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We combine experimental investigation with numerical simulation to explore fundamental hydrodynamic effects of an inner droplet on deformation of a double emulsion droplet under shear. The transient deformation oscillation is found to be intensified by the inner droplet. Especially, we demonstrate that the double emulsion droplet can exhibit both larger and smaller steady deformation than the single-phase droplet, which arises from the competition between the coexisting enhancing and suppressing effects by the inner droplet on the deformation. We further provide a regime diagram to quantitatively recognize the respective dominant regime of these two effects, depending on the capillary number and radius ratio of the inner droplet to the outer one.

We investigate emulsion droplet formation in coflowing liquid streams based on a computational fluid dynamics simulation using the volume-of-fluid method to track the interface motion with a focus on the dynamics of the dripping and jetting regimes. The simulations reproduce dripping, widening jetting and narrowing jetting simultaneously in a coflowing microchannel in agreement with the experimental observations in this work. The result indicates that the dripping regime, rather than the jetting regime, is a favorable way to producing monodisperse emulsions. We find that, in dripping and widening jetting regimes, the breakup of a drop is induced by higher pressure in the neck which squeezes liquid into the lower-pressure region in subsequent and primary droplets, while the breakup in the narrowing jetting regime is due to slow velocity at the back end of the trough with respect to the leading end of the trough. In addition, the capillary number of the outer fluid and the Weber number of the inner fluid not only determine the drop diameter and generation rate but also the regime of emulsification.

A flow-focusing microfluidic chip based on assembled capillary method is developed and the solid-core encapsulation behaviors of double emulsion in a flow-focusing microchannel are visually observed via high-speed microscopic imaging system. The experimental results identify the encapsulation of single, double and triple cores in a flow-focusing microchannel arising from the competition among the inertial force of inner phase, the shear force of outer phase fluid, and the interfacial tension. In addition, the solid-core encapsulation of double emulsion in flow-focusing microchannel is either in the stable state or in the transition state, depending on the flow rates of inner and outer phase fluid. The stable microencapsulation includes the stable single-core, double-cores and triple-cores encapsulation states, while the transition microencapsulation includes the single-double-cores and double-triple-cores transition encapsulation state. Irrespective of encapsulation state, the solid-core encapsulation process can be classified into four stages, including entering, neck stretch, neck shrinking and breakup. [ABSTRACT FROM AUTHOR]

Microfluidic systems with splitting structures are excellent for increasing emulsion production. However, traditional two-dimensional (2D) lithographic systems require complex modification of the microchannel surfaces and achieve only 2D splitting of the droplets. Herein, we present new glass capillary microfluidic devices that perform three-dimensional (3D) splitting of droplets. These devices are simply constructed using different structural glass capillaries as the collection microchannels of the droplet microfluidic systems. We demonstrate that the devices are able to produce a 3D split of both single emulsions and double emulsions into two and three portions, respectively. These emulsions, after the splitting process, still have high monodispersity. We believe that this new technique for 3D splitting could be widely used, not only in the field of microfluidics but also in chemical/biological applications (e.g., drug delivery, micro-dispersion, etc.). [ABSTRACT FROM AUTHOR]

Highlights • A gravity-driven microfluidic system with stable pressure input is developed. • The microfluidic system is constructed with multi-unit assembly method. • High monodisperse emulsions with different sizes is generated through adjusting the overflow height differences. • An economical and efficient approach is proposed to prepare single, double and multi-core emulsions. Abstract Droplet microfluidics generally employed syringe pumps to infuse fluids into microchannels that created unavoidable interface fluctuation during emulsification. They also caused droplet differentiation among different batches due to the reloading of syringes. We herein reported a new gravity-driven approach to supply a steady pressure input on the microchannels and continuous liquid supplement due to the semi-open design. This microfluidic system combined by several overflow units could effectively reduce the disturbance on the multiphase interface, and single emulsions with high monodispersity could be continuously generated. By changing the overflow height, different sizes of single emulsions could be obtained. In addition, double emulsions could also be realized by simply adding another unit without effecting further changes in the system. These features indicated that the gravity-driven overflow microfluidic system could infuse fluids steadily and continuously, and could widely contribute to fields requiring less manual power. [ABSTRACT FROM AUTHOR]