A Statically Oleophilic but Dynamically Oleophobic Smooth Nonperfluorinated Surface

Abstract

Extensive research on the preparation of superhydrophobic surfaces has been reported in the past decade, ranging from basic research to practical applications. Such surfaces usually exhibit water contact angles (CAs) larger than 1508. However, studies on superoleophobic surfaces, on which nonpolar (NP) liquid droplets (in particular, small volumes ca. 3-5 mL) that have appreciably low surface tensions (LSTs) can easily roll across and off, and which have a minimum tilt angle (TA) for droplet motion of less than 108, [1][2][3] are rare, despite of the practical advantages they offer. It is very challenging to fabricate surfaces that strongly promote dewetting of NP liquids, such as n-hexadecane and n-dodecane, because of their LSTs (g lv = 27.5 and 25.4 dyn cm À1 at 20 8C, respectively [4] ). When most superhydrophobic surfaces encounter such NP-LST liquids, they exhibit Wenzel wetting behavior: the liquid droplet sticks to the surface and does not roll off or dewet at any TA (poor dynamic oleophobicity). [5] Although textured surfaces with low surface energy usually maintain high advancing CAs (q A ), when the high-surface-tension (HST) liquid is replaced with a NP-LST one, receding CAs (q R ) are greatly decreased, which results in marked increases in CA hysteresis values (Dq = q A Àq R ). However, a few reports have described the formation of superoleophobic surfaces that are capable of preserving the excellent mobility of NP-LST liquid droplets. [2,3] Tuteja et al. [2] successfully fabricated perfluorinated patterned surfaces with a q A value of more than 1508 and TA = 158 for a 10 mL drop of n-octane (g lv = 21.8 dyn cm À1 at 20 8C [4] ). Zhang and Seeger [3] also prepared a superoleophobic surface consisting of CF 3 -terminated oxidized-silicone nanofilaments with topographical features similar to the "perfectly hydrophobic surface" reported by Gao and McCarthy. [6a] In that case, the minimum TAs needed for movement of 5 mL droplets of NP-LST liquids, such as n-decane (g lv = 23.8 dyn cm À1 at 20 8C [4] ) and nhexadecane, was only approximately 2-58. [3] For the prepara-tion of surfaces with such excellent dynamic oleophobicity, Zhang and Seeger [3] emphasized the necessity of producing weak interactions between the NP-LST droplets and the substrate surfaces.In contrast to such topographically modified surfaces, there have only been a few reports of flat or smooth surfaces with a low CA hysteresis value for both HST and NP-LST liquids. Proper control of both the molecular architecture (branched or ring-shaped molecules) and the physical nature of the modified surface (liquid-like surface that results from high mobility of functional groups in the surface-tethered molecules) are required to realize excellent dynamic dewetting behavior and to allow HST/NP-LST liquid droplets to easily dewet from the surface at low TAs, independent of the magnitude of the CA (this is a practical definition of "ultralyophobic"). [6b] Fadeev and McCarthy [7] and several other groups [8] have previously reported ultralyophobic ...

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