5,490 research outputs found

    The superhydrophobicity of polymer surfaces: Recent developments

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    Superhydrophobicity is the extreme water repellence of highly textured surfaces. The field of superhydrophobicity research has reached a stage where huge numbers of candidate treatments have been proposed and jumps have been made in theoretically describing them. There now seems to be a move to more practical concerns and to considering the demands of individual applications instead of more general cases. With these developments, polymeric surfaces with their huge variety of properties have come to the fore and are fast becoming the material of choice for designing, developing, and producing superhydrophobic surfaces. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1203–1217, 201

    An introduction to superhydrophobicity

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    This paper is derived from a training session prepared for COST P21. It is intended as an introduction to superhydrophobicity to scientists who may not work in this area of physics or to students. Superhydrophobicity is an effect where roughness and hydrophobicity combine to generate unusually hydrophobic surfaces, causing water to bounce and roll off as if it were mercury and is used by plants and animals to repel water, stay clean and sometimes even to breathe. The effect is also known as The Lotus EffectÂź and Ultrahydrophobicity. In this paper we introduce many of the theories used, some of the methods used to generate surfaces and then describe some of the implications of the effect

    Is superhydrophobicity robust with respect to disorder?

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    We consider theoretically the Cassie-Baxter and Wenzel states describing the wetting contact angles for rough substrates. More precisely, we consider different types of periodic geometries such as square protrusions and disks in 2D, grooves and nanoparticles in 3D and derive explicitly the contact angle formulas. We also show how to introduce the concept of surface disorder within the problem and, inspired by biomimetism, study its effect on superhydrophobicity. Our results, quite generally, prove that introducing disorder, at fixed given roughness, will lower the contact angle: a disordered substrate will have a lower contact angle than a corresponding periodic substrate. We also show that there are some choices of disorder for which the loss of superhydrophobicity can be made small, making superhydrophobicity robust

    Unraveling the Salvinia paradox: design principles for submerged superhydrophobicity

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    The complex structure of the Salvinia molesta is investigated via rare event molecular dynamics simulations. Results show that a hydrophilic/hydrophobic patterning together with a re-entrant geometry control the free energy barriers for bubble nucleation and for the Cassie-Wenzel transition. This natural paradigm is translated into simple macroscopic design criteria for engineering robust superhydrophobicity in submerged applications

    Superhydrophobicity: Localized Parameters And Gradient Surfaces

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    The use of Cassie and Baxter's equation and that of Wenzel has 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 small-scale 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 self-actuated 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

    Superhydrophobic Surface by Replication of Laser Micromachined Pattern in Epoxy/Alumina Nanoparticle Composite

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    Superhydrophobic surfaces were obtained by superposition of microstructure—defined by replication of laser micromachined masters, with nanostructure—created by durable epoxy/Îł-Al₂O₃ nanoparticle composite, used for replication. Hierarchical surface topography thus obtained consisted of hexagonally spaced microcavities and nanoparticle agglomerates, exposed on the replica surface by radio frequency (RF) air plasma etching. Surface topography was further enhanced by rims around the microcavity edges, resulting from nanosecond laser micromachining defects in aluminum masters. Subsequent wet chemical hydrophobization with 1H,1H,2H,2H-perfluorotetradecyltriethoxysilane (PFTDTES) provided superhydrophobic behavior in replicas with a microcavity spacing of 30 Όm, as indicated by a water contact angle of 160° and a sliding angle of 8°. The preparation method is relatively simple, inexpensive, and potentially scalable.This work was supported by the National Science Centre of Poland through projects nos. 2011/03/N/ST8/05879 and UMO-2012/05/B/ST8/02876. The authors are grateful to Professor Weimin Liu and Professor Feng Zhou of the Lanzhou Institute of Chemical Physics, Chinese Academy of Science, for making the laser micromachining, JEOL SEM, and Drop Shape Analysis equipment available

    Function-led design of multifunctional stimuli-responsive superhydrophobic surface based on hierarchical graphene-titania nanocoating

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    Multifunctional smart superhydrophobic surface with full-spectrum tunable wettability control is fabricated through the self-assembly of the graphene and titania nanofilm double-layer coating. Advanced microfluidic manipulative functions, including directional water transport, adhesion & spreading controls, droplet storage & transfer, and droplet sensing array, can be readily realized on this smart surface. An in-depth mechanism study regarding the underlying secrets of the tunable wettability and the UV-induced superhydrophilic conversion of anatase titania are also presented