Research On The Micro Droplet Generation Method And Its Application Based On The Plateau-Rayleigh Instability

Plateau-Rayleigh instability describes the fluid column breaks into small droplets under the influence of interfacial tension. As feature size of fluid decreases, interfacial tension begins to occupy the dominant position. As a result, Plateau-Rayleigh instability plays a more important role. It has a wide range of potential applications, such as micro-nano particles manufacture, microfluidics and inkjet technology.However, there are some limitations of Tomotika model which is the existing mainstream theory of Plateau-Rayleigh instability. Tomotika model assumes a viscous infinitely long liquid column is embedded in a static incompatible liquid and does not consider the impact of external thickness, neglects the effect of interfacial tension on the wavelength. Besides, one of the theoretical model is based on the assumption of no-slip boundary. However, in the microfluidic environment slip velocity cannot be ignored. Nowadays, many applications involve a uniform or non-uniform temperature field, and the current theoretical model of Plateau-Rayleigh instability did not consider the influence of temperature. Because of the cost of experiments, numerical simulation has become an effective research tool.In the support of the National Natural Science Foundation of China (No. 51375444), the micro-droplet generation based on Plateau-Rayleigh instability is studied in this paper. The work mainly completed is as follows:(1) Under specific micro-flow condition, the Plateau-Rayleigh instability model is established to describe the micro-droplets generation. In the modeling process, the following difficulties are solved:the applicability of the continuity hypothesis is demonstrated under the microfluidic environment; the interfacial tension is integrated into the momentum equation as the volume force to facilitate solving; due to dramatic changes in the topology of breakup droplet, improved level set method is selected. Phase reinjection method is adopted to reduce the quality leak; slip condition is added.(2) Based on the above model, Plateau-Rayleigh instability in microfluidic environment is solved at the aid of the numerical simulation. First, the results of the simulation are compared with the literature experimental data to verify the correctness of the method. Then, the problems of Tomotika model are mainly studied by simulation. Relation between wavelength and droplet diameter is obtained by theoretical derivation. The results show that the inner diameter and interfacial tension are linearly correlated with instability growth rate. Interfacial tension and thickness are inversely proportional to the wavelength of the instability.(3) The flow field is coupled with temperature field to study the effects of the two-phase temperature and temperature gradient on instability. The simulation results confirm temperature instability effect on the instability matches with the viscosity effect, while the surface tension remains the same. The existence of critical viscosity ratio point is confirmed, when it is larger than critical point, wavelength was positively correlated with the viscosity ratio. Compared to a unified temperature field, the temperature gradient is more likely to induce the instability.(4) Flow focusing channel is studied based on the Plateau-Rayleigh numerical model. First, the simulation results are compared with the experimental data to validate our model. Next, simulation results show that Plateau-Rayleigh instability begins to dominate after the formation of the discrete phase flow. The effect of some factors on droplet generation, such as the two-phase velocity, viscosity, interfacial tension, temperature and other factors. The critical parameters and effective design are obtained. Finally, based on the simulation results, flow focusing channel is designed and tested, experimental and simulation results are in good agreement.These studies show that the methods and results of this paper are conducive to reveal the mechanism of flow instabilities in micro/nano droplet generation process. It is contributed to the development of the more precise control method for micro-nano droplet generation and manipulation. It also provides theoretical basis for the invention of new micro-nano devices which is based on the microfluidic manipulation.