Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea.
Langmuir. 2013 Jul 23;29(29):9118-25. doi: 10.1021/la401801u. Epub 2013 Jul 9.
This study investigates the effects of drop size and viscosity on spreading dynamics, including response time, maximum velocity, and spreading pattern transition, in response to various DC voltages, based on both experiment and theoretical modeling. It is experimentally found that both switching time (i.e., time to reach maximum wetted radius) and settling time (i.e., time to reach equilibrium radius) are proportional to 1.5th power of the effective base radius. It is also found that the maximum velocity is slightly dependent on drop size but linearly proportional to the electrowetting number. The viscosity effect on drop spreading is investigated by observing spreading patterns with respect to applied voltages, and the critical viscosity at which a spreading pattern changes from under- to overdamped response is obtained. Theoretical models with contact angle hysteresis predict the spreading dynamics of drops with low and high viscosities fairly well. By fitting the theoretical models to experimental results, we obtain the friction coefficient, which is nearly proportional to 0.6th power of viscosity and is rarely influenced by applied voltage and drop size. Finally, we find that drop viscosity has a weak effect on maximum velocity but not a clear one on contact line friction.
本研究基于实验和理论建模，研究了液滴大小和粘度对润湿动力学（包括响应时间、最大速度和润湿模式转变）的影响，这些润湿动力学响应于各种直流电压。实验发现，切换时间（即达到最大润湿半径的时间）和稳定时间（即达到平衡半径的时间）均与有效基半径的 1.5 次幂成正比。还发现，最大速度略微取决于液滴大小，但与电润湿数呈线性关系。通过观察不同电压下的铺展模式，研究了粘度对液滴铺展的影响，并获得了铺展模式从欠阻尼响应变为过阻尼响应的临界粘度。带有接触角滞后的理论模型可以很好地预测低粘度和高粘度液滴的铺展动力学。通过将理论模型拟合到实验结果中，我们获得了摩擦系数，该摩擦系数几乎与粘度的 0.6 次幂成正比，且很少受到施加电压和液滴大小的影响。最后，我们发现液滴粘度对最大速度的影响较弱，但对接触线摩擦的影响不明显。
