Abstract

A novel triple-line stick–slip behavior, manifested by “sawtooth oscillations” of the contact angle (CA), was observed during sessile droplet advance by electrowetting-on-dielectric (EWOD) for DC voltages and during droplet retreat for AC voltages. The onset of stick–slip occurred on polished substrate surfaces when the applied potential approached the EWOD saturation voltage and at lower voltages on rougher surfaces. Stick–slip was reduced at higher AC frequencies (> 1 kHz), not significantly influenced by pH or voltage polarity and did not occur with AC polarization on substrates with a Parylene coating but no hydrophobic top-layer. The different triple-line pinning behaviors under DC and AC polarization are shown to be consistent with heterogeneous wetting associated with immobilization of charged species—referred to as charge trapping—near the triple-line at saturation. These experiments and insights offer a new approach for understanding and addressing EWOD device limitations related to CA saturation and charged species trapping leading to improved performance in micro-/nanofluidic pumps, digital microfluidic chips, and electret devices.

Graphical Abstract

中文翻译：

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电介质上电润湿过程中的粘滑行为：极化和基底效应

摘要

在通过直流电的电介质上电润湿（EWOD）进行固着液滴前进过程中，对于交流电压进行液滴后退过程中，观察到了一种新颖的三线粘滑特性，表现为接触角（CA）的“锯齿形振荡”。当施加的电势接近EWOD饱和电压时，粘滑开始发生在抛光的基材表面上，而在较粗糙的表面上则发生较低的电压。在较高的交流频率（> 1 kHz）下，粘滑现象减少，不受pH或电压极性的显着影响，并且在具有聚对二甲苯涂层但无疏水顶层的基材上，由于交流极化而不会发生。直流和交流极化下不同的三线钉扎行为显示与与带电物质的固定化相关的异质润湿（称为电荷俘获）一致，饱和状态下靠近三线。这些实验和见识提供了一种新方法，用于了解和解决与CA饱和和带电物质捕获有关的EWOD设备局限性，从而改善了微/纳米流体泵，数字微流体芯片和驻极体设备的性能。

图形概要