Liquid
fouling can reduce the functionality of critical engineering
surfaces. Recent studies have shown that minimizing contact angle
hysteresis is a promising strategy for achieving omniphobic (all-liquid
repellent) properties, thereby inhibiting fouling. Prior omniphobic
films can repel a broad range of liquids, but the applicability of
these coatings has always been limited to silicon wafers or smooth
glass. Here we develop a facile procedure to generate an omniphobic
coating on any surface, including metals, paper, ceramics, etc. The
coating involves depositing an ultrasmooth, silicon wafer-like silica
layer and then treating this layer with a highly reactive chlorosilane,
which grafts polydimethylsiloxane chains onto the surface. Negligible
contact angle hysteresis (≤1°) for various liquids, including
ultralow surface tension oils, alcohols, and fluoro-solvents, was
achieved on many different substrates regardless of their initial
roughness or chemistry. In fact, the contact angle hysteresis was
so low we were forced to propose an alternate measurement technique,
using tilt angles, that reduced the inherent errors associated with
traditional contact angle goniometry. The coating’s durability
was characterized and, when it was damaged, could be repeatedly repaired,
fully restoring the omniphobic properties to their initial state.
We report macroscopic evidence of the liquidlike nature of surface-tethered poly(dimethylsiloxane) (PDMS) brushes by studying their adhesion to ice. Whereas ice permanently detaches from solid surfaces when subjected to sufficient shear, commonly referred to as the material's ice adhesion strength, adhered ice instead slides over PDMS brushes indefinitely. When additionally methylated, we observe Couette-like flow of the PDMS brushes between the ice and silicon surface. PDMS brush ice adhesion displays a shear-rate-dependent shear stress, rheological behavior reminiscent of liquids, and is affected by ice velocity, temperature, and brush thickness, following scaling laws akin to liquid PDMS films. This liquidlike nature allows ice to detach solely by self-weight, yielding an ice adhesion strength of 0.3 kPa, 1000 times less than a low surface energy, perfluorinated monolayer. The methylated PDMS brushes also display omniphobicity, repelling essentially all liquids with vanishingly small contact angle hysteresis. Methylation results in significantly higher contact angles than previously reported, nonmethylated brushes, especially for polar liquids of both high and low surface tension.
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