![]() Based on the study of single step-emulsification device, Stolovicki generated droplet in parallel microchannels with volcanic structure, and studied the variation of droplet volume with the flow rate of dispersed phase. However, the transition of flow pattern and the effect of resistance in chamber have not been studied, which seriously rely on the interface evolution and determine the monodispersity. In addition, the prediction model of bubble volume under constant injection pressure of gas is proposed, for the viscosity ratio between dispersed and continuous phase μ d/ μ c ∈ . It’s found that the bubble volume is more affected by the liquid viscosity, rather than the injection pressure of gas. Stoffel et al used glycerol aqueous solution with mass fraction less than 60 % as the continuous phase, and nitrogen as the dispersed phase, to study the bubble formation in 256 parallel microchannels within a step-emulsificator. When the capillary number of internal phases exceeds a certain value, droplets are mainly produced under the Balloon mechanism. Li et al found that when the capillary number of the internal phase is small, droplets are generated under the Dripping mechanism. The Balloon regime is mainly caused by the convective instability of the liquid–liquid interface, and the pinch-off of the neck of the dispersed phase occurs at downstream of the step. Dripping is mainly caused by the absolute instability of the liquid–liquid interface, , and the pinch-off of the neck of the dispersed phase occurs within the step plane. clarified the flow patterns-Dripping and Balloon for droplet formation by using fluorinated oil of 1.4 mPa.s as the dispersed phase and deionized water with viscosity of 1.0 mPa.s as the continuous phase. The bubble (droplet) volume and formation frequency, and the evolution of gas (liquid) -liquid interface during bubble (droplet) formation are the important parameters for the manipulation of bubbles (droplets). If the variation magnitude of viscosity is great in high-viscosity liquid, the enough outcome of effect of viscosity can be obtained, which is different from the case in low-viscosity liquid. Viscous effect can be changed greatly when viscosity is higher. ![]() The research of viscous effect is helpful for us to achieve design of device and to optimize the performance of flow system. The viscosity of fluid, as an important property parameter, affects the system performance including multi-phase flow. Some Non-Newtonian fluids usually have high viscosity that can vary with the shear rate. Additionally, the high-viscosity liquid is ubiquitous in industry and research, like complex fluids of high-concentration polymer solutions, sol solutions, and nanoparticle suspensions. Therefore, it is necessary to study the generation of bubbles in high-viscosity liquids in step-emulsification microdevices. However, how bubbles are generated in high-viscosity fluids within step-emulsification devices remains still unclear. It has been acknowledged that how much geometry configuration of step-emulsification device determines the bubble formation is related closely to the liquid viscosity. The common geometry configuration of step-emulsification is characterized by the fact that the gas at a microchannel is driven into the liquid-filled chamber through a step, which relies on the sudden change of restriction on bubble formation. Among passive equipment, the step-emulsification microdevice plays an important role in bubble and droplet formation, dominated by surface tension and geometry of devices, , and has gained massive attention. Therefore, the passive-dominant equipment for the generation of bubbles has been emerged in recent years, thanks to the controlling of the bubble size by the geometry of the device. However, the monodispersity of bubble or droplet is significant in most applications, such as biomedicine and petrochemical industry, since the monodispersity determines the quality of product. ![]() Active-dominant methods for the formation of bubbles include T-junction, , flow-focusing junction, in which bubble volume mostly depends on operating conditions and other external energy source such as electric or magnetic field. The formation of bubble or droplet is usually realized by active-dominant and passive-dominant methods. Microfluidics technology is widely used in biotechnology test, medicine separation, and food cosmetics.
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