Fang Weidong, Tao Zhi, Li Haiwang, Yin Shuai, Xu Tiantong, Huang Yi, Wong Teckneng
National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Research Institute of Aero-Engine, Beihang University, Beijing, 100191, China.
School of Mechanical and Power Engineering, Nanjing Tech University, Nanjing 211816, China.
Lab Chip. 2023 May 2;23(9):2341-2355. doi: 10.1039/d3lc00086a.
Droplet coalescence with fast response, high controllability and monodispersity has been widely investigated in industrial production and bioengineering. Especially for droplets with multiple components, programmable manipulation of such droplets is crucial for practical applications. However, precise control of the dynamics can be challenging, owing to the complex boundaries and the interfacial and fluidic properties. AC electric fields, with their fast response and high flexibility, have attracted our interest. We design and fabricate an improved flow-focusing microchannel configuration together with a non-contact type of electrode featuring asymmetric geometries, based on which we conduct systematic investigations of the AC-electric-field-controlled coalescence of multi-component droplets at the microscale. Parameters such as flow rates, component ratio, surface tension, electric permittivity and conductivity were given our attention. The results show that droplet coalescence in different flow parameters can be achieved in milliseconds by adjusting the electrical conditions, which shows high controllability. Specifically, both the coalescence region and reaction time can be adjusted by a combination of applied voltage and frequency, and unique merging phenomena have appeared. One is contact coalescence with the approach of paired droplets, while the other is squeezing coalescence, which occurs in the start position and promotes the merging process. The fluid properties, such as the electric permittivity, conductivity and surface tension, present a significant influence on merging behavior. The increasing relative dielectric constant leads to a dramatic reduction of the start merging voltage from the original 250 V to 30 V. The range of effective voltage for coalescence decreases with the addition of surfactant, offering a stricter and yet higher selectivity on electrical conditions, about 1500 V. The conductivity presents a negative correlation with the start merging voltage due to the reduction of the dielectric stress, from 400 V to 1500 V. Finally, we achieve the precise fabrication process of the Janus droplet implementation of the proposed method, where the components of the droplets and the coalescence conditions are well controlled. Our results can serve as a potent methodology to decipher the physics of multi-component droplet electro-coalescence and contribute to applications in chemical synthesis, bioassay and material synthesis.
在工业生产和生物工程中，具有快速响应、高可控性和单分散性的液滴聚并已得到广泛研究。特别是对于具有多种成分的液滴，这种液滴的可编程操纵对于实际应用至关重要。然而，由于复杂的边界以及界面和流体性质，精确控制动力学可能具有挑战性。交流电场具有快速响应和高灵活性，引起了我们的兴趣。我们设计并制造了一种改进的流动聚焦微通道结构以及一种具有不对称几何形状的非接触式电极，在此基础上，我们对微尺度下交流电场控制的多组分液滴聚并进行了系统研究。我们关注了流速、组分比、表面张力、介电常数和电导率等参数。结果表明，通过调整电学条件，不同流动参数下的液滴聚并可在毫秒内实现，具有高可控性。具体而言，聚并区域和反应时间均可通过施加电压和频率的组合进行调整，并且出现了独特的合并现象。一种是成对液滴靠近时的接触聚并，另一种是挤压聚并，它发生在起始位置并促进合并过程。流体性质，如介电常数、电导率和表面张力，对合并行为有显著影响。相对介电常数的增加导致起始合并电压从原来的250 V急剧降低到30 V。聚并的有效电压范围随着表面活性剂的添加而减小，在电学条件上提供了更严格但更高的选择性，约为1500 V。由于介电应力的降低，电导率与起始合并电压呈负相关，从400 V降至1500 V。最后，我们通过所提出方法的实施实现了Janus液滴的精确制造过程，其中液滴的组分和聚并条件得到了很好的控制。我们的结果可作为一种有效的方法来解读多组分液滴电聚并的物理过程，并有助于化学合成、生物测定和材料合成等应用。
