Hydrophobicity and drag reduction properties of surfaces coated with silica aerogels and xerogels

Superhydrophobic surfaces have application in self-cleaning, anti-fouling and drag reduction. Most superhydrophobic surfaces are constructed using complex fabrication methods. An alternative method is to use sol–gel methods to make hydrophobic aerogel and xerogel surfaces. In this work, hydrophobic silica aerogels and xerogels were made from the silica precursors tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in volume ratios MTMS/TMOS of 0–75 % using a base-catalyzed recipe. Overall hydrophobicity was assessed using contact angle measurements on surfaces prepared from crushed aerogel and xerogel powders. The surfaces made from aerogels were super-hydrophobic (with contact angles of 167°–170°) for all levels of MTMS (10–75 %). Of the xerogel-coated surfaces, those made with 50 % MTMS were hydrophobic and with 75 % MTMS were superhydrophobic. Chemical hydrophobicity was assessed using Fourier transform infrared spectroscopy, which showed evidence of Si–CH₃ and Si–C bonds in the aerogels and xerogels made with MTMS. Morphological hydrophobicity was assessed using SEM imaging and gas adsorption. The drag characteristics of the aerogel- and xerogel-coated surfaces were measured using a rotational viscometer. Under laminar flow conditions all of the hydrophobic aerogel-coated surfaces (10–75 % MTMS) were capable of capturing an air bubble, thereby reducing the drag on a horizontal rotating surface by 20–30 %. Of the xerogel-coated surfaces, only the one made from 75 % MTMS could capture a bubble, which led to 27 % drag reduction. These results imply that morphological differences between silica aerogels and xerogels, rather than any differences in their chemical hydrophobicity, give rise to the observed differences in hydrophobicity and drag reduction for the sol–gel-coated surfaces.