured Surfaces
International Conference on Liquid Atomization and Spray Systems (ICLASS)
The flourishing of applications in need of self-cleaning mechanisms increased the search for water repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale interface phenomena on the wetting behavior is essential to design efficient hydrophobic materials with defined topography. On the other hand, the spreading behavior concerning the formation of thin films on a surface is required to assure the quality of spray cooling and coatings. The contact angle undoubtedly plays an important role in the droplet impact, providing different outcomes. Moreover, an open question is, how surface topography can affect the impact process. Therefore, to evaluate these matters, different surface patterns were manufactured to assess the surface topography influence on the impact dynamic behavior. Additionally, the wettability of the micro-structured surfaces was flexibly influenced through plasma activation and plasma polymerization. The impact of distilled water and isopropanol droplets on the different surface patterns was captured from three perspectives providing high-quality images of the phenomena. Different surface morphologies can be obtained depending on the surface micro-structures and wettability, affecting spreading shape and evolution. The fluid penetration within the micro-structures is a key feature influencing not only the structures of the outcomes but also the transition between regimes.
Direct and accurate measurement of size dependent wetting behaviors for sessile water droplets
Scientific reports, 2015
The size-dependent wettability of sessile water droplets is an important matter in wetting science. Although extensive studies have explored this problem, it has been difficult to obtain empirical data for microscale sessile droplets at a wide range of diameters because of the flaws resulting from evaporation and insufficient imaging resolution. Herein, we present the size-dependent quantitative change of wettability by directly visualizing the three phase interfaces of droplets using a cryogenic-focused ion beam milling and SEM-imaging technique. With the fundamental understanding of the formation pathway, evaporation, freezing, and contact angle hysteresis for sessile droplets, microdroplets with diameters spanning more than three orders of magnitude on various metal substrates were examined. Wetting nature can gradually change from hydrophobic at the hundreds-of-microns scale to super-hydrophobic at the sub-μm scale, and a nonlinear relationship between the cosine of the contact ...
Procedia Engineering, 2015
Designing hydrophobic surfaces with controllable wettability has attracted much interest in the recent times. The present study seeks to simulate the static and dynamic wetting behavior of liquid droplets on horizontal flat and microgrooved surfaces and compare the findings with experimentally obtained data. Using an open-source software, a 3D drop-shape model is developed to numerically analyze the shape of liquid droplets and anisotropic wetting for a wide range of parametric space. The effects of droplet volume, variation in the microgroove geometry and wettability gradient along the parallel and perpendicular directions of the microgrooved surfaces etc. on the drop shape and apparent contact angle are examined. Simulation and analysis are extended to analyze the wetting behavior of V-grooved geometry and are compared with the findings on rectangular microgrooved surfaces. For creating wettability gradient along the parallel direction of the grooves, periodic PDMS (Polydimethylsiloxane) coating is considered and increased hydrophobicity of the surface is observed with significant increase in the parallel contact angle for this case. The simulated results manifest considerable differences in the wetting pattern of the microgrooved and flat surfaces and are found out to be in good agreement with the experimental findings.
Small droplets on superhydrophobic substrates
We investigate the wetting behavior of liquid droplets on rough hydrophobic substrates for the case of droplets that are of comparable size to the surface asperities. Using a simple three-dimensional analytical free-energy model, we have shown in a recent letter ͓M. Gross, F. Varnik, and D. Raabe, EPL 88, 26002 ͑2009͔͒ that, in addition to the well-known Cassie-Baxter and Wenzel states, there exists a further metastable wetting state where the droplet is immersed into the texture to a finite depth, yet not touching the bottom of the substrate. Due to this new state, a quasistatically evaporating droplet can be saved from going over to the Wenzel state and instead remains close to the top of the surface. In the present paper, we give an in-depth account of the droplet behavior based on the results of extensive computer simulations and an improved theoretical model. In particular, we show that releasing the assumption that the droplet is pinned at the outer edges of the pillars improves the analytical results for larger droplets. Interestingly, all qualitative aspects, such as the existence of an intermediate minimum and the "reentrant transition," remain unchanged. We also give a detailed description of the evaporation process for droplets of varying sizes. Our results point out the role of droplet size for superhydrophobicity and give hints for achieving the desired wetting properties of technically produced materials.
Simultaneous dropwise and filmwise condensation on hydrophilic microstructured surfaces
International Journal of Heat and Mass Transfer, 2017
While wicking or spreading of a liquid through microstructures has been found to be promising for applications such as textiles, microelectronics or heat sinks, the effects of such structured surfaces on condensation phase change has received less attention. On a hydrophilic surface and for a fixed micropillar aspect ratio (height/diameter), the spacing between pillars is found to have a strong impact on the dynamics of condensation and on the final morphology of the condensate. In the case of micropillars with a large spacing between pillars, the condensate grows initially dropwise, and thereafter, as condensation develops, the condensate overcomes the pillars' height flooding the substrate, and condensation continuous in a filmwise condensation (FWC) fashion. In contrast, filmwise condensation and the continuous nucleation, growth, and departure of drops at the pillars' tops in a dropwise condensation (DWC) fashion occurs when the spacing between pillars is decreased. In this configuration, the geometry of the microstructures constrains the condensate between the pillars and rise of the condensate interface above the micropillars' height is not thermodynamically favorable, while the top of the pillars act as nucleation sites. We refer to this latter condensation behavior as simultaneous dropwise/filmwise condensation. These observations were enabled by the excellent spatial and temporal resolution of Environmental Scanning Electron Microscopy. A heat transfer model is proposed to demonstrate the greater heat transfer performance of the simultaneous dropwise/filmwise condensation behavior on these surfaces when compared to solely filmwise condensation. The enhanced heat transfer is realizable due to the ability to maintain a thin film within the microstructures and to the active dropwise condensation at the micropillars' tops. We report for the first time the occurrence of dropwise condensation on a completely hydrophilic wettability configuration without the assistance of a hydrophobic coating. Our findings pave the way to the development of microstructures for enhanced condensation heat transfer.
Physical Review Letters, 2012
Evaporation of a sessile droplet is a complex, nonequilibrium phenomenon. Although evaporating droplets upon superhydrophobic surfaces have been known to exhibit distinctive evaporation modes such as a constant contact line (CCL), a constant contact angle (CCA), or both, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive. We show that the onset time for the CCL-CCA transition and the critical base size at the Cassie-Wenzel transition exhibit remarkable dependence on the surface roughness. Through global interfacial energy analysis we reveal that, when the size of the evaporating droplet becomes comparable to the surface roughness, the line tension at the triple line becomes important in the prediction of the critical base size. Last, we show that both the CCL evaporation mode and the Cassie-Wenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.