INTERFACIAL RHEOLOGY AND MASS TRANSFER KINETICS IN COMPLEX, MICROCHANNEL FLOWS
Jeffrey Martin and Steven Hudson
Complex Fluids Group, Polymers Division, MSEL, NIST
Multiphase liquid systems are essential to everyday life, e.g., foods, pharmaceutics, cosmetics, oil recovery, etc. The morphology and stability of such systems depend on dynamic interfacial properties and processes. Typical methods used to measure such properties often employ simpler flows and larger drops than those encountered in typical processing applications. Mass transfer mechanisms are governed by drop size; therefore experimentation at length scales typical of those encountered in applications is desirous. Using a microfluidic approach, dynamic structure and kinetics are measured in multiphase systems using drop sizes comparable to those seen in applications and easily adjustable flow complexity. Through drop deformation dynamics, the dynamic interfacial tension of aqueous, surfactant-containing drops in mineral oil is probed as a measure of surfactant mass transfer kinetics. Using particle tracers, the drop internal circulation velocity is used as a measure of interfacial mobility. Deformation dynamics, interfacial tension, and circulation patterns are measured in Poiseuille and transient elongational flows in a microchannel.
Experiments performed on drops of aqueous n-butanol solutions in mineral oil show that the interfacial tension decreases with interface age due to the diffusion of butanol from the drop to the oil. A shift from diffusion-controlled mass transfer to a regime where adsorption / desorption kinetics at the interface become limiting is verified for small drop sizes (tens of microns). Significant interfacial immobilization is seen at low surfactant concentrations, with remobilization at high concentrations. Internal drop circulation patterns are studied in more detail using a nearly index-matched system of ethylene glycol / water drops in silicone oil. Thus our microfluidic approach facilitates measurement of mass transfer kinetics and Marangoni effects in the same experiment utilizing industrially-relevant flows and drop sizes.