American Chemical Society
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How To Reduce Resistance to Movement of Alkane Liquid Drops Across Tilted Surfaces Without Relying on Surface Roughening and Perfluorination

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posted on 2012-12-21, 00:00 authored by Chihiro Urata, Benjamin Masheder, Dalton F. Cheng, Atsushi Hozumi
Alkylsilane-derived monolayer-covered surfaces generally display a reasonably good level of hydrophobicity but poor oleophobicity. Here, we demonstrate that the physical attributes of alkylsilane-derived surfaces (liquid-like or solid-like) are dependent on the alkyl chain length and density, and these factors subsequently have significant influence upon the dynamic dewetting behavior toward alkanes (CnH2n+2, where n = 7–16). In this study, we prepared and characterized hybrid films through a simple sol–gel process based on the cohydrolysis and co-condensation of a mixture of a range of alkyltriethoxysilanes (CnH2n+1Si­(OEt)3, where n = 3, 6, 8, 10, 12, 14, 16, and 18) and tetramethoxysilane (TMOS). Surprisingly, when the carbon number (Cn) of alkyl chain was 10 and below, the produced hybrid films were all smooth, highly transparent, and showed negligible contact angle (CA) hysteresis. On these hybrid surfaces, 5 μL drops of alkanes (n-hexadecane, n-dodecane, and n-decane) could move easily at low tilt angles (<5°) without pinning. On the other hand, when the Cn exceeded 12, both transparency and mobility of probe liquids significantly worsened. In the former case, TMOS molecules played key roles in both forming continuous films (as a binder) and improving flexibility of alkyl chains (as a molecular spacer), resulting in the smooth liquid-like surfaces. Silylation of the hybrid film and subsequent dynamic CA measurements proved the presence of silanol groups on the outermost surfaces and demonstrated that the dynamic dewettability of hybrid films worsened as packing densities increased. Additionally, solvent effects (high affinity) between the alkyl chains and alkane liquids imparted a more liquid-like character to the surface. Thanks to these simple physical effects, the resistance to the alkane droplet motion across tilted surfaces was markedly reduced. With the longer carbon chains, the chain mobility was strictly inhibited by mutual interactions between neighboring alkyl chains even in the presence of TMOS molecules. The achieved surfaces displayed a solid-like nature along with surface defects, leading to inferior dynamic oleophobicity. Therefore, the critical Cn of alkyl chain used for determining final dynamic dewetting behavior against alkane liquids was 12. Furthermore, our hybrid surfaces exhibited excellent antifingerprint properties, particularly demonstrating low adhesion and easy removal from the surface.