Ion mass spectrometry of the fluorinated side chains

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ion mass spectrometry) of the fluorinated side-chains to the nickel interface before crosslinking (S.Abbott,personal communication). On such flat solids,the so-called (static) advancing contact angle of water measured using a goniometer was found to be θ = 110 ± 3 ° (confirming the hydrophobic nature of the material), and the (static) receding angle was found to be 80 ± 3 ° . Τ he contact-angle hysteresis θ was therefore 30 ° .A water drop deposited on the microtextured surface (first snapshot in Fig.2) was indeed found to have a much larger angle, at θ *=164 ± 3 ° .In addition,the hysteresis was observed to be extremely low: θ * = 5°,which is a sign of air trapping 3 :because the drop sits on a cushion of air, its pinning on the solid (which is responsible for the hysteresis) is highly reduced. However, as is usually the case with solids decorated with a single pattern, the substrate is not very rough, and its hydrophobicity is moderate ( θ = 110°). We should thus be in the metastable part of the Cassie regime defined by the dotted line in Fig. 1; we tried to understand if a transition could be induced between this metastable state and the Wenzel regime. Our first attempt consisted of measuring the contact angle of a drop obtained by condensing water vapour on the same textured solid(a flux of oversaturated vapour was directed towards the substrate until a macroscopic drop was obtained).In this situation,water should explore the cavities of the solid, and the condensation should lead to a Wenzel drop.We indeed found that such a drop has an advancing contact angle significantly smaller than in a deposition experiment. Instead of 164 ° , we observed θ *=141 ± 3 ° .Moreover,the contact angle hysteresis θ * is greatly affected: it was found to be 100–105 ° (instead of 5 ° ).Both these measurements confirm that we induced a Wenzel situation.In particular, the contact-angle hysteresis should indeed increase by a large amount in this state:as it recedes,the drop is in contact with a textured solid surface 130 135 140 145 150 155 160 165 170 0 50 100 150 200 250 300 P (Pa) θ * ( ° ) Figure 2 Compression of a millimetric water drop between two identical microtextured hydrophobic surfaces. The apparent contact angle θ * is measured as a function of the imposed pressure P (open points).The filled points are obtained by varying the size of drops deposited on a single microtextured surface,which modifies the pressure P applied by the drop on the surface.The upper and lower dotted lines are,respectively,the values measured for a large drop initially formed either by deposition or condensation. © 2003 Nature Publishing Group
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LETTERS nature materials | VOL 2 | JULY 2003 | 459 filled with liquid,and the contact angle is then given by a Cassie average between θ ,the angle on the solid,and 0 the angle of the liquid on itself.
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  • Fall '19
  • Surface tension, Contact angle, Surface energy, Hysteresis, Wenzel, Cassie regime

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