Hydrodynamic Analysis of Polymer–Salt Aqueous Two-Phase Systems in a Millifluidic Channel

The integration of aqueous two-phase systems (ATPSs) with microfluidics delves into the possibility of a wide spectrum of applications. The effectiveness of these integrated processes depends on the hydrodynamic study of two immiscible aqueous streams. This work investigates the hydrodynamics of three different polymer–salt ATPSs in a millifluidic channel. The flow patterns noted during the flow experiments are categorized as droplet, slug, jetting, irregular, and stratified flows. The interplay of forces between two liquids at the junction is the determining factor for the type of flow pattern in the channel. Dimensional analysis was carried out to propose a universal flow regime map based on unified dimensionless numbers (We.Ca^0.5) to satisfactorily segregate the observed flow patterns of different polymer–salt ATPSs studied. Further, the slug length (L_s) is an integral parameter that defines the effectiveness of transport and encapsulation processes. The predictive empirical correlations for the calculation of the slug length (L_s) of the stable continuous-dispersed flow patterns based on competing stresses, flow rate ratio (Q_R), and viscosity ratio (μ_R) are proposed for polymer–salt ATPSs in general. The proposed correlations predict the slug length with an average standard deviation of ±5%. Further, two empirical correlations were proposed for Q_R < 1 and Q_R ≥ 1, which predict the slug length with an average deviation of ±3.44 and ±3.87%, respectively.