Theoretical Investigation on Flow Instability of Liquid Jets in Co-flow Focusing

Microcapsules with core-shell structures have greatly potential applications in various areas including medicine, pharmaceutical industry, material science, food and agriculture. Liquid driven co-flow focusing is an emerging technique for producing monodisperse microcapsules due to the break up of compound liquid jets. Experimental studies have been performed to capture the morphology of compound jets. A linear instability analysis of three-phase water-oil-water jets has been carried out based on a simplified physical model. The results indicate that the interfacial tension of both inner and outer liquids suppresses the stability of the jets, while the liquid viscosities promote it. A lower inner interfacial tension and higher liquid viscosities result in a longer wavelength of perturbations for the coaxial jet breakup. The coupling effects of the inner and outer interfaces and the encapsulation process of the compound droplets are both closely related to the radius ratio of inner and outer liquid jets. The critical Weber number for the absolute/convective transition increases as the Reynolds number and the inner interfacial tension increase. The results will provide theoretical guidance to practical applications by improving the process control of liquid driven co-flow focusing.