Contact Angle Hysteresis Generated by Strong Dilute Defects

Reyssat, Mathilde; Quéré, David

doi:10.1021/jp8066876

Cited by 180 publications

(273 citation statements)

References 28 publications

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“…This pinning force also provides an estimate for the roll-off angle α. Therefore, we compare the pinning force to the force caused by the weight when tilting the plate (45). The component of the weight of the drop parallel to the surface is F k = V ρ g sin α.…”

Section: Resultsmentioning

confidence: 99%

How superhydrophobicity breaks down

Papadopoulos

Mammen

Deng

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

420

442

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A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar's circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water-air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.micropillars | pinning | wetting transition | contact angle | lithography | self-cleaning | hysteresis

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Section: Resultsmentioning

confidence: 99%

How superhydrophobicity breaks down

Papadopoulos

Mammen

Deng

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

420

442

View full text Add to dashboard Cite

show abstract

“…A large number of papers have focused on superhydrophobic surfaces. [32][33][34][51][52][53][54][55][56]60,61 Those studies have employed microscale patterning with hydrophobic pillars and voids where liquid did not penetrate. They are different from what we report here.…”

Section: ■ Discussionmentioning

confidence: 99%

Drop Detachment and Motion on Fuel Cell Electrode Materials

Gauthier

Hellstern

Kevrekidis

et al. 2012

ACS Appl. Mater. Interfaces

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Liquid water is pushed through flow channels of fuel cells, where one surface is a porous carbon electrode made up of carbon fibers. Water drops grow on the fibrous carbon surface in the gas flow channel. The drops adhere to the superficial fiber surfaces but exhibit little penetration into the voids between the fibers. The fibrous surfaces are hydrophobic, but there is a substantial threshold force necessary to initiate water drop motion. Once the water drops begin to move, however, the adhesive force decreases and drops move with minimal friction, similar to motion on superhydrophobic materials. We report here studies of water wetting and water drop motion on typical porous carbon materials (carbon paper and carbon cloth) employed in fuel cells. The static coefficient of friction on these textured surfaces is comparable to that for smooth Teflon. But the dynamic coefficient of friction is several orders of magnitude smaller on the textured surfaces than on smooth Teflon. Carbon cloth displays a much smaller static contact angle hysteresis than carbon paper due to its two-scale roughness. The dynamic contact angle hysteresis for carbon paper is greatly reduced compared to the static contact angle hysteresis. Enhanced dynamic hydrophobicity is suggested to result from the extent to which a dynamic contact line can track topological heterogeneities of the liquid/solid interface. KEYWORDS: contact angle, hydrophobic, carbon, wetting, carbon fibers, contact angle hysteresis, Teflon ■ INTRODUCTIONWater transport is an essential element of polymer electrolyte membrane (PEM) fuel cell operation. 1−5 Liquid water that is formed at a catalyst/membrane interface is pushed through porous electrodes into gas flow channels. The water drops emerge from the largest pores in the electrode, and grow in the channel. Initially, the drops are held in place by adhesion to the water column in the electrode pore, but as the drops grow surface contact with the porous electrode and surface contact with the walls of the flow channel are the dominate adhesive forces. The drop must be detached and pushed through the gas flow channel in contact with the electrode surface to be removed from the fuel cell. Efficient fuel cell operation requires removal of liquid water drops from the fuel cell with minimal work. The surface properties of the porous electrode play a key role in the energy required to remove water drops from the gas flow channels of the fuel cell. We have been devising experiments to isolate the flow resistances for water transport through the porous electrode and gas flow channel. 6−8 The porous electrode is required to carry the electron current from the electrocatalyst to the external circuit. 9 It must also permit transport of gas reactants and liquid product between the gas flow channels and the catalyst/PEM interface. 4,10,11 The porous electrode, or gas diffusion layer (GDL), of the fuel cell is typically made from carbon fibers, either as a random sheet of fibers in the form of carbon paper or as a woven arr...

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“…There is evidence that defects, contact line perimeter and sharp points induce hysteresis [50,51]. Indeed one of the main theories for the existence of hysteresis of any sort is the presence of areas of different contact angle within the surface of a sample caused by local slope or chemistry, suggesting that a perfect single crystal could have very low hysteresis.…”

Section: Superhydrophobicity and Contact Angle Hysteresismentioning

confidence: 99%