Comment on How Wenzel and Cassie Were Wrong by Gao and McCarthy.
About: This article is published in Langmuir.The article was published on 2007-11-15. It has received 104 citations till now.
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TL;DR: The definition of superhydrophilic substrates has not been clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in the surface chemistry community.
Abstract: The term superhydrophilicity is only 11–12 years old and was introduced just after the explosion of research on superhydrophobic surfaces, in response to the demand for surfaces and coatings with exceptionally strong affinity to water. The definition of superhydrophilic substrates has not been clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in the last few years in the surface chemistry community. In this review, we take a close look into major definitions of hydrophilic surfaces used in the past, before we review the physics behind the superhydrophilic phenomenon and make recommendation on defining superhydrophilic surfaces and coatings. We also review chemical and physical methods used in the fabrication of substrates on surfaces of which water spreads completely. Several applications of superhydrophilic surfaces, including examples from the authors' own research, conclude this review.
702 citations
TL;DR: In this paper, the authors introduce the formalism and models for implementing contact angle hysteresis into relevant physical phenomena, such as sliding drops, coffee stain phenomenon (in general evaporative self-assembly), and curtain and wire coating techniques.
Abstract: Contact angle hysteresis is an important physical phenomenon. It is omnipresent in nature and also plays a crucial role in various industrial processes. Despite its relevance, there is a lack of consensus on how to incorporate a description of contact angle hysteresis into physical models. To clarify this, starting from the basic definition of contact angle hysteresis, we introduce the formalism and models for implementing contact angle hysteresis into relevant physical phenomena. Furthermore, we explain the influence of the contact angle hysteresis in physical phenomena relevant for industrial applications such as sliding drops, coffee stain phenomenon (in general evaporative self-assembly), and curtain and wire coating techniques
533 citations
TL;DR: This work fabricates a range of model superoleophobic surfaces with controlled surface topography in order to correlate the details of the local texture with the experimentally observed apparent contact angles, and modify the classical Cassie-Baxter relation to include a local differential texture parameter which enables it to quantitatively predict the apparent advancing and receding contact angles.
487 citations
TL;DR: The review begins with a discussion of superhydrophobicity, and then explores biomedical applications that are utilizingsuperhydrophobic biomaterials for medical applications in depth including material selection characteristics, in vitro performance, and in vivo performance.
307 citations
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TL;DR: It is demonstrated that the 1D (three-phase, solid/liquid/vapor) contact line perspective is simpler, more intuitive, more useful, and more consistent with facts than the disproved but widely held-to-be-correct 2D view.
Abstract: We review our 2006-2009 publications on wetting and superhydrophobicity in a manner designed to serve as a useful primer for those who would like to use the concepts of this field. We demonstrate that the 1D (three-phase, solid/liquid/vapor) contact line perspective is simpler, more intuitive, more useful, and more consistent with facts than the disproved but widely held-to-be-correct 2D view. We give an explanation of what we believe to be the reason that the existing theoretical understanding is wrong and argue that the teaching of surface science over the last century has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. We review our analyses of the phenomena of contact angle hysteresis, the lotus effect, and perfect hydrophobicity and suggest that needlessly complex theoretical understandings, incorrect models, and ill-defined terminology are not useful and can be destructive.
289 citations
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11,004 citations
TL;DR: In this paper, the authors present an attempt towards a unified picture with special emphasis on certain features of "dry spreading": (a) the final state of a spreading droplet need not be a monomolecular film; (b) the spreading drop is surrounded by a precursor film, where most of the available free energy is spent; and (c) polymer melts may slip on the solid and belong to a separate dynamical class, conceptually related to the spreading of superfluids.
Abstract: The wetting of solids by liquids is connected to physical chemistry (wettability), to statistical physics (pinning of the contact line, wetting transitions, etc.), to long-range forces (van der Waals, double layers), and to fluid dynamics. The present review represents an attempt towards a unified picture with special emphasis on certain features of "dry spreading": (a) the final state of a spreading droplet need not be a monomolecular film; (b) the spreading drop is surrounded by a precursor film, where most of the available free energy is spent; and (c) polymer melts may slip on the solid and belong to a separate dynamical class, conceptually related to the spreading of superfluids.
6,042 citations
TL;DR: In this paper, the wetting behavior on surfaces with a single, circular heterogeneous island was studied, where small sessile drops were deposited on the center of an island and liquid was sequentially added, eventually forcing the contact line to advance beyond the island perimeter onto the surrounding area.
Abstract: Wetting behavior was studied on surfaces with a single, circular heterogeneous island. Lyophobic islands were created on lyophilic Si wafers using polystyrene. Alternately, lyophobic perfluoroalkoxy fluoropolymer film was etched to make lyophilic domains. Contact angles and hysteresis were measured with water and hexadecane. Small sessile drops were deposited on the center of an island and liquid was sequentially added, eventually forcing the contact line to advance beyond the island perimeter onto the surrounding area. Even though the underlying contact area contained a mixture of lyophilic and lyophobic domains, the contact angles, both advancing and receding, were equal to the angles exhibited by the homogeneous periphery. Or in other words, if the heterogeneity was completely contained with the contact area and did not intersect the contact line, then no area averaging of the contact angles occurred. These findings suggest that interactions at the contact line, not the contact area, control wetting of...
365 citations
TL;DR: It is argued that Cassie-Baxter and Wenzel models are not so much wrong as have assumptions that define the limitations on their applicability and that with suitable generalization they can be used with surfaces possessing some types of spatially varying defect distributions.
Abstract: The properties of superhydrophobic surfaces are often understood by reference to the Cassie-Baxter and Wenzel equations. Recently, in a paper deliberately entitled to be provocative, it has been suggested that these equations are wrong; a suggestion said to be justified using experimental data. In this paper, we review the theoretical basis of the equations. We argue that these models are not so much wrong as have assumptions that define the limitations on their applicability and that with suitable generalization they can be used with surfaces possessing some types of spatially varying defect distributions. We discuss the relationship of the models to the previously published experiments and using minimum energy considerations review the derivations of the equations for surfaces with defect distributions. We argue that this means the roughness parameter and surface area fractions are quantities local to the droplet perimeter and that the published data can be interpreted within the models. We derive versions of the Cassie-Baxter and Wenzel equations involving roughness and Cassie-Baxter solid fraction functions local to the three-phase contact line on the assumption that the droplet retains an average axisymmetry shape. Moreover, we indicate that, for superhydrophobic surfaces, the definition of droplet perimeter does not necessarily coincide with the three-phase contact line. As a consequence, the three-phase contact lines within the contact perimeter beneath the droplet can be important in determining the observed contact angle on superhydrophobic surfaces.
314 citations
01 May 2002
108 citations