Favakeh Amirhossein, Bijarchi Mohamad Ali, Mohammadrashidi Mahbod, Yaghoobi Mohammad, Shafii Mohammad Behshad
Center of Excellence in Energy Conversion (CEEC), Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran.
Sharif Energy, Water, and Environment Institute (SEWEI), Tehran, Iran.
PLoS One. 2025 May 23;20(5):e0321099. doi: 10.1371/journal.pone.0321099. eCollection 2025.
Digital microfluidics for ferrofluids enables the manipulation of discrete droplets on open surfaces and has garnered significant interest as an alternative to traditional continuous-flow microfluidic systems. However, droplet generation within digital microfluidics remain underdeveloped. This study introduces a novel method for droplet generation using a wedge-shaped surface with hydrophilic-hydrophobic patterning, which functions as a two-dimensional flat nozzle. We first demonstrated the concept by investigating gravity-driven water droplet generation on a sloping surface, revealing that smaller droplets form at higher tilting angles, while droplet size remains constant with increasing flow rate. Frequency of droplet formation decreases by 60% with decreasing the tilting angle from 90° to 30°. The proposed method results in significant improvement in frequency (10 Hz) compared to nozzle-based droplet generation (1-5 Hz). We then extend this approach to ferrofluid droplets under an external magnetic field, observing five distinct steps in the formation process. Additionally, a scale analysis of both water and ferrofluid droplet generation provides a deeper theoretical understanding of the governing forces, showing a strong correlation between non-dimensional droplet diameter and the Bond number, following a -1/3 power law (R2 > 0.95). The derived empirical factor offers precise droplet diameter predictions, with an average error of 3.9%. Finally, inspired by cactus structures, we demonstrate parallelization of the flat nozzles, highlighting the potential for high-throughput droplet generation in digital microfluidic applications.
用于铁磁流体的数字微流控技术能够在开放表面上操控离散的液滴，作为传统连续流微流控系统的替代方案，已引起了广泛关注。然而，数字微流控中的液滴生成技术仍不够成熟。本研究介绍了一种利用具有亲水-疏水图案的楔形表面生成液滴的新方法，该表面可作为二维平面喷嘴。我们首先通过研究倾斜表面上重力驱动的水滴生成来验证这一概念，结果表明，在较高倾斜角度下形成的液滴较小，而液滴尺寸随流速增加保持不变。将倾斜角度从90°减小到30°时，液滴形成频率降低60%。与基于喷嘴的液滴生成（1-5Hz）相比，该方法使频率显著提高（10Hz）。然后，我们将此方法扩展到外部磁场下的铁磁流体液滴，观察到形成过程中的五个不同步骤。此外，对水和铁磁流体液滴生成的尺度分析提供了对支配力更深入的理论理解，表明无量纲液滴直径与邦德数之间存在强相关性，遵循-1/3幂律（R2>0.95）。导出的经验因子可精确预测液滴直径，平均误差为3.9%。最后，受仙人掌结构的启发，我们展示了平面喷嘴的并行化，突出了数字微流控应用中高通量液滴生成的潜力。
