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Drop impact upon micro- and nanostructured superhydrophobic surfaces  [PDF]
Peichun Tsai,Sergio Pacheco,Christophe Pirat,Leon Lefferts,Detlef Lohse
Physics , 2009, DOI: 10.1021/la900330q
Abstract: We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number \We and the roughness of the surface. At small \We, i.e. small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large \We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small \We $\apprle 120$ but an important one for large \We $\apprge 120$. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime \We $\apprle 150$
Introduction to microstructured optical fibers  [PDF]
M. Yan
Physics , 2005,
Abstract: This article recounts definition, classification, history, and applications of microstructured optical fibers.
Tailoring surface interactions, contact angles, drop topologies and self-assembly using laser irradiation  [PDF]
John Canning,Hadrien Weil,Masood Naqshbandi,Kevin Cook,Matthieu Lancry
Physics , 2012, DOI: 10.1364/OME.3.000284
Abstract: UV laser irradiation (lambda = 193 nm), below and above damage thresholds, is used to both alter and pattern the surface properties of borosilicate slides to tune and control the contact angle of a water drop over the surface. Large variation exceeding 25 deg using laser processing alone, spanning across both sides of the original contact angle of the surface, is reported. An asymmetric contact angle distribution, giving rise to an analogous ellipsoidal-like drop caplet, is shown to improve convective self-assembly of silica nanoparticles into straighter microwires over a spherical caplet.
Stability of a granular layer on an inclined "fakir plane"  [PDF]
Jesica Benito,Yann Bertho,Irene Ippolito,Philippe Gondret
Physics , 2012, DOI: 10.1209/0295-5075/100/34004
Abstract: We present here experimental results on the effect of a forest of cylinder obstacles (nails) on the stability of a granular layer over a rough incline, in a so-called "fakir plane" configuration. The nail forest is found to increase the stability of the layer, the more for the densest array, and such an effect is recovered by a simple model taking into account the additional friction force exerted by the pillar forest onto the granular layer.
Superhydrophobicity from microstructured surface
Lijun Zheng,Xuedong Wu,Zeng Lou,Dan Wu
Chinese Science Bulletin , 2004, DOI: 10.1007/BF03183400
Abstract: Superhydrophobicity is referred to the wettability of a solid surface which has a water apparent contact angle greater than 150°. It has attracted great interest in both fundamental researches and practical applications. This paper discusses two models: Wenzel model and Cassie model, to describe the superhydrophobic states of surface. The effects of surface morphology and microstructure on superhydrophobicity are discussed, and the internal relationship between Wenzel and Cassie states is presented. These two superhydrophobic states can coexist and they present different properties on contact angle hysteresis. It is reported that the irreversible transition can be realized from Cassie state to Wenzel state under some certain conditions. This paper also gives a review of recent progresses in the strategies of fabricating superhydrophobic surfaces by designing microstructured or microtextured surfaces. Finally, the fundamental research and applications of superhydrophobic surfaces are prospected.
Drops on soft solids: Free energy and double transition of contact angles  [PDF]
Luuk A. Lubbers,Joost H. Weijs,Lorenzo Botto,Siddhartha Das,Bruno Andreotti,Jacco H. Snoeijer
Physics , 2013, DOI: 10.1017/jfm.2014.152
Abstract: The equilibrium shape of liquid drops on elastic substrates is determined by minimising elastic and capillary free energies, focusing on thick incompressible substrates. The problem is governed by three length scales: the size of the drop $R$, the molecular size $a$, and the ratio of surface tension to elastic modulus $\gamma/E$. We show that the contact angles undergo two transitions upon changing the substrates from rigid to soft. The microscopic wetting angles deviate from Young's law when $\gamma/Ea \gg 1$, while the apparent macroscopic angle only changes in the very soft limit $\gamma/ER \gg 1$. The elastic deformations are worked out in the simplifying case where the solid surface energy is assumed constant. The total free energy turns out lower on softer substrates, consistent with recent experiments.
