Figure 1 summarizes this discussion the apparent

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Figure 1 summarizes this discussion: the apparent contact angle θ * is plotted as a function of the Young angle θ ,and the expected behaviours are shown by a solid line. We also represented the Cassie regime at a moderate hydrophobicity (90 ° < θ < θ c ; dotted line) to stress its metastability in this region. Superhydrophobic states AURÉLIE LAFUMA AND DAVID QUÉRÉ* Laboratoire de Physique de la Matière Condensée,UMR 7125 du CNRS,Collège de France,75231 Paris Cedex 05,France *e-mail:[email protected]de-france.fr Published online:22 June 2003; doi:10.1038/nmat924 1 1 1 + s φ θ 0 cos * cos c θ cos θ Wenzel Cassie Figure 1 The two models of superhydrophobicity. For a moderate hydrophobicity (90 ° < θ < θ c ,where θ is the contact angle on a flat surface,and θ c is fixed by the texture design,as defined in the text),the apparent contact angle θ * should be given by the Wenzel model (equation 1).If θ is larger than θ c ,air remains trapped below the drop,which sits on a composite surface made of solid and air; φ s is the fraction of solid in contact with the liquid (Cassie regime,equation 2).However it has often been reported that the Cassie regime can also be observed for θ < θ c ,in spite of a higher energy.This metastable situation is represented by a dotted line. © 2003 Nature Publishing Group
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LETTERS 458 nature materials | VOL 2 | JULY 2003 | In many cases, it turns out that drops on textured materials are observed to be in the Cassie regime,even for a moderate hydrophobicity or, equivalently, for a moderate roughness 5,7,9,10 . This is revealed in particular by a discontinuity of the contact angle when entering the hydrophobic region (equation 2 predicts that the angle should then be given by cos θ * = φ s 1),as reported previously 5 .This suggests that two superhydrophobic states might coexist,which raises the question of the transitions between these states.It has been reported that pressing on a drop may change the contact angle 7 , and that the drop size can also influence θ * (ref.10,also Pantankar,private communication).Here we discuss the existence of such transitions and the differences between both states. We stress in particular that even if the contact angles are comparable, their fluctuations (that is, the so-called hysteresis of the contact angle) are dramatically affected,which leads to a huge difference in adhesion properties,and can deeply affect the self-cleaning properties generally associated with superhydrophobic states. These experiments thus reveal a natural limit of microtextured surfaces, and also suggest a condition on the design of the surface for achieving a robust anti-adhesive state. For our experiments we used surfaces decorated with a square lattice of triangular spikes (of typical height and spacing 2 µ m; Autotype, Oxford). These textures were created by ultraviolet microreplication against a nickel master structure.The ultraviolet-curable material was a complex mixture of perfluoroacrylates and non-fluorinated acrylates that had been optimized to ensure migration (confirmed by secondary-
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  • Fall '19
  • Surface tension, Contact angle, Surface energy, Hysteresis, Wenzel, Cassie regime

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