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The release of trapped droplets in pore-throat structures is of great significance to study multiphase flow in porous media. In this paper, the effects of nanoparticle surfactants on the release behavior of trapped droplets in micro-pore throat are investigated using microfluidic visualization system and fluorescence techniques. We demonstrate a droplet control technique in microchannel and observe the release states of trapped droplets in pore-throat. We obtain the phase diagram of droplet states and establish mathematical models describing the critical transition condition by mechanism analysis. Based on the analysis of force on the trapped droplets, the breakup mechanism and the release mechanism are also obtained when droplets move through the pore-throat. The breakup of droplets is dominated by capillary pressure, with the critical capillary number of breakup being negatively correlated with droplet size. Conversely, the release of droplets is controlled by capillary pressure and hydrostatic pressure, with the critical capillary number of release exhibiting a positive correlation with droplet size. In addition, this research reveals the effect of nanoparticle surfactants on droplet release behavior by analyzing the variation of droplet length with flow velocity and capillary number. Nanoparticle surfactant reduces the critical flow velocity of droplet release but significantly increases the critical capillary number, and this phenomenon becomes more pronounced with the increase of concentrations of nanoparticle surfactants. Fluorescence experiments further elucidate the mechanism by which nanoparticle surfactants inhibit the release of trapped droplets in pore-throat by inducing interfacial viscoelasticity. Nanoparticles react with polymers at the interface to form the viscoelastic film. This film-induced interfacial viscoelasticity hinders droplet deformation and increases the viscous resistance between droplets and wall, thereby impeding the release of trapped droplets in pore-throat.
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Keywords:
- pore throat /
- droplet release /
- trapped droplet /
- nanoparticle surfactants
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图 1 微流控芯片结构及纳米颗粒表面活性剂分布示意图 (a)微孔喉芯片结构; (b) 液滴生成、加速和堵塞束缚实验示意图; (c) 纳米颗粒与表面活性剂油水界面反应组装示意图
Figure 1. Schematic diagram of the structure of the microfluidic chip and the distribution of nanoparticle surfactants: (a) Structure of the micro-pore throat chip; (b) schematic diagram of the experiments of droplet generation, acceleration and trapping confinement; (c) schematic diagram of the assembly of nanoparticle surfactants at the oil-water interface.
图 2 微孔喉中单个束缚液滴的典型释放过程 (a)束缚液滴尺寸 L/w1 = 8的释放过程; (b) 束缚液滴尺寸L/w1 = 5的释放过程; (c) 束缚液滴尺寸L/w1 = 3.5的释放过程
Figure 2. Typical release processes of single confined droplets in the micro-pore throat: (a) Release process of the confined droplet with a size of L/w1 = 8; (b) release process of the confined droplet with a size of L/w1 = 5; (c) release process of the confined droplet with a size of L/w1 = 3.5.
图 3 干净油水界面下单个液滴的束缚、破碎释放和直接释放相图, 图中的水平虚线从上至下分别为L/w1 = 7.7(受限液滴额外受到孔隙壁面的摩擦阻滞作用), L/w1 = 5(由于通道高度有限, 液滴被微通道挤压成饼状液滴), L/w1 = 4.1(液滴破碎释放与直接释放的尺寸界限), 序号①, ②, ③, ④分别对应(3)式、(6)式、(8)式、(10)式的曲线形式
Figure 3. Phase diagram of confinement, fragmentation and direct release of a single droplet with clean oil-water interface. In this diagram, the horizontal dashed lines from top to bottom represent the following scenarios: L/w1 = 7.7 (restricted droplets additionally experience frictional resistance from the pore wall), L/w1 = 5 (Owing to the limited channel height, droplets are squeezed into pie-shaped droplets by the microchannel), and L/w1 = 4.1 (this value marks the size threshold between droplet breakup release and direct release). The serial numbers ①, ②, ③, and ④ correspond to the curve forms of Eqs. (3), (6), (8), and (10), respectively.
图 4 不同试剂作用下微孔喉中单个束缚液滴(相同大小L/w1 = 3.5)的临界释放状态 (a)纯水纯油组; (b)仅添加纳米颗粒组; (c)纳米颗粒表面活性剂组
Figure 4. Critical release state of a single confined droplet with the same size (L/w1 = 3.5) in the micro-pore throat under different reagents: (a) Pure water and pure oil group; (b) only nanoparticles group; (c) nanoparticles surfactant group.
图 5 纳米颗粒表面活性剂作用下单个液滴的束缚、破碎和释放相图.
Figure 5. Phase diagram of confinement, fragmentation and direct release of a single droplet under the action of nanoparticles surfactant.
图 6 不同浓度纳米颗粒表面活性剂对单液滴的临界破碎毛细数(实心)和临界释放毛细数(空心)的影响
Figure 6. Effects of different concentrations of nanoparticles surfactant on the critical capillary number of breakup (solid) and critical capillary number of releasing (hollow) of a single droplet.
图 7 纳米颗粒表面活性剂作用下液滴破碎的荧光示意图 (a)纳米颗粒表面活性剂作用下的液滴破碎过程; (b)液滴破碎过程中的纳米颗粒荧光示意图
Figure 7. Fluorescence diagram of droplet fragmentation under the action of nanoparticle surfactant: (a) Droplet breaking process under the action of nanoparticle surfactant; (b) fluorescence diagram of nanoparticles during droplet fragmentation.
表 1 不同流体系统的界面张力
Table 1. The interfacial tension for three different fluid systems.
试剂组别 质量百分比/% γ/(mN·m–1) COOH-PS NH2-PDMS-NH2 纯水纯油 0 0 25.2±0.6 仅添加纳米颗粒 0.5 0 24.7±0.4 纳米颗粒表活剂 0.001 1 12.0±0.7 0.005 1 10.3±0.7 0.05 1 6.4±0.8 
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