Abstract
Emulsion stability in a flow field is an extremely important issue relevant for many daily-life applications such as separation processes, food manufacturing, oil recovery etc. Microfluidic studies can provide micro-scale insight of the emulsion behavior but have primarily focussed on droplet breakup rather than on droplet coalescence. The crucial impact of certain conditions such as increased pressure or elevated temperature frequently used in industrial processes is completely overlooked in such micro-scale studies. In this work, we investigate droplet coalescence in flowing oil-in-water emulsions subjected to higher than room temperatures namely between 20 to 70 ^{circ }C. We use a specifically designed lab-on-a-chip application for this purpose. Coalescence frequency is observed to increase with increasing temperature. We associate with this observation the change in viscosity at higher temperatures triggering a stronger perturbation in the thin aqueous film separating the droplets. Using the scaling law for rupture time of such a thin film, we establish a mechanism leading to a higher coalescence frequency at elevated temperatures.
Highlights
Emulsion stability in a flow field is an extremely important issue relevant for many daily-life applications such as separation processes, food manufacturing, oil recovery etc
Effects of some of the industrial process conditions such as oil volume fraction, droplet size and oil viscosity on the emulsion stability have previously been studied in an empirical fashion[16], but microscopic insight of what such a specific process condition e.g., elevated temperature does to the thinning of the aqueous film between two droplets leading to the droplets’ coalescence is quite limited
The same emulsion flows to the part of the microfluidic chip known as coalescence chamber where we study the oil droplets’ coalescence as a function of temperature, surfactant concentration, initial oil droplet size and the adsorption time allowed to the surfactant molecules to get adsorbed to the oil–water interface
Summary
Emulsion stability in a flow field is an extremely important issue relevant for many daily-life applications such as separation processes, food manufacturing, oil recovery etc. The complex interplay of droplet interactions coupled with the process parameters and the flow conditions make such a system extremely difficult to study Understanding this system very much requires a description of the micro-scale phenomena since the dispersed phase droplets are of the order of a few hundred nanometers to a few tens of microns. Effects of some of the industrial process conditions such as oil volume fraction, droplet size and oil viscosity on the emulsion stability have previously been studied in an empirical fashion[16], but microscopic insight of what such a specific process condition e.g., elevated temperature does to the thinning of the aqueous film between two droplets leading to the droplets’ coalescence is quite limited. We propose a mechanistic explanation of the film drainage at elevated temperatures and explain how that will lead to the coalescence trends observed in our experiments
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