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

Abstract

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.

The growth and shrinkage of water droplets at the oil-solid interface

“…The roughness of the coated substrates was measured by AFM and gave a root mean square roughness (RMS) of 8.30 nm over an area of 10 × 10 μm 2 , (topography image inFig. S2) which was consistent with previously reported values[6]. No detectable effect was found for the influence of roughness on the formation of interfacial droplets.…”

“…Here we have focused on water droplets forming at the oil/solid interface, which has a number of important commercial applications, particularly enhanced oil recovery [55][56][57][58] and has been the subject of a series of interesting contributions addressing more fundamental aspects [4][5][6]. The somewhat 'unexpected' behaviour of hydrophilic water molecules spontaneously diffusing through a sparingly water-soluble oil to form droplets on hydrophobic substrates was first reported in 2010 [4].…”

“…The protocol was based on that reported by Yang et al [6]. Culture dishes were prepared with a 10 mm diameter hole cut out of the centre of the base and the OTSglass test piece was glued in place across this hole (see Supplementary Fig.…”

“…It is easy to accept that these three phases are immiscible and well separated or involves very little diffusion, however water droplets have been observed to form spontaneously over a period of hours to days at the oil/solid interface [4,5]. Yang et al used confocal microscopy to track the growth of droplets with diameters typically between microns to tens of microns on a number of hydrophobic surfaces (contact angle greater than 90°) [6]. The droplets increased in size by steady growth, although there was coalescence (accompanied by a decrease in number) when the droplets grew and touched each other.…”

Modified Voronoi Analysis of Spontaneous Formation of Interfacial Droplets on Immersed Oil–Solid Substrates

“…[15][16][17][18] Studies on single or a few droplets have enabled advancements in our understanding of processes such as contact line pinning 19,20 , droplet growth [21][22][23] and coalescence. [24][25][26][27] However, the droplet behavior in realistic systems is controlled by complex many-body interactions and is extremely sensitive to initial conditions, [28][29][30][31] making a statistical analysis via automatic techniques necessary. A valuable technique for the automatic image analysis of large number of droplets on solid substrates is the Voronoi diagram analysis, used by Zhang, Lohse and collaborators, for example, to study the size and spatial distribution of surface nanobubbles and nanodroplets.…”