Ultrahydrophobic and Ultralyophobic Surfaces:  Some Comments and Examples

Abstract

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.

Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces

“…This perspective is a consolidation of views that we have presented in one 2008, 9 two 2006, 10,11 and two 1999 12,13 papers. This perspective could be gleaned from these manuscripts, but its formulation would be difficult.…”

Hydrophobic and Ultrahydrophobic Multilayer Thin Films from Perfluorinated Polyelectrolytes

“…[3,4] These studies address the critical issue of inducing surface roughness, which is required for true ultrahydrophobic behavior. [5][6][7][8][9][10][11][12][13][14][15] Soeno et al [3] described a multilayered polyelectrolyte/silica nanoparticle system which was heated to sinter the particles and burn off the polymer, then treated with a fluorosilane to induce the required hydrophobicity. Zhai et al [4] reported the formation of a pH-sensitive multilayer which undergoes a porosity-inducing phase transition in acidic solutions.…”

“…In a recent critical evaluation of the "superhydrophobic" or "ultrahydrophobic" surface literature, Chen et al provide numerous examples of systems that are poorly defined because they fail to clearly present contact angle hysteresis. [14] Even very recent work falls short in this respect. [27] Following the recommendation of Chen et al, [14] we reserve the term "ultrahydrophobic" for those surfaces exhibiting both large water contact angles and small hysteresis between q a and q r .…”

“…[14] Even very recent work falls short in this respect. [27] Following the recommendation of Chen et al, [14] we reserve the term "ultrahydrophobic" for those surfaces exhibiting both large water contact angles and small hysteresis between q a and q r .…”

“…[14] Ultrahydrophobicity, however, appears to require surface roughness, although the length scales are not well understood. Regularly patterned pillars, produced by lithography, with diameters of approximately 10 mm and about the same spacing generated ultrahydrophobic surfaces.…”