Elastic superhydrophobic and water glass-based silica aerogels and applications

Silica aerogels with high elasticity and superhydrophobicity have several scientific and technological applications such as in multidisciplinary subjects dealing with liquid/solid interfacial energies, long-range attractive interactions, frictionless flow of liquids through nano and microchannels, pipes, and storage of mechanical energy. In addition, the aerogels are being used as light weight super thermal, acoustic and electrical insulating materials especially in microelectronics, aeronautics, Space, in high-energy physics; as bandages for leaks in oil supply pipes, and underwater pressure sensitive devices. The present paper describes the preparation, physicochemical properties along with applications of elastic superhydrophobic silica aerogels with a water contact angle as high as 175o and Young’s modulus as low as 3 × 104 N/m2 with one of the highest reverse compressibility of around 60% known for any solid state material. The Young’s modulus is measured by uniaxial compression. The water droplets placed on surfaces coated with the superhydrophobic aerogel powder at 55° of angle of inclination showed velocities as high as 1.44 m s−1 (free fall velocity ≈ 1.5 m s−1). Water intrusion into the superhydrophobic aerogels at pressures greater than the Laplace pressure exhibited hysteresis resulting in the storage of water in the aerogels, and indicating the sizes of the pores. It has been shown that the elastic superhydrophobic aerogels are very efficient adsorbents of oil and organic compounds and hence useful for oil-spill clean-up applications, in addition to their potential use as laser induced X-ray and plasma emission sources. Moreover, the recent developments in the field of aerogels have shown a great potential for the combination of ambient pressure drying and a low-cost inorganic, and easily available precursor such as sodium silicate, which is also called as water–glass, for the production of hydrophobic silica aerogels on a large industrial scale for commercial applications. The properties of such aerogels are comparable to those obtained by the conventional supercritical drying methods. So far, only powders and translucent and transparent grains of sizes 2–4 mm, are being produced. The experimental results on the preparation and properties along with various applications of elastic superhydrophobic and sodium silicate-based silica aerogels, are presented in this paper. Several sol–gel and drying parameters affecting the quality of the aerogels, in terms of low-density ( 90% in the visible range), are also presented and discussed.