Abstract
Drop-based microfluidic devices are becoming more common, and molecular mass transfer and drop circulation are issues that often affect the performance of such devices. Moreover, interfacial properties and surfactant mass transfer rates govern emulsion behavior. Since these phenomena depend strongly on drop size, measurement methods using small drops and flow typical of applications are desired. Using mineral oil as a continuous phase, water droplets and an alcohol surfactant, we demonstrate here a microfluidic approach to measure the interrelated phenomena of dynamic interfacial tension, surfactant mass transfer and interfacial retardation that employs droplet flows in a microchannel with constrictions/expansions. Interfacial flow is influenced markedly by adsorption of surfactant: severe interfacial retardation (by a factor of 30) is observed at low surfactant concentrations and interface remobilization is observed at higher surfactant concentrations. The interfacial tension is described by Langmuir kinetics and the parameters for interfaces with mineral oil (studied here) compare closely with those previously found at air interfaces. For the conditions explored, the surfactant mass transfer is described well by a mixed kinetic-diffusion limited model, and the desorption rate coefficients are measured to be both approximately 70 s−1. The transition from a diffusion-controlled to mixed diffusion-kinetic mass transfer mechanism predicted with reducing drop size is verified. This experimental approach (i.e. adjustable geometry and drop size and height) can therefore probe interfacial dynamics in simple and complex flow.
Highlights
The fundamental basis of this investigation is particle and drop tracking. Quantities derived from these primary measurements of particle position, and drop position and shape are the drop velocity, the rate of strain in the continuous fluid, the interface age, the internal circulation rate and the interfacial tension, as described in the previous section
These experiments demonstrate that interfacial tension, surfactant mass transfer and interfacial retardation can be measured in a single experiment, so that interfacial properties and mobility can be correlated directly
The interface is remobilized at higher surfactant concentration
Summary
In two-phase systems with surface-active solutes, many techniques have been employed to measure the interfacial tension as a function of time. Our microfluidic approach allows generation of droplets with diameters of the order of tens of micrometers, facilitating experimentation at droplet length scales smaller than RD−K At these length scales, the transition from diffusion-controlled to mixed kinetically controlled surfactant mass transfer can be verified. Beyond the ability to measure dynamic interfacial tension on droplets with diameters of the order of tens of micrometers, our microfluidic approach has the advantage of facilitating the study of drops in complex flow conditions like that which emulsions encounter in many industrial processes. This is accomplished by introducing flow complexity through variation of the microchannel geometry (see the appendix). Our microfluidic approach allows us to image drops under flow and, by placing particle tracers inside the drops, directly measure interfacial immobilization caused by Marangoni effects
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