Investigating the Role of Glass and Quartz Substrates on the Formation of Interfacial Droplets

In our daily life, interface between or multiple interfaces among gas, liquid, and solid are usually involved. In a particular scenario, water/oil/solid interface was studied and we found that water droplets can be spontaneously formed at oil solid interfaces. Though the underlying mechanism is not fully revealed nowadays, it is known that solid substrates play an important role, especially in driving water molecules to accumulate at the oil solid interfaces. Here, we exploited glass and quartz, which are widely used materials and investigated how the surface property would influence the interfacial droplets formation. The work was divided into two parts; first, piranha-cleaned glass and quartz and octadecyltrichlorosilane (OTS)-treated glass and quartz were prepared respectively and systematically characterized. Second, these substrates were used to grow interfacial droplets, and the influence of surface property on interfacial droplets was discussed. The key conclusions were as follows: glass and quartz mainly containing SiO2 possessed different stability to acid etching, resulting in different initial Si-OH density. Such microscopic surface chemistry determined the morphology and number of interfacial droplets. In general, piranha-cleaned substrates tended to be more hydrophilic, wetted interfacial droplets to irregular or island-like shape. The OTS-treated substrates, owing to its high hydrophobicity, dewetted interfacial droplets to spherical shape. As glass was relatively easier to be etched than quartz, producing more Si-OH after piranha cleaning, in microscopic scale, the interfacial droplets wetted better on glass and showed a lower contact angle than that on quartz. The residual Si-OH on OTS-treated substrates was proven to be the main chemistry to promote interfacial droplets formation and OTS-treated glass contained more Si-OH, leading to formation of much more interfacial droplets than those on OTS-treated quartz. This work provides a new angle to the old materials, e.g., glass and quartz, and draws attention that the very similar composition materials can possess a much different surface property and eventually causes a drastic difference in confining interfacial droplets formation.