We discuss dynamic hydrophobicity from the perspective of the force required to move a water droplet on a surface and argue that the structure of the three-phase contact line is important. We studied the wettability of a series of silicon surfaces that were prepared by photolithography and hydrophobized using silanization reagents. Hydrocarbon, siloxane, and fluorocarbon surfaces were prepared. The surfaces contain posts of different sizes, shapes, and separations. Surfaces containing square posts with X-Y dimensions of 32 µm and less exhibited ultrahydrophobic behavior with high advancing and receding water contact angles. Water droplets moved very easily on these surfaces and rolled off of slightly tilted surfaces. Contact angles were independent of the post height from 20 to 140 µm and independent of surface chemistry. Water droplets were pinned on surfaces containing square posts with larger dimensions. Increasing the distance between posts and changing the shape of the posts from square to staggered rhombus, star, or indented square caused increases in receding contact angles. We ascribe these contact angle increases to decreases in the contact length and increases in tortuosity of the three-phase contact line. The maximum length scale of roughness that imparts ultrahydrophobicity is ∼32 µm.

The preparation of ultrahydrophobic and ultralyophobic surfaces using several techniques is described. Plasma polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate on poly(ethylene terephthalate) yields surfaces with water contact angles of θA/θR = 174°/173°. Argon plasma etching of polypropylene in the presence of poly(tetrafluoroethylene) renders surfaces with water contact angles as high as θA/θR = 172°/169°. Surfaces of compressed pellets of submicrometer, variable-diameter spherical particles of PTFE oligomers exhibit water contact angles of θA/θR = 177°/177°, methylene iodide contact angles of θA/θR = 140°/138°, and hexadecane contact angles of θA/θR = 140°/125°. We emphasize that contact angle hysteresis is more important in characterizing lyophobicity than is the maximum achievable contact angle. These surfaces are rough at the micrometer and submicrometer scales, and water drops roll easily on all of them. We make an intuitive argument that the topology of the roughness is important and controls the continuity of the three-phase contact line and thus the hysteresis. We also report smooth ultralyophobic surfaces that are prepared by silanization of silicon wafers with Cl(SiMe2O) n SiMe2Cl (n = 0, 1, 2, and 3), (Me3SiO)3SiCH2CH2Si(CH3)2Cl, and (Me3SiO)2Si(CH3)CH2CH2Si(CH3)2Cl. These surfaces exhibit much lower contact angles but little or no hysteresis, and droplets of water, hexadecane and methylene iodide slide easily off them. We propose that these covalently attached monolayers are flexible and liquidlike and that droplets in contact with them experience very low energy barriers between metastable states.

We argue using experimental data that contact lines and not contact areas are important in determining wettability. Three types of two-component surfaces were prepared that contain "spots" in a surrounding field: a hydrophilic spot in a hydrophobic field, a rough spot in a smooth field, and a smooth spot in a rough field. Water contact angles were measured within the spots and with the spot confined to within the contact line of the sessile drop. Spot diameter and contact line diameter were varied. All of the data indicate that contact angle behavior (advancing, receding, and hysteresis) is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the contact perimeter is irrelevant. The point is made that Wenzel's and Cassie's equations are valid only to the extent that the structure of the contact area reflects the ground state energies of contact lines and the transition states between them.

A view of contact angle hysteresis from the perspectives of the three-phase contact line and of the kinetics of contact line motion is given. Arguments are made that advancing and receding are discrete events that have different activation energies. That hysteresis can be quantified as an activation energy by the changes in interfacial area is argued. That this is an appropriate way of viewing hysteresis is demonstrated with examples.

Silicon-supported alkylsiloxane layers were prepared by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with silicon wafers under two conditions: (1) in the vapor phase and (2) in toluene in the presence of ethyldiisopropylamine. Covalent attachment of di-and trichlorosilanes to the surface of silicon/silicon oxide through SiS-O-Si bonds occurs for the amine-catalyzed reactions. This sets apart this reaction from the self-assembly process that occurs in the reaction between certain trichlorosilanes and hydrated silica with no amine present. The thickness of the layers formed from dichloro-and trichlorosilanes (as assessed by ellipsometry) is on the order of the single molecule sizes and increases gradually with alkyl chain length. The thickness values are considerably smaller (by a factor of ∼0.75) than the length of the fully stretched alkyl chain, which argues for disordered structures of the monolayers. Dynamic advancing and receding contact angles for water, methylene iodide, and hexadecane argue for interaction between the probe fluids and accessible silanol groups (Si-OH) on the surface. Water contact angles increase with alkyl chain length and level at θA/θR ) ∼103°/∼90°for relatively long alkyl chains (∼C6 and longer), indicating that these surfaces project disordered methyl and methylene groups toward the probe fluid. n-Hexadecane and methylene iodide contact angles show more complex behavior, which is discussed in the paper. The vapor-phase reaction of di-and trichlorosilanes with silicon wafers yields surfaces that depend dramatically on the alkyl chain of the silane. Alkylsilanes with short and medium chains form polymeric grafted layers with thicknesses ranging from a few nanometers for dichlorosilanes up to tens of nanometers for trichlorosilanes. We suggest a mechanism that involves polycondensation of chlorosilanes into 3-D alkylsiloxanes in the presence of adsorbed water. Dynamic advancing and receding contact angles of water, methylene iodide, and hexadecane on these surfaces are consistently higher than for surfaces prepared in the liquid phase. Alkylsilanes with long alkyl moieties yield approximately monomolecular layers that exhibit wettabilities similar to those for surfaces prepared in the liquid phase.