Controlling the Accumulation of Water at Oil–Solid Interfaces with Gradient Coating

Abstract

In this work, we demonstrate a strategy to control the accumulation of water in the oil-solid interface using a gradient coating. Gradient chemistry on glass surface is created by vapor diffusion of organosilanes, leading to a range of contact angles from 110 to 20°. Hexadecane is placed on the gradient substrate as an oil layer, forming a "water/hexadecane/gradient solid substrate" sandwich structure. During incubation, water molecules spontaneously migrate through the micrometer-thick oil layer and result in the formation of micrometer-sized water droplets at the oil-solid interface. It turns out that water droplets at more hydrophobic regions tend to be closer to a regular spherical shape, which is attributed to their higher contact angle with the hydrophobic substrate. However, along the gradient from hydrophobic to hydrophilic, the water droplets gradually form more irregular shapes, as hydrophilic surfaces pin the edges of droplets to form a distorted morphology. It indicates that more hydrophilic surfaces containing more Si-OH groups lead to a higher electrostatic interaction with water and a higher growth rate of interfacial water droplets. This work provides further insights into the mechanism of spontaneous water accumulation at oil-solid interfaces and assists in the rational design for controlling such interfacial phenomenon.

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

“…However, the ζ-potential of OTS-glass was much higher than that of OTSquartz; the difference in surface charges can be attributed to different Si−OH densities. Based on our previous report, Si− OH was the main driving force for interfacial droplets formation; 49 therefore, the more Si−OH residues after OTS modification on OTS-glass promoted more interfacial droplets' formation on the surface. Combining all experiments, we conclude that the piranha-cleaned substrates showed very similar interfacial droplets formation trend; in contrast, the OTS-glass can significantly promote droplet formation than OTS-quartz.…”

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

“…Here we refer to substrates as hydrophobic when the contact angle is greater than 90°, and hydrophilic when the contact angle is less than 90°. The assumption of spherical droplets on hydrophobic surfaces is supported by the observations of Li et al [33], who measured contact angles of water droplets on a range of surfaces in bulk n-hexadecane and in water/n-hexadecane sandwiches: those formed on hydrophobic surfaces were spherical. The values on hydrophilic surfaces, however, differed owing to contact line 1 The capillary length, lc, is the length scale at which hydrostatic pressure is equal to capillary pressure:…”

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

“…In this current paper we focus on the analysis of interfacial droplets grown spontaneously at an oil-solid interface by water diffusion through an oil layer. 26,34,35 In a typical experiment, several tens of micrometers in thickness of a n-hexadecane layer is deposited onto a hydrophilic or hydrophobic substrate and then submerged in water. Just after a few minutes, water diffuses through the oil layer and condenses on the hydrophilic or hydrophobic glass in the form of tiny interfacial droplets.…”

“…A valuable technique for the automatic image analysis of a 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 the spatial distribution of surface nanobubbles and nanodroplets. ,, In their experiments, nanobubbles and nanodroplets were produced by a solvent exchange process, and Voronoi diagram analysis helped them gather information on the mechanism and formation of nanobubbles and nanodroplets. In this paper, we focus on the analysis of interfacial droplets grown spontaneously at an oil–solid interface by water diffusion through an oil layer. ,, In a typical experiment, an n -hexadecane layer of several tens of micrometer thickness is deposited onto a hydrophilic or hydrophobic substrate and then submerged in water. Just after a few minutes, water diffuses through the oil layer and condenses on the hydrophilic or hydrophobic glass in the form of tiny interfacial droplets.…”