Hassan Md Rifat, Zhang Jie, Wang Cheng
Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, 400 W. 13th Street, Rolla, Missouri 65409, United States.
Langmuir. 2021 May 18;37(19):5823-5837. doi: 10.1021/acs.langmuir.1c00141. Epub 2021 May 7.
Magnetic digital microfluidics is advantageous over other existing droplet manipulation methods, which exploits magnetic forces for actuation and offers the flexibility of implementation in resource-limited point-of-care applications. This article discusses the dynamic behavior of a pair of sessile droplets on a hydrophobic surface under the presence of a permanent magnetic field. A phase field method-based solver is employed in a two-dimensional computational domain to numerically capture the dynamic evolution of the droplet interfaces, which again simultaneously solves the magnetic and flow fields. On a superhydrophobic surface (i.e., θ = 150°), the nonuniform magnetic field forces the pair of sessile droplets to move toward each other, which eventually leads to a jumping off phenomenon of the merged droplet from the solid surface after coalescence. Also, there exists a critical magnetic Bond number , beyond which no coalescence event between droplets is observed. Moreover, on a less hydrophobic surface (θ ≤ 120°), the droplets still coalesce under a magnetic field, although the merged droplet does not experience any upward flight after coalescence. Also, the merging phenomenon at lower contact angle values (i.e., θ = 90°) appears significantly different than at higher contact angle values (i.e., θ = 120°). Additionally, if the pair of sessile droplets is dispersed to a different surrounding medium, the viscosity ratio plays a significant role in the upward flight of the merged droplet, where the coalesced droplet exhibits increased vertical migration at higher viscosity ratios.
磁数字微流控技术优于其他现有的液滴操控方法，它利用磁力进行驱动，并在资源有限的即时医疗应用中具有实施灵活性。本文讨论了在永磁体存在的情况下，疏水性表面上一对静止液滴的动态行为。在二维计算域中采用基于相场法的求解器，以数值方式捕捉液滴界面的动态演变，该求解器同时求解磁场和流场。在超疏水表面（即θ = 150°）上，非均匀磁场力迫使这对静止液滴相互靠近，最终导致合并后的液滴在聚结后从固体表面跳跃离开。此外，存在一个临界磁邦德数，超过该值后未观察到液滴之间的聚结事件。而且，在疏水性较低的表面（θ≤120°）上，液滴在磁场作用下仍会聚结，尽管合并后的液滴在聚结后不会向上飞起。此外，在较低接触角值（即θ = 90°）时的合并现象与在较高接触角值（即θ = 120°）时明显不同。另外，如果将这对静止液滴分散到不同的周围介质中，粘度比在合并后液滴的向上飞行中起重要作用，在较高粘度比下，合并后的液滴垂直迁移增加。
