Fabrication and Evaluation of Superhydrophobic Surfaces Using Carbon Soot and Different Adhesives

Introduction

Hydrophobic and superhydrophobic surfaces can repel water and thus show more immense potential to be used in self-cleaning applications. The development of these special glass surfaces has become an active research area due to the increasing need for solar and other applications (Barati Darband et al., 2020; Das et al., 2018; Hooda et al., 2020; Jumrus et al., 2020; Kumar et al., 2020; Latthe et al., 2019; Lin et al., 2019; Wang et al., 2017; M. Xu et al., 2019; Zhi & Zhang, 2018). The state of hydrophobicity of a surface is investigated by the water contact angle (WCA) of the water droplet with the surface. A surface that shows a contact angle greater than 150^o and a roll-off angle less than 10° is referred to as a superhydrophobic surface (SHS) (Kumar, Verma, Sharma, et al., 2022). To achieve a state of being superhydrophobic, the surface must satisfy two conditions. Firstly, the surface should be rough and secondly, the surface material should have low surface energy (Lei et al., 2014; Nosonovsky & Bhushan, 2007).

In view of the application SHS, Seo et al. coated glass slides with candle paraffin wax and then sooted with a candle flame (Seo et al., 2014). Contact angles (CAs) (static, advancing, and receding) were measured with the help of a contact angle analyzer (Phoenix 300, SEO). The WCA of a water droplet on the glass surface after paraffin wax coating was recorded as 106⁰, and after candle sooting, it was increased to 162⁰. The contant angle hysteresis (CAh) was reported as 2⁰, whereas advancing and receding angles were found to be 163⁰ and 161⁰, respectively. Moreover, the roll-off angle (ROA) was measured as less than 1⁰. The prepared surfaces thermal stability, pH tolerance, and compression tolerance were also tested. For the thermal stability test, the coatings on the glass slides were annealed for 1 hour at different temperatures up to 300⁰C. After cooling at room temperature, static contact angle (SCA) and ROAs were found to be 162° and 1°, respectively. For the pressure tolerance test, the Perfluorochemicals (PFCS) coatings were covered with a silicon wafer and pressed at up to 5 bar of gauge pressure for 20 sec. During the pH reactivity test, several buffer solutions of different pH values were used to measure SCA (163°) and ROA (1°). The PFCS coatings were immersed in the buffer solutions for over 3 hours with different pH values in the pH tolerance test. To check the durability of the PFCS coating, water droplets were allowed to fall onto the coating from a height of 10 cm to 60 cm. The substrate was tilted at 45⁰ and then fixed. It was found that the developed superhydrophobic surfaces were stable. The soot layer was durable with paraffin wax when used as adhesive. As a result, the durability in terms of the energy required to break the coating of the PFCS coating was improved up to a factor of 50 compared to a bare soot-coated surface without paraffin wax. In another study carried out by Ramachandran et al. candle soot was deposited on Vaseline coated glass substrates (Ramachandran et al., 2013). The soot was obtained by burning incense sticks directly in the candle flame. Followed by, the stability of the soot coating was examined by a water immersion test. The soot deposited on vaseline-coated glass slideware was found to be more stable than direct soot-coated plain glass slides. The soot on the Vaseline coated glass slides was retained even after repeated dipping in a beaker of water. The fabricated surface showed excellent water repellency behavoiur even after prolonged exposure to the ambient conditions. During an interesting work reported by Xu et al., coatings of polydimethylsiloxane (PDMS) were deposited on glass slides followed by candle soot. In their study, single and double PDMS coatings were applied on two different substrates of glass slides of the same grade (C. Xu et al., 2017). After coating, the WCA value was evaluated via an optical contact angle meter (OCA20, data physics, German). The substrates with single and double coating show WCA of 135^o and 160^o, respectively.