Bian Xiongheng, Huang Haibo, Chen Liguo
Robotics & Microsystem Center, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University Suzhou 215123 China
RSC Adv. 2019 Oct 10;9(55):32278-32287. doi: 10.1039/c9ra05135j. eCollection 2019 Oct 7.
Due to the superior operability and good anti-interference, the prospect of controlling microdroplets using a parallel plate structure (PPS) is very promising. However, in practical applications, droplets in such structures are often affected by various factors, resulting in deformation, evaporation, stress rupture and other phenomena, leading to equipment failure. Therefore, how to simply and effectively transfer liquid droplets to PPS to maintain the stable and efficient operation of the system has become an urgent problem to be solved. In this paper, a simple and effective ratchet-like strategy (relaxing and squeezing actions) is introduced to transfer droplets. To analyze the mechanism of the strategy and optimize the control, we conduct this study from three aspects. First, the droplet movement trend is obtained by analyzing the pressure between SPS and PPS. Second, the reasons why the droplet can achieve this inward motion are investigated. Through theoretical analysis, which is also proven by simulations and experiments, we creatively put forward that the asymmetric change of the contact angle (CA) induced by the asymmetric structure is the fundamental cause of this kind of motion. Due to the asymmetric change of the contact angle, the CA in the PPS will reach the advancing angle first in the squeezing process, and the CA in the SPS will reach the receding angle first in the relaxing process, thus causing the inward movement of the droplet. Third, to optimize this strategy, the effects of the following governing parameters are researched individually based on the corresponding simulations and experiments: the control parameters (the initial gap width of the PPS and the amount of squeezing and relaxing of Δ) and the thickness of the top plate. Subsequently, an optimized ratchet-like cycle is achieved. In summary, these findings not only provide a new method by which to realize the movement of droplets toward hydrophobic PPSs but also creatively point out the cause of the ratchet strategy, which can be applied in many microfluidics fields.
由于具有卓越的可操作性和良好的抗干扰性，利用平行板结构（PPS）控制微滴的前景十分广阔。然而，在实际应用中，此类结构中的液滴常常受到各种因素的影响，导致变形、蒸发、应力破裂等现象，进而造成设备故障。因此，如何简单有效地将液滴转移至PPS以维持系统的稳定高效运行已成为亟待解决的问题。本文引入了一种简单有效的棘轮状策略（松弛和挤压动作）来转移液滴。为分析该策略的机制并优化控制，我们从三个方面开展了这项研究。首先，通过分析SPS和PPS之间的压力来获取液滴的运动趋势。其次，研究液滴能够实现这种向内运动的原因。通过理论分析，并经模拟和实验验证，我们创造性地提出由不对称结构引起的接触角（CA）不对称变化是这种运动的根本原因。由于接触角的不对称变化，在挤压过程中PPS中的CA会先达到前进角，而在松弛过程中SPS中的CA会先达到后退角，从而导致液滴向内运动。第三，为优化该策略，基于相应的模拟和实验分别研究了以下控制参数的影响：控制参数（PPS的初始间隙宽度以及Δ的挤压和松弛量）和顶板厚度。随后，实现了优化的棘轮状循环。综上所述，这些发现不仅提供了一种实现液滴向疏水PPS移动的新方法，还创造性地指出了棘轮策略的成因，可应用于众多微流控领域。
