The Numerical Simulation About Motion Of W/O Micro-droplets Guided By Pillar Array

Since the90s of last century, an important trend of the natural sciences andengineering technology development is moving towards miniaturization. Attentionstransfer away from the familiar conventional scale to the unfamiliar micro/nano-scale,especially in micro-mechanical systems (MEMS) and nano-material fields.This article applied phase-field method and established a two-phase microscalemodel to achieve the water-liquid two-phase fluid dynamic simulation，which was basedon the laminar multiphase flow theory. It studied the manipulation technology ofwater-in-oil (W/O) droplets immersed in silicone oil. The detail contents are listed asbelow:Firstly, the surface tension was introduced into N-S equation on the basis oftwo-phase laminar flow theory，and the phase interface between two liquids wasdetermined through the function Φ. Secondly，considering the impacts of micropillararrays arranged inside microchannel，we investigated the trajectory and deformation ofdroplets, the velocities of droplets with different sizes, the positions of droplets releasedfrom the last micropillar. Next, we studied the merging of two droplets as well as themixing of two different concentration sub-regions inside one droplet. Finally, we alsoinvestigated the dividing of droplets by a wedge-shaped obstacle in the case that thesurface tension is dominant force.The results show that the micro-pillar array embedded in microchannel has a greatimpact for the movement of droplets. For the droplet shape，the background oil flow andpillar array will change the trajectory of droplets and force droplets to be deformed and stretched. For the velocities，droplets have the same velocity under the same flowcondition approximately, which is independent of the size of droplet. However, dropletsown the distinct releasing positions because they were deformed and affected by thepillars when leaving from the last pillar. For two droplets that are placed closely at theinitial stage, they will deform and merge together when they move into and touch withmicropillar array. The wedge-shaped obstacle located in the downstream of channel canmake different size droplets to be cut into two parts, and the size of one of daughterdroplets was almost the same. Combined the model of dilute species transfer, the mixinginside the droplet with different concentrations was simulated. Flow field inside thedroplet affected by micropillar array was changed and induce the uniform mixing ofdifferent concentration parts within the droplet. Through above a series of researchworks, we basically realized different functions of micro-droplets manipulation bysimulation, and the simulation agrees well with the previous experiments.In short, micropillar array enables to control the movement of micro-droplets, andto achieve the fusion, dividing, merging of droplets. Numerical method overcomes theshortcoming of experimental method with the very limited parameter range, and will behelpful to design a better W/O droplets manipulation. The present method provides amore robust research tool to explore the behavior of micro-droplet in future.