Interfacial instability and transition of jetting and dripping modes in a co-flow focusing process

Abstract

The breakup dynamics of coaxial liquid interfaces into compound droplets in a co-flow focusing process is studied systematically. In experiments, the jetting and dripping modes downstream the focusing orifice are identified within the parametric regime where a coaxial liquid cone can be established steadily, and the phase diagram is plotted under different flow rates of inner, outer, and driving liquids. The force balance for the jet interface is analyzed numerically to explore the critical conditions for the jetting-dripping transition. It is found that the instability of the inner interface is much easier to trigger the modes transition, and the transition criterion is decided by the balance of inertia force, shear stress, and interfacial tension at the local inner jet. The linear spatiotemporal instability analysis is further carried out to study the convective and absolute instabilities of the coaxial jets. The effects of main process parameters on jet instability are accessed, and the boundary between the absolute/convective instabilities is further compared with the experimental and numerical results, which achieves good agreement. Finally, the energy budget analysis of the instability of coaxial liquid jets is performed to provide more understanding of physical mechanisms for the mode transition.

Modulation of coaxial cone-jet instability in active co-flow focusing

“…In this work, the values of Q 3 vary within 1150 and 2100 ml h −1 , corresponding to the change of Re from 27.9 to 49.1. The experimental results within this parameter range have shown that the coaxial liquid jets maintain the axisymmetric evolution without occurrence of non-axisymmetric perturbation (Mu et al 2020a(Mu et al , 2021b. The numerical code has been carefully validated by experiments reported in previous studies (Mu et al 2020a(Mu et al , 2021b(Mu et al , 2022, where good agreements of the coaxial cone-jet interface profiles and the generation of compound droplets can be reached.…”

“…Therefore, the geometric parameters are chosen moderately and kept constant in our work. The liquids are chosen as a water-in-oil-in-water system, which is the same as considered in our previous studies (Mu et al 2020a(Mu et al , 2021b. Therefore, the coaxial cone-jet flow corresponds to a two-phase liquid system (i.e.…”

“…The parameters chosen for numerical simulations correspond to our previous experimental set-up without flow rate actuation (Mu et al. 2020 a , 2021 b , 2022), where the core and focusing liquids are chosen as the distilled water ( kg m and Pa s), and the shell liquid is chosen as silicone oil with constant dynamic viscosity ( kg m and Pa s). The interfacial tension coefficient between water and silicone oil is mN m.…”

The liquid–liquid flow dynamics and droplet formation in a modified step T‐junction microchannel

“…Different droplet breakup regimes have been demonstrated of possessing vastly different droplet dispersion mechanisms 26 . In dripping regime, the viscous force that deforms the immiscible interface overcomes the interfacial tension and neck the growing dispersed liquid into highly monodisperse microdroplets whose diameter relies on the dimension of microchannel 27 . On the other hand, when the flow rate ratio of continuous and dispersed phase overpasses certain critical value, the flow conditions within microchannel would land in jetting regime.…”

“…26 In dripping regime, the viscous force that deforms the immiscible interface overcomes the interfacial tension and neck the growing dispersed liquid into highly monodisperse microdroplets whose diameter relies on the dimension of microchannel. 27 On the other hand, when the flow rate ratio of continuous and dispersed phase overpasses certain critical value, the flow conditions within microchannel would land in jetting regime. Herein, the dispersed liquid jets and ultimately breaks up into microdroplets at high throughput under the effects of the Rayleigh-Plateau instability.…”