Influence of Interfacial Gas Enrichment on Controlled Coalescence of Oil Droplets in Water in Microfluidics

Abstract

Interfacial gas enrichment (IGE) of dissolved gases in water is shown to govern the strong attraction between solid hydrophobic surfaces of an atomic force microscopy (AFM) colloidal probe and a solid substrate. However, the role of IGE in controlling the attraction between fluid-fluid interfaces of foam films and emulsion films is difficult to establish by AFM techniques due to the extremely fast coalescence. Here, we applied droplet-based microfluidics to capture the fast coalescence event under the creeping flow condition and quantify the effect of IGE on the drainage and stability of water films between coalescing oil droplets. The amount of dissolved gases is controlled by partially degassing the oil phase.When the amount of dissolved gases (oxygen) in oil decreases (from 7.89 mg/L to 4.59 mg/L), the average drainage time of coalescence significantly increases (from 19 ms to 50 ms). Our theoretical quantification of the coalescence by incorporating IGE into the multilayer van der Waals attraction theory confirms the acceleration of film drainage dynamics by the van der Waals attractive force generated by IGE. The thickness of the IGE layer decreases from 5.5 nm to 4.9 nm when the amount of dissolved gas decreases from 7.89 mg/L to 4.59 mg/L. All these results establish the universal role of dissolved gases in governing the strong attraction between particulate hydrophobic interfaces.

Microfluidic method for determining drop-drop coalescence and contact times in flow

“…Most likely the increased viscosity of dodecane, also decreasing the interfacial mobility, led to an increase of coalescence time as suggested by other authors 23,37 . By comparison, Wang et al reported the coalescence time of squalene droplets (eight times the viscosity of dodecane) in the range of 15-60 ms, depending on the concentration of dissolved oxygen 31 . Krebs et al also reported longer coalescence times for increasing viscosities of mineral and silicone oils 23 .…”

“…Lin et al used a collision chamber to investigate coalescence of asphaltene stabilized water-in-oil emulsions in the presence of de-emulsifiers 30 , however, they focused on comparing coalescence rates, rather than coalescence times. Finally, a recent paper by Wang et al presented the effect of dissolved oxygen on coalescence kinetics of squalene droplets in water 31 . While the experimental approach is similar to the one presented here, they reported coalescence of only one size class of droplets with far less coalescence times detected.…”

Microfluidic Droplet Detection via Region-Based and Single-Pass Convolutional Neural Networks with Comparison to Conventional Image Analysis Methodologies

“…In addition to just droplet detection, we show that the models can be trained to discriminate inter-droplet interaction morphologies, such as various coalescence events instead of the actual droplets themselves. Measurement of coalescence time is an important topic in the literature concerning emulsions, and several microfluidic techniques have been reported previously [71,73,74,75]. In our previous work [53], we have also reported a microfluidic method for measuring coalescence time.…”

Effect of Dodecane-Oleic Acid Collector Mixture on the Evolution of Wetting Film between Air Bubble and Low-Rank Coal

“…Wang et al [7] discussed the effect of gas enrichment on wetting film, and they found that a gassed system had faster coalescence due to the strong surface force. Besides, there are multiple theories trying to interpret the mechanism of the film evolution.…”