Hydrodynamics of aqueous two-phase systems (ATPS) in millichannels

Aqueous Two-Phase Systems (ATPS) have been recently employed for purification, concentration, synthe-sis, and separation of biomaterials and nanomaterials. Depending on the application, either parallel flow or segmented flow in microchannels is preferred. These ATPS are characterized by interfacial tensions, which are two orders of magnitude lower than typical organic aqueous systems. This results in only par-allel flows in microchannels as the capillary numbers are high (Ca >> 3). Consequently, the flow observed here is stratified flow or core annular. The internal circulations in segmented flows in microchannels offer enhanced mixing. In the context of nanoparticle synthesis, this results in monodisperse particles. In this work, we show that Ca can be lowered to Ca<<1 by using a millichannel. In this work, we have studied the different flow patterns of ATPS (Polyethylene glycol 6000-trisodium citrate system). Experiments were carried out in a milli-channel to span different flow patterns. Two cases were investigated i) the feed containing pure PEG and pure Citrate and ii) the feed containing the PEG-rich and citrate-rich solutions, which are at equilibrium. In the former case, there is a range of operating conditions when a single-phase flow is observed, while in the latter, we are always guaranteed a two-phase flow. The pattern formation was analysed and classified into three main categories, i.e., slug flow (interfacial tension dominated), transition flow, and core annular flow (inertia dominated). Flow regime maps based on the Reynolds number, Capillary number, and Weber number of each phase were found to be qualitatively similar to those of the Organic Aqueous System (OAS). Slug lengths and film thickness measured were validated with of numerical simulation.