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

The nucleation and growth of liquid droplets on solid substrates have received much attention because of the significant relevance of these multiphase processes to both nature and practical applications. There have been extensive studies on the condensation of water from the air phase on solid substrates. Here, we focus on water diffusion through the oil phase and subsequent settlement on solid substrates because such interfacial droplets are formed. Voronoi diagram analysis is proposed to statistically characterize the size distribution of the growing droplets. It is found that modification of the standard Voronoi diagram is required for systems of interfacial droplets which have a noncircular shape and/or whose centers change with time. The modified Voronoi analysis of the growing droplets provides an automatic quantification of the droplet distribution and reveals that (i) during the nucleation stage, the interfacial droplets do not nucleate at the same time because the nucleation of newly formed droplets competes with the growth of the existing ones; (ii) the growth of interfacial droplets comes from water diffusion from the bulk water layer, and/or from adjacent interfacial droplets, and/or from coalescence of interfacial droplets; and (iii) the sizes of interfacial droplets become more polydispersed on P-glass but more monodispersed on OTS-glass as time goes. This work opens a new perspective on the formation of interfacial droplets at the interface between oil and the solid substrate and demonstrates the capability of an automatic analysis method, which can be potentially applied to similar interfacial multiphase systems.

A facile method for preparation of emulsion using the high gravity technique.

This study demonstrates a new process for preparation of oil-in-water (O/W) emulsion using the high gravity technique. This involves a mixture of oil and water that are passed through a rotating packed bed, under a high shear force, from which oil is emulsified into tiny droplets and subsequently dispersed in water. The process is cycled in order to break the droplets repeatedly and achieve an emulsion with small size and low polydispersity index (PDI). The advantage of the high gravity technique is that the emulsions with a desired size and polydispersity can be rapidly obtained by tuning experimental parameters, such as relative centrifugal force (RCF), cycle times (CT), liquid flow rate (VL) and surfactant concentration (Csurfactant). The size of emulsions is shown to decrease with increasing RCF, CT, VL and Csurfactant. The PDI of emulsion prepared by high gravity technique is also much improved in comparison to that prepared by conventional sonication, which is further confirmed with dynamic light scattering and confocal imaging characterization. To provide an additional perspective on the high gravity technique as a tool to make O/W emulsions, uniformly distributed liquid crystal droplets were prepared by using the high gravity technique, which have been well studied for their in situ chemical and biological detection. In short, the high gravity technique for preparing emulsions is facile, fast and can be potentially applied for large scale industry applications.

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

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.