Slug flow hydrodynamics of immiscible fluids within a rectangular microchannel towards size-controllable fabrication of dextran-based cryogel beads

Cryogel beads are novel adsorbents with potential applications in biotechnology, environmental and tissue engineering. However, the controllable preparation of cryogel beads with expected small and uniform diameters is a challenging work. The micro-flow-focusing method combining with the cryo-polymerization has been suggested as a new approach for the fabrication of cryogel beads. In order to form the uniform droplet, the slug flow is generated in the microchannel, so the slug flow hydrodynamics are crucial and fundamental for this process. In the present work, the dextran-methacrylate (Dex-MA-PEGDA) solution and ethyl acetate containing surfactant of Span 80 were used as the aqueous phase and the water-immiscible, respectively. The fabrication of dextran-based cryogel beads and the slug flow hydrodynamics in a rectangular microchannel were investigated experimentally. The flow regime map was proposed and the typical regime boundaries were measured and correlated, and the formation mechanisms of slugs were also discussed. The regime boundaries were correlated by Weber number and Capillary number, i.e., We o  = 58.16 Ca w 0.92 for the boundary between the annular and the slug flow, and We o  = 4.12 Ca w 0.09 for the slug and the bubbly flow, respectively. Several crucial parameters of the droplets generated by the slug flow, i.e., the aqueous slug length, the aqueous slug velocity and the aqueous droplet size, were measured and correlated. The Dex-MA-PEGDA cryogel beads were prepared successfully under different conditions and their diameters were correlated to the sizes of the droplets. Eventually, the uniform size distributions and supermacroporous microstructures of dextran-based cryogel beads were demonstrated.