Mechanical Engineering Department, Assiut University , Assiut, Egypt.
Mechanical Engineering Department, Taif University , Taif, Saudi Arabia.
Biomicrofluidics. 2015 Feb 10;9(1):014115. doi: 10.1063/1.4907977. eCollection 2015 Jan.
We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results.
我们提出了一种电润湿系统中接触角饱和现象的新模型。这个新模型将接触角饱和归因于在三相接触线附近的液滴的帽和基底表面上的俘获电荷之间的排斥，这阻止了这些表面在接触角减小期间收敛。这种基于排斥的饱和类似于在导电液滴的表面上积累的电荷之间的排斥，这导致了众所周知的库仑分裂和泰勒锥形成现象。在我们的模型中，与更常见的假设即完美导电的液滴和完全绝缘的介电层相反，液滴和介电涂层都被视为有损耗的介电介质（即具有有限的电导率和介电常数）。我们使用理论分析和数值模拟来找到液滴表面上的实际电荷分布，计算排斥能，并将总系统的能量最小化作为液滴接触角的函数。得到的饱和曲线与先前报道的实验结果吻合良好。我们使用这个提出的模型来预测改变液体性质（例如电导率）和系统参数（例如介电层的厚度）对饱和角的影响，这也与实验结果相符。
