1.Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China 2.College of Electrical, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51725602, 51906039), the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK20180405, BK20180102), and the Fundamental Research Funds for the Central Universities (Grant No. 2242019k1G008)
Received Date:22 July 2020
Accepted Date:13 November 2020
Available Online:18 March 2021
Published Online:05 April 2021
Abstract:Asymmetric droplet splitting is a common method to obtain micro-droplets of different sizes. The study of droplet asymmetric splitting behaviors is of great significance to the fields of biomedicine, energy, chemical industry and food engineering. In this paper, the control flow is introduced into a branch of the T-shaped microchannel to control the pressure distribution in the channel and precisely control the size of the daughter droplets. The method is simple to operate and is a preferred method for asymmetric microfluidic splitting. Existing studies have analyzed droplet splitting modes, critical conditions for flow pattern transitions, and splitting dynamics, but the theoretical prediction of droplet asymmetric splitting behaviors needs to be strengthened. Moreover, compared with tunnel splitting and obstructed splitting, which are more abundantly studied, neither semi-obstructed splitting as an intermediate state of tunnel splitting nor obstructed splitting is analyzed sufficiently. Therefore, a microfluidic T-junction chip is designed and fabricated, with which asymmetrical splitting behaviors of droplets with a tunnel in a microfluidic T-junction are investigated experimentally. The influence of flow rate regulation on the droplet splitting ratio is studied. And a theoretical model is also established to predict the splitting ratio. The results are concluded as follows: 1) the process of asymmetrical droplet splitting is divided into three stages i.e. early squeezing, late squeezing and rapid pinch-off stage. In the early stage of squeezing, the radius of curvature of the droplet neck is sizable, and the additional pressure of interfacial tension is minor. Compared with the additional pressure that hinders neck contraction, the upstream continuous phase driving force is dominant, and the width of the neck changes linearly with time; in the process of late squeezing, the upstream pressure driving effect is still greater than the hindering effect of the additional tension, and the neck width changes exponentially with time; However, in the rapid pinch-off stage, the interfacial tension pointing to the center of the cross section of droplet neck dominates the pinch-off stage. Then, the droplet neck shrinks sharply. 2) Adjusting the flow rate of the branch channel can effectively control the asymmetric splitting ratio of the droplets, and under the current semi-obstructed asymmetric splitting of the droplets, the regulation effect is less affected by the size of the mother droplet, but more affected by the capillary number. 3) The prediction model of droplet splitting ratio based on the pressure drop model can effectively predict the droplet splitting ratio. Keywords:microfluidic/ droplet/ asymmetric/ breakup
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2.1.芯片结构
如图1(a)所示, 本文所用微流控芯片由4部分组成: 流动聚焦微通道、辅助微通道、T型分裂微通道和调控微通道. 液滴在流动聚焦微通道内生成, 流经辅助微通道, 最后进入T型分裂微通道, 在调控微通道的控制作用下发生分裂. 流动聚焦微通道为十字形, 宽600 μm, T型分裂微通道宽w = 420 μm, 在辅助微通道处完成宽度过渡. 整块芯片的微通道深度h = 200 μm. 芯片由两块分别表面蚀刻有微槽道和表面平整的玻璃键合而成, 芯片具有良好的亲水疏油性. 辅助微通道内注入连续相流体以调整液滴流速, 进而调控主通道毛细数. T型分裂微通道的两个分支长度比为1∶2, 在较短分支下游的U型弯中部接入调控微通道以实现对液滴分裂比的调控. 流体最终从出口流出芯片.?液滴半阻塞不对称分裂示意图和实验系统图如图2和图3所示. 图 1 微流控芯片结构 (a)示意图, I-流动聚焦微通道, II-辅助微通道, III-T型分裂微通道, IV-调控微通道; (b)实物图, 其中Qc为连续相体积流量, Qd为离散相体积流量, Qf为辅助流量, Qt为调控流量 Figure1. Geometric structure of the microfluidic chip: (a) Schematic diagram, I-flow focusing microchannel, II-T-shaped splitting microchannel, III-tuning microchannel; (b) actual chip, Qc is volumetric flow rate of continuous phase, Qd is volumetric flow rate of dispersed phase, Qf is volumetric flow rate of supporting continuous phase, Qt is volumetric flow rate of controlling continuous phase
实验中所用连续相为甘油水溶液, 添加有0.5 wt%的十二烷基硫酸钠(SDS), 溶液密度ρc = 1062.6 kg/m3, 动力黏度μc = 3.8 mPa·s. 实验发现, 温度较低时SDS会发生沉淀析出, 导致液体浑浊、相界面模糊, 因此实验环境温度维持在20 °C. 实验中离散相为二甲基硅油, 密度ρd = 897.9 kg/m3, 动力黏度μd = 5.0 mPa·s. 两相界面张力σ = 10.00 mN/m. 图 2 液滴半阻塞不对称分裂示意图(u为流体速度, lm为母液滴长度, w为微通道宽度, Δx为子液滴头部位移, d为液滴颈部宽度, ld为子液滴长度, lc为子液滴间距) Figure2. Schematic of droplet asymmetrical splitting with a tunnel in a T-junciton (u is velocity of the fluid, lm is length of the mother droplet, w is width of the microchannel, Δx is displacement of the head of the daughter droplets, d is width of the droplet neck, ld is length of the daughter droplet, lc is interval of daughter droplets).
图 3 实验系统图 Figure3. Schematic of the experimental system.