Abstract
Drops contained in an immiscible liquid phase are attractive as microreactors, enabling sound statistical analysis of reactions performed on ensembles of samples in a microfluidic device. Many applications have specific requirements for the values of local shear stress inside the drops and, thus, knowledge of the flow field is required. This is complicated in commonly used rectangular channels by the flow of the continuous phase in the corners, which also affects the flow inside the drops. In addition, a number of chemical species are present inside the drops, of which some may be surface-active. This work presents a novel experimental study of the flow fields of drops moving in a rectangular microfluidic channel when a surfactant is added to the dispersed phase. Four surfactants with different surface activities are used. Flow fields are measured using Ghost Particle Velocimetry, carried out at different channel depths to account for the 3-D flow structure. It is shown that the effect of the surfactant depends on the characteristic adsorption time. For fast-equilibrating surfactants with a characteristic time scale of adsorption that is much smaller than the characteristic time of surface deformation, this effect is related only to the decrease in interfacial tension, and can be accounted for by the change in capillary number. For slowly equilibrating surfactants, Marangoni stresses accelerate the corner flow, which changes the flow patterns inside the drop considerably.
Highlights
Drop microreactors have been successfully used for activities including the measurement of kinetic constants of chemical and biochemical reactions [2,3,4], drug–protein association constants [5], cell screening [6,7,8], synthesis of nanoparticles [9,10,11] and hydrogel particles [12,13,14]
This was obtained by subtracting the average drop velocity from the Eulerian Ghost Particle Velocimetry (GPV) flow fields, which are shown in the Supplementary Materials, Figure S1
The sink and source in the middle plane indicate the presence of a recirculatory flow in the drop cross-section, parallel to the side walls, with flow moving in the direction of drop motion around the middle plane and in the opposite direction near the top/bottom wall
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Using microfluidic drops as microreactors is a rapidly developing area that enables reaction studies and optimization using a statistically relevant number of reactors. This platform enables tightly controlled conditions, with minimal cross-contamination and advective dispersion, whilst using small amounts of reagents [1]. This enables a considerable reduction of the ecological impact of research and development, and potentially increases the rate of production of new formulations. Drop microreactors have been successfully used for activities including the measurement of kinetic constants of chemical and biochemical reactions [2,3,4], drug–protein association constants [5], cell screening [6,7,8], synthesis of nanoparticles [9,10,11] and hydrogel particles [12,13,14]
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