Abstract: Micro-droplet formation in the crossing micro-channel was numerically simulated by the 3D VOF model. Three flow regimes to generate droplet with monodispersity (squeezing, dripping and jetting) and other regimes (turbulent jetting, nodular flow, tubular flow, slip flow) were obtained. The simulation results agreed well with experiment observations, which validated the numerical model. The flow regimes were mainly affected by flow rate, surface tension and viscosity of two phases. With increasing flow rate of the continuous phase, droplet size would decrease and generation frequency would increase, whereas the effect of flow rate of the disperse phase was the opposite. Surface tension and viscosity of the continuous phase played a leading role under the conditions of high and low Ca number of the continuous phase, respectively. When Ca number was low, droplet size decreased in the squeezing and dripping regimes and increased in the jetting regime with decreasing surface tension, while generation frequency shows the reverse changing tendency compared with droplet size. When Ca number of the continuous phase was high, however, droplet size decreased with increasing the viscosity of the continuous phase, while generation frequency changed inversely. In addition, contact angel had little effect on the squeezing regime, but droplet could not be generated in the dripping and jetting regimes when contact angel decreased.

Key words: microfluidics, two-phase flow, CFD, micro-droplet, flow regime

摘要： 采用VOF模型对十字交叉微通道内微液滴的生成进行三维数值模拟,获得了拉伸挤压、滴状剪切、单分散射流等单分散微液滴的生成机制以及紊乱射流、节状形变流、管状流和滑移流等两相流型,模拟与实验结果相吻合验证了模拟的有效性。液液两相流型主要受两相流速、两相界面张力以及连续相黏度的影响,发现随着连续相的流量增大,微液滴的生成尺寸减小,生成频率增大;而离散相流量的影响则相反。两相表面张力与连续相黏度分别在低连续相Ca数和高连续相Ca数条件下分别起主导作用。在低连续相Ca数(U_d<0.03 m·s^-1)的拉伸挤压和滴状剪切流流型下,微液滴生成尺寸随着表面张力系数的减小而减小,在射流条件下反而增大,微液滴的生成频率变化则相反。在高连续相Ca数(U_d>0.03 m·s^-1)下,微液滴的生成尺寸随着连续相黏度的增大而减小,微液滴的生成频率变化则相反。另外,壁面接触角在拉伸挤压流型下对微液滴生成无太大影响,但在滴状剪切和单分散射流流型下,接触角减小会导致微液滴无法稳定生成。

关键词: 微流体学, 两相流, 计算流体力学, 微液滴, 流型