Son ChangHee, Yang Zhengyu, Kim Seungbeom, Ferreira Placid M, Feng Jie, Kim Seok
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 37673, South Korea.
ACS Nano. 2023 Dec 12;17(23):23702-23713. doi: 10.1021/acsnano.3c07360. Epub 2023 Oct 19.
Droplet manipulation has garnered significant attention in various fields due to its wide range of applications. Among many different methods, magnetic actuation has emerged as a promising approach for remote and instantaneous droplet manipulation. In this study, we present the bidirectional droplet manipulation on a magnetically actuated superhydrophobic ratchet surface. The surface consists of silicon strips anchored on elastomer ridges with superhydrophobic black silicon structures on the top side and magnetic layers on the bottom side. The soft magnetic properties of the strips enable their bidirectional tilting to form a ratchet surface and thus bidirectional droplet manipulation upon varying external magnetic field location and strength. Computational multiphysics models were developed to predict the tilting of the strips, demonstrating the concept of bidirectional tilting along with a tilting angle hysteresis theory. Experimental results confirmed the soft magnetic hysteresis and consequential bidirectional tilting of the strips. The superhydrophobic ratchet surface formed by the tilting strips induced the bidirectional self-propulsion of dispensed droplets through the Laplace pressure gradient, and the horizontal acceleration of the droplets was found to be positively correlated with the tilting angle of the strips. Additionally, a finite element analysis was conducted to identify the critical conditions for dispensed droplet penetration through the gaps between the strips, which hinder the droplet's self-propulsion. The models and findings here provide substantial insights into the design and optimization of magnetically actuated superhydrophobic ratchet surfaces to manipulate droplets in the context of digital microfluidic applications.
由于其广泛的应用，液滴操控在各个领域都引起了极大的关注。在众多不同的方法中，磁驱动已成为一种有前途的远程和即时液滴操控方法。在本研究中，我们展示了在磁驱动超疏水棘轮表面上的双向液滴操控。该表面由锚固在弹性体脊上的硅条组成，顶部具有超疏水黑色硅结构，底部具有磁性层。硅条的软磁特性使其能够双向倾斜以形成棘轮表面，从而在改变外部磁场位置和强度时实现双向液滴操控。开发了计算多物理场模型来预测硅条的倾斜，证明了双向倾斜的概念以及倾斜角滞后理论。实验结果证实了硅条的软磁滞和相应的双向倾斜。由倾斜硅条形成的超疏水棘轮表面通过拉普拉斯压力梯度诱导了所分配液滴的双向自推进，并且发现液滴的水平加速度与硅条的倾斜角呈正相关。此外，进行了有限元分析以确定所分配液滴穿过硅条之间间隙的临界条件，这些间隙会阻碍液滴的自推进。这里的模型和发现为在数字微流体应用背景下设计和优化磁驱动超疏水棘轮表面以操控液滴提供了重要的见解。
