4,738 research outputs found
The superhydrophobicity of polymer surfaces: Recent developments
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: 1203â1217, 201
An introduction to superhydrophobicity
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
Perpetual superhydrophobicity
A liquid droplet placed on a geometrically textured surface may take on a âsuspendedâ state, in which the liquid wets only the top of the surface structure, while the remaining geometrical features are occupied by vapor. This superhydrophobic CassieâBaxter state is characterized by its composite interface which is intrinsically fragile and, if subjected to certain external perturbations, may collapse into the fully wet, so-called Wenzel state. Restoring the superhydrophobic CassieâBaxter state requires a supply of free energy to the system in order to again nucleate the vapor. Here, we use microscopic classical density functional theory in order to study the CassieâBaxter to Wenzel and the reverse transition in widely spaced, parallel arrays of rectangular nanogrooves patterned on a hydrophobic flat surface. We demonstrate that if the width of the grooves falls below a threshold value of ca. 7 nm, which depends on the surface chemistry, the Wenzel state becomes thermodynamically unstable even at very large positive pressures, thus realizing a âperpetualâ superhydrophobic CassieâBaxter state by passive means. Building upon this finding, we demonstrate that hierarchical structures can achieve perpetual superhydrophobicity even for micron-sized geometrical textures
Is superhydrophobicity robust with respect to disorder?
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
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
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Recovering superhydrophobicity in nanoscale and macroscale surface textures
Here, we investigate complete drying of hydrophobic cavities in order to elucidate its dependence on the size of confinement, its geometry, and the degree of hydrophobicity. Two complementary theoretical approaches are adopted: a macroscopic one based on classical capillarity and a microscopic classical density functional theory. This combination allows us to pinpoint unique drying mechanisms at the nanoscale and to clearly differentiate them from the mechanisms operational at the macroscale. Nanoscale hydrophobic cavities allow the thermodynamic destabilization of the confined liquid phase over an unexpectedly broad range of conditions, including pressures as large as 10 MPa and contact angles close to 90°. On the other hand, for cavities on the micron scale, such destabilization occurs only for much larger contact angles and close to liquid-vapor coexistence. These scale-dependent drying mechanisms are used to propose design criteria for hierarchical superhydrophobic surfaces capable of spontaneous self recovery over a broad range of operating conditions. In particular, we detail the requirements under which it is possible to realize perpetual superhydrophobicity at positive pressures on surfaces with micron-sized textures by exploiting drying, facilitated by nanoscale coatings. Concerning the issue of superhydrophobicity, these findings indicate a promising direction both for surface fabrication and for the experimental characterization of perpetual surperhydrophobicity. From a more basic perspective, the present results have an echo on a wealth of biological problems in which hydrophobic confinement induces drying, such as in protein folding, molecular recognition, and hydrophobic gating
Function-led design of multifunctional stimuli-responsive superhydrophobic surface based on hierarchical graphene-titania nanocoating
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
Nano-scale superhydrophobicity: suppression of protein adsorption and promotion of flow-induced detachment
Wall adsorption is a common problem in microfluidic devices, particularly when proteins are used. Here we show how superhydrophobic surfaces can be used to reduce protein adsorption and to promote desorption. Hydrophobic surfaces, both smooth and having high surface roughness of varying length scales (to generate superhydrophobicity), were incubated in protein solution. The samples were then exposed to flow shear in a device designed to simulate a microfluidic environment. Results show that a similar amount of protein adsorbed onto smooth and nanometer-scale rough surfaces, although a greater amount was found to adsorb onto superhydrophobic surfaces with micrometer scale roughness. Exposure to flow shear removed a considerably larger proportion of adsorbed protein from the superhydrophobic surfaces than from the smooth ones, with almost all of the protein being removed from some nanoscale surfaces. This type of surface may therefore be useful in environments, such as microfluidics, where protein sticking is a problem and fluid flow is present. Possible mechanisms that explain the behaviour are discussed, including decreased contact between protein and surface and greater shear stress due to interfacial slip between the superhydrophobic surface and the liquid
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