| Abstract
| - Very rough surfaces can suspend small water drops, prevent wetting, and cause contact angles to approach180°. A criterion based on contact line density is proposed for predicting the conditions that produce theseultrahydrophobic surfaces, where, above a critical value, drops are suspended by asperities. Critical valuesof the contact line density can be calculated from contact angles, asperity shape, and information aboutthe contact liquid, such as density and surface tension. The criterion was found to correctly predict suspensionfor several model surfaces prepared by lithography techniques. Apparent contact angles from suspendedand collapsed drops also were modeled by accounting for rough edges and employing a linear average ofcontact angle along the perimeter of the drop, rather than an area average of cosine. This linear modelsuggests that, for suspended drops, both advancing and receding angles should increase. Alternatively,for drops that have collapsed over surface asperities, advancing contact angles should increase, whilereceding angles should decrease. These findings agree with the observations of a number of investigatorsthat have suggested that asperity height may be less important than asperity shape in determining wetting.
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