The physics of a coflow micro-extractor: Interface stability and optimal extraction length

In biotechnology and chemistry, extraction of target molecules from a primary liquid and concentration of these molecules in a secondary liquid are now recognized as essential operations before analysis and recognition processes. Many continuous-flow chemical processing (CFCP) micro-devices have been developed in the last years, using the principle of diffusion across an interface maintained stable between the two liquids. So far, the development of such devices is mostly experimental. In this work, we focus on the physical phenomena governing the stability of the interfaces and the extraction. Our system is constituted of two adjacent microchannels geometrically separated by vertical micro-pillars. The primary and secondary liquids are immiscible, and vertical interfaces attached to the pillars separate the two fluids. Two main constraints apply for such systems: first, the interfaces must be stable and remain attached to the pillars at all times, second the interfacial area must be sufficient to provide an efficient mass transfer. In this work, we develop a model for the stability of interfaces attached to pillars and a model for the determination of the efficiency of the mass transfer. Optimization rules are deduced from these two models.