Eriments And Theories Of Droplet-Based Microfluidics

We start this thesis with the overview of rapid development of microfluidics. The simplest soft-lithography method based on a portable UV LED, which may greatly decreases the fabrication cost of PDMS chips, is introduced here. Since microdroplets have great advantage in various of biochemical analysis, our focus is also about droplets. In experimental part, we uses PDMS flow-focusing microchannel and a3D easily assembled "off the shelf device (Micro-Cross) to generate highly monodisperse droplets. Both of the two devices can achieve droplet passive breakup to obtain smaller droplets or add the droplet concentration. We find the commercial Micro-Cross is also capable of perfectly producing double emulsion by serially connecting two of them. Cell capsulation based on microdroplets is also studied, and further work still need to be done to fulfill the single cell capsulation technology. The instability of a droplet passing through a sudden contraction cross-section/nozzle is also investigated experimentally here, from which we draw a conclusion that the higher the Capillary and Weber number is, the more easily the droplet breakup into smaller droplets.In theoretical part, we firstly deduce the thread size which is the most significant parameter in theoretical analysis according to the assumption of fully developing flow. When the breakup occurs near the exit of the orifice, the thread maintains its size all through the orifice and the droplet grows and expands at the thread head. We analogy this as the process of droplet formation in co-flowing capillary tube, and describe the droplet breakup as the competition of deathching force and holding force. As for the breakup occurring at the entrance of the orifice, we explain that the relativeness of Laplace pressure and viscous pressure drop between two fluids play a main role. We try to find the critical condition to judge the breakup occurring at the entrance of orifice or not. Our obsevations about the evolution of thread also confirm that the3D collapse is more faster than2D case.In simulational part, we study droplet formation in co-flowing and T-shape shear flowing microfluidics basing on VOF method in FLUENT. Various flowing and material parameters are explored in detail in numerical method, and the pressure distribution of two phase can be also understanded in VOF. Double emulsion and droplet passive breakup are also simulated. In addition, we introduce the Level Set method in COMSOL, and point out that the conservation of the Level Set equation must be solved to obtain the cogent results.In the end, we summarize the other research work about applications based on microfluidics such as microfluidic fuel cell, mixing strengthen, blood typing and separation, and so on.