Hydrodynamic studies of liquid–liquid slug flows in circular microchannels

The aim of the work presented was to clarify the existence of a wall film and its influence on the hydrodynamics of liquid–liquid slug flow capillary microreactor.The methodology of the laser induced fluorescence (LIF) was adopted for visualisation purposes. The measurement of the light intensity profiles revealed a fully developed wall film for a variety of aqueous–organic two-phase systems in glass and PTFE capillaries of 1 mm internal diameter. In addition an acid as a quenching agent enabled the observation of the internal circulation patterns within the liquid slugs, as the fluorescent dye was deactivated by the acid diffusing in from the dye-free phase. A well-defined internal circulation pattern was always present in the wetting phase, i.e. that forming the wall film, leading to uniform mixing in the slugs of this phase. Stagnant zones and local circulation vortices, indicated by variations in the concentrations of the quenched dye, were observed in the non-wetting dispersed phase. These more complex flow structures varied little with the slug velocity, but were strongly dependent on the physical properties of the liquid–liquid system. To predict slug shape and hydrodynamics within the liquid slugs, CFD simulations were carried out using the volume-of-fluid method (VOF) based on the incompressible Navier–Stokes equation with appropriate boundary conditions between the two phases. The slug generation process was studied in a T-junction with 1 mm internal diameter inlets. The implementation of the wetting contact angle, measured in the visualisation experiments for the various systems, led to realistic slug lengths and shapes. The velocity vector plot indicated a fully developed internal circulation pattern within the simulated slugs. Calculations for a single slug with a non-wetting condition gave rise to a wall film in the simulated system.The results obtained demonstrate the significance of the wall film in the hydrodynamics and mass transfer liquid–liquid slug flow and reveal the presence of hitherto unsuspected complex patterns in place of simple single Taylor vortex flow assumed in the past.