Effect of Surfactant Dynamics on Flow Patterns Inside Drops Moving in Rectangular Microfluidic Channels

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.

Effect of surfactants during drop formation in a microfluidic channel: a combined experimental and computational fluid dynamics approach

“…The gutter flow can also reverse the direction of the flow fields after the drop has formed (Mießner et al. 2020; Kovalchuk & Simmons 2021).

Figure 4.Numerical results for the surfactant-free case generated with and ml min.

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“…As seen in previous work, the small characteristic time scales that are present in microfluidics can lead to interfacial tension values different from the equilibrium ones, which are often used in the calculation of capillary numbers (Kalli & Angeli 2022) and in predictive equations for drop size. Additionally, shear stresses from high continuous phase velocities can redistribute surfactant molecules at the interface, which will cause interfacial tension gradients and thus Marangoni stresses (Kovalchuk & Simmons 2021). For surfactants that result in a large decrease in interfacial tension, the effect of interface retardation at low surfactant concentrations can be sufficiently large to change the velocity fields close to the interface and inside the drop, due to Marangoni stresses in the opposite direction to the flow.…”