Droplet bouncing and skipping on microstructured hydrophobic surfaces  [PDF]
Rajat Saksena,Andrew Cannon,Julio Manuel Barros Jr.,William Paul King,Kenneth T. Christensen
Physics , 2009,
Abstract: The impact of a jet of droplets upon surfaces of varying hydrophobicity is studied via high-speed imaging. Microstructures on silicone surfaces consisting of cylindrical pillars of varying sizes and spacings are utilized to enhance hydrophobicity. Comparison of droplet motion after impact with these microstructured surfaces is contrasted with that noted for plain glass (hydrophilic) and flat silicone surfaces. Fluid dynamics videos are captured at 6000 fps and played back at 30 fps over a field of view of 1.35 cm (height) X 2.7 cm (width) that is back-illuminated with an LED array for 800-micron diameter droplets impinging the surfaces at 2.5 m/s with an angle of incidence of 38 degrees (relative to the surface). Bouncing of droplets after impact is not apparent for the glass and unstructured silicone cases, though many droplets were observed to roll along the surface in the latter case which is consistent with its slightly hydrophobic nature. In contrast, droplets were found to both skip and bounce upon impacting the microstructured surfaces which indicates a significant enhancement in hydrophobicity due to these surface features.
Hydrodynamic friction of fakir-like super-hydrophobic surfaces  [PDF]
Anthony M. J. Davis,Eric Lauga
Physics , 2010, DOI: 10.1017/S0022112010003460
Abstract: A fluid droplet located on a super-hydrophobic surface makes contact with the surface only at small isolated regions, and is mostly in contact with the surrounding air. As a result, a fluid in motion near such a surface experiences very low friction, and super-hydrophobic surfaces display strong drag-reduction in the laminar regime. Here we consider theoretically a super-hydrophobic surface composed of circular posts (so called fakir geometry) located on a planar rectangular lattice. Using a superposition of point forces with suitably spatially-dependent strength, we derive the effective surface slip length for a planar shear flow on such a fakir surface as the solution to an infinite series of linear equations. In the asymptotic limit of small surface coverage by the posts, the series can be interpreted as Riemann sums, and the slip length can be obtained analytically. For posts on a square lattice, our analytical results are in excellent quantitative agreement with previous numerical computations.
Quantitative testing of robustness on super-omniphobic surfaces by drop impact  [PDF]
Thi Phuong Nhung Nguyen,Philippe Brunet,Yannick Coffinier,Rabah Boukherroub
Physics , 2010,
Abstract: The quality of a liquid-repellent surface is quantified by both the apparent contact angle $\theta_0$ that a sessile drop adopts on it, and the value of the liquid pressure threshold the surface can withstand without being impaled by the liquid, hence keeping a low-friction condition. We designed surfaces covered with nano-wires obtained by the vapor-liquid-solid (VLS) growth technique, that are able to repel most of the existing non-polar liquids including those of very low surface tension, as well as many polar liquids of moderate to high surface tension. These super-omniphobic surfaces exhibit apparent contact angles ranging from 125 to 160$^{\circ}$ depending on the liquid. We tested the robustness of the surfaces against impalement by carrying out drop impact experiments. Our results show how this robustness depends on the Young's contact angle $\theta_0$ related to the surface tension of the liquid, and that the orientational growth of NWs is a favorable factor for robustness.
Impalement transitions in droplets impacting microstructured superhydrophobic surfaces  [PDF]
J. Hyv?luoma,J. Timonen
Physics , 2008, DOI: 10.1209/0295-5075/83/64002
Abstract: Liquid droplets impacting a superhydrophobic surface decorated with micro-scale posts often bounce off the surface. However, by decreasing the impact velocity droplets may land on the surface in a fakir state, and by increasing it posts may impale droplets that are then stuck on the surface. We use a two-phase lattice-Boltzmann model to simulate droplet impact on superhydrophobic surfaces, and show that it may result in a fakir state also for reasonable high impact velocities. This happens more easily if the surface is made more hydrophobic or the post height is increased, thereby making the impaled state energetically less favourable.
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