液珠受介電潤濕影響於超疏水表面之動態分析

Dynamics analysis of droplets on super-hydrophobic surface under influence of dielectrowetting effect

白耘翰(Yun-Hun Bai)

指導教授 ： 江宏仁

國立臺灣大學/工學院/應用力學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201900500

摘要

所謂的實驗室晶片（Lab-on-chip）為近幾年的研究主流，有許多使用電潤濕原理，來驅動數位式微流體的相關文獻，目前最新的設計是採用指叉狀電極陣列，以介電潤濕效應（Dielectrowetting）作為驅動力。此種類型的裝置在製造和操作上，皆享有優勢。但目前為止，我們尚未完全明瞭介電潤濕在驅動的過程中，影響液珠動態行為的各種因素。我們希望能對此有更佳深入的了解。本篇論文主要探討使用指叉狀電極設計的數位式微流體制動器，在驅動液珠的過程中，影響液珠動態行為的種種因素。
首先我們製作出一具有超疏水表面之介電潤濕裝置，用該裝置量測了水珠（DI Water）在靜態之下接觸角。也在施加交流電場的情況下，找出了電壓與接觸角之間的關係，以及水珠受電場影響後達到平衡態所需的反應時間。接著讓水珠在以不同傾斜角度在裝置表面滾落，過程中經過和中軸夾45度角之電極，和未施加電壓的情況相比，其路徑會產生橫向偏折，偏折量和電壓大小呈正相關。在我們分析了偏折量與電壓間的關係和橫向作用力，發現水珠在離開電極的當下偏折最為明顯，且不能只用介面間接觸角差異來解釋，還有其他因素的影響。
我們更進一步，重新設計電極，讓水珠通過與中軸呈0度及90度夾角之電極，觀察到兩者之間運動情形，有明顯差異。分析結果發現在介電潤濕效應中，方向性的親疏水，對水珠動態行為有很大的影響。
我們最後在介電潤濕裝置上方2mm處增加一片平板，將水珠固定於此。接著等速移動下方的電極，比較不同速度、不同電壓下水珠的動態接觸角，並且用分子動力理論（MKT）進行分析。結果發現在水珠行徑方向與電場平行時，若是將介電泳力等校於毛細力來建立模型，可以有效描述動態接觸角受電場的影響，但當行徑方向與電極垂直時，還沒辦法精準的使用模型分析。

關鍵字

指叉狀電極 ； 介電潤濕 ； 超疏水 ； 動態接觸角 ； 接觸角遲滯

並列摘要

Lab-on-chip has been a popular research topic in recent years. The latest actuator is driving droplets on interdigitated electrodes using the mechanism so called “dielectrowetting”. This kind of device only require a single plate of electrodes.
This system does not require the top plate (cover), allowing for the handling of a much wider range of liquid volumes with easily accessible and simplified structures. Bus so far, we don’t fully understand the dynamics behavior of droplets during actuating process. This paper is focus on the dynamics behavior of droplets in digital microfluidic actuator driven by dielectrowetting.
In this experiment, we represented a dielectrowetting device with super-hydrophobic treatment and placed a drop of DI water (30μl) on its surface. We measured the advancing angle, receding angle, contact angle hysteresis and critical sliding angle of droplet under stationary condition. By applying an external electric field (AC), we observed how the magnitude of voltage influences the contact angle and the reaction time for droplet to get to new equilibrium state.
After that, we let droplets roll freely down on the surface of dielectrowetting device which was inclined at a certain angle. Its path was deflected due to the dielectrowetting effect created by electrodes which placed on the surface with 45 degree of included angle. We measured the lateral displacement of droplet and it has positive correlation with magnitude of voltage. Results also showed that the force which caused droplet to deflect mainly happened on the edge of electrode and it can’t be contributed to difference of contact angle between two surfaces.
Hence, we redesigned the alignment of electrodes, now with parallel and vertical to the path of droplet. The behaviors of droplet were significantly different when passing through two kind of electrode that mentioned above. Proofing that the phenomenon of directional wettability in dielectrowetting has huge impact on dynamics of droplet.
In order to obtain more accurate data on dynamics contact angle, we fixed the droplet on a hydrophilic upper surface. By moving the electrodes on lower plate with constant velocity, we have measured the dynamics contact angle under different applied voltage. Finally, we used molecular kinetic theory to analyze dynamics contact angle. We discovered that our model is suitable to describe the dynamics angle change due to the influence of dielectrowetting, but only comply when applied electric field is parallel to the path of droplet. Further research is needed when applied electric field is vertial to the path of droplet.

並列關鍵字

Interdigitated electrodes ； Dielectrowetting ； Super-hydrophobic surface ； Dynamics Contact Angle ； Contact Angle Hysteresis

參考文獻

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