Contact angle hysteresis on rough hydrophobic surfaces
ABSTRACT In this short note, we report a quantitative investigation of the hysteresis of the Cassie and Wenzel drops on a given rough surface. The Cassie drop shows much less hysteresis compared to a Wenzel drop and is therefore preferred in applications involving moving droplets. The experimental measurements are compared with the various theoretical models for the apparent contact angles and recommendations are made.
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ABSTRACT: In the present study, we design, fabricate and characterize robust hydrophobic surfaces on which four different shapes (rectangle, circle, triangle and cross) of a micropillar array are patterned. Various micropillar arrays are designed to give rise to the same Wenzel and Cassie contact angles (CAs) of the surfaces through changes in the shape and size of the micropillar and the spacing between adjacent micropillars. The designed surfaces are fabricated using polydimethylsiloxane (PDMS) replica molding with photolithographically manufactured SU-8 masters. The hydrophobicities of the various PDMS surfaces are evaluated through theoretical and experimental measurements of the water CAs. The theoretical and experimental CAs are strongly correlated to each other, as expected. The generalization of the suggested design rule was carried out by introducing effective parameters of square-arranged micropillar arrays. The apparent contact angle of the robust hydrophobic surface monotonically increases as the top surface area decreases for a given perimeter and height.Journal of Micromechanics and Microengineering 01/2010; 20(2):025028. · 1.79 Impact Factor
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ABSTRACT: Roughness-based superhydrophobic surfaces have been extensively studied over the past decade. The primary objective in most of those studies has been to mimic nature, e.g. lotus leaves, to produce low-drag and/or self-cleaning surfaces. However, other applications of such engineered surfaces are being explored. Recent work on using superhydrophobic or superhydrophilic surfaces for dropwise condensation or nucleate boiling heat transfer applications is highlighted here. It is known that nucleate boiling as opposed to film boiling, and dropwise condensation as opposed to filmwise condensation can lead to much higher heat transfer coefficients. This can significantly improve the energy efficiency in variety of engineering applications. The key idea, e.g. in boiling applications, is to design supernucleating surfaces that provide ample nucleation sites for vapour bubbles to form but at the same time delay the formation of a vapour film next to the surface. It has been shown that this can be achieved by using rough superhydrophilic surfaces that provide roughness-based cavities or defects as nucleation sites. Similarly, superhydrophobic surfaces that encourage dropwise condensation are also envisaged. The goal of this article is also to summarize the key issues and challenges in designing supernucleating surfaces and to speculate approaches to overcome the challenges.Soft Matter 01/2010; 6(8). · 3.91 Impact Factor
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ABSTRACT: In this work, side view images of liquid–gas–solid interfaces are observed during the evaporation of liquid water droplets on various commercially available untreated gas diffusion layers (GDLs). The change in contact diameter as a function of evaporative volume loss is measured to quantify the unpinning rates of micro-sized droplets. This contact diameter pinning behaviour during evaporation is correlated to the material topography, which is quantified through profilometry measurements. The carbon fibre paper with the smallest average roughness (15 μm) exhibits the strongest degree of pinning (unpinning at a rate of 0.13 mm/μL). Higher average surface roughnesses for felt (30 μm) and cloth yarn (32 μm) result in higher unpinning rates, 0.21 mm/μL and 0.19 mm/μL, respectively. These results indicate that common GDL materials exhibit Cassie–Baxter wetting behaviour, and reduced GDL roughness promotes droplet pinning. The material-specific droplet contact diameter progression should be considered during GDL selection for polymer electrolyte membrane (PEM) fuel cells. This work provides insight into the effect of GDL material properties on gas channel water management, as water droplets are expected to experience similar pinning to that observed in this work within the cathode gas channels of a PEM fuel cell.International Journal of Hydrogen Energy. 01/2010;