微液滴制备及其在静止流体中的浮升过程

采用自制微流控共轴流装置制备出粒径dp=100~600 mm且球形度良好的单一粒径微液滴，对大豆油及甲苯两种微液滴在静止流体中的浮升过程和速度进行了研究. 结果表明，生成液滴的粒径dp主要由微通道锥口直径Dt (40~450 mm)确定；既定锥口直径下，液滴粒径随连续相流量Qc增大稳定减小，而增加分散相流量只改变液滴产生频率(生成颗粒群)，对粒径影响微弱，建立了dp与Qc的关联式，关联度大于0.99. 由于表面张力作用导致微液滴具有弹性球行为，微液滴的液滴阻力系数CD与雷诺数Re的关系与刚性颗粒一致；因尾流效应颗粒群中各颗粒在交替追赶中前行，导致颗粒群浮升速度高于单颗粒，颗粒群比单颗粒浮升速度高约30%.

Preparation of Micro-droplets and Their Rising Process in Stagnant Liquid

A co-axial microfluidic device was designed and assembled, and the micro-droplets of 100~600 mm with good spheric shape were successfully prepared. The test shows that the droplet diameter dp was mainly determined by the tapered tip Dt (40~450 mm) of the microfluidic devices. For a given tapered tip, the dp decreased with increasing flow rate Qc of the continuous phase. The increase in the flow rate of the dispersed phase only had a minor effect on the droplet diameter, but increased the frequency of micro-droplet generation. Based on the test, the correlations between dp and Qc for the device of different sizes are presented and their correlation coefficients are all above 0.99. Then the rising velocities of micro-droplets of soybean oil and toluene in stagnant liquid were measured. The results show that, unlike the millimeter-scale droplets which usually have lower drag coefficient CD than the rigid particles due to deformation, the relation of CD versus Re for micro-droplets is consistent with that for the rigid particles and the reason for this finding is given in terms of interfacial tension. Observation shows that, due to the wake effect, the bulk velocity of the droplet swarms is 30% higher than the single droplet with the same diameter.