Hydrodynamic characteristics of core/shell microdroplets formation: Map of the flow patterns in double-T microchannel

Abstract

Liquid-liquid–liquid (L/L/L) flows in microchannels involving three-phase interactions are important in various chemical applications that entail liquid–liquid–liquid reactions. This work comprehensively investigates core (silicone oil)/shell (Polycaprolactone) microdroplet formation using a continuous third phase (distilled water) in a double T-junction microchannel with a diameter of 600 µm, employing Volume-of-Fluid (VOF) simulations and high-speed imaging experimental observation. Due to the satisfactory agreement between computational and experimental techniques, the theoretical flow patterns were identified and categorized into three distinct categories at various flow rates of three phases: core/shell dripping, core/shell slug, and core/shell jetting. The shell thickness increases as the ratio of shell to core phase velocity (ushell:ucore) increases. Based on the continuous phase Capillary number (Cac) and core and shell phases Weber number (Wecore,shell), a flow pattern map was generated that segregates the distinct flow patterns into different regions. When the flow of the core and shell phases fails to form a droplet due to the inertia of the continuous phase, a core/shell dripping/slug pattern is observed. The channel walls encompass the droplets that form in the slug pattern (2×10-5≤Cac≤5×10-4, 0<Weshell≤1.5×10-3, and 0<Wecore≤5×10-5) because the continuous phase has less inertia, whereas, in the dripping pattern (5.5×10-5≤Cac≤9×10-4, 7×10-5≤Weshell≤2.5×10-4, and 1×10-5≤Wecore≤7×10-5), the shell phase fluid is enclosed by the continuous phase. The transition from dripping to jetting (Cac≥0.5, 0<Weshell≤0.15, and 1×10-2≤Wecore≤8×10-2) occurs as the flow rate of a continuous fluid or shell fluid increases. A dimensionless correlation for shell thickness estimation with a 12 % tolerance was proposed with a core/shell/continuous phase's physical properties, surface tensions, and microdroplet size, using Buckingham's theory based on 200 CFD simulations. Additionally, the flow rate and viscosity substantially impact the internal fluid velocity and vorticity of core/shell microdroplets, whereas the influence of interfacial tension can be disregarded. This research lays the groundwork for future studies on mass transfer and reaction behaviors in liquid–liquid–liquid three-phase flow in double T-junction microchannels by exploring various hydrodynamic characteristics under different operating conditions for core/shell microdroplet formation.