Abstract The dynamic modulation of contact angle on switchable hydrophilic-hydrophobic surfaces is one of the important factors in designing miniaturized optical devices. A spreading and retention of a nano water droplet placed on an insulator surface covering two planar electrode layers and perpendicularly in contact with a conductive tip is studied using atomistic molecular simulation. The change in the droplet shape is initiated by modifying interaction energy between the insulator atoms and the water molecules. The contact angle analysis shows a unique dependence on the nature of the underlying substrate. Time correlation function analysis based on the calculation of variation of the height of the drop’s center of mass with time estimates the rate of relaxations for wetting-dewetting behavior subject to the condition of surface hydrophilic and hydrophobic nature. For the hydrophilic case, the relaxation rate of the drop is found to be twice slower than the hydrophilic one. The difference in the rate of relaxations is analyzed by considering the effects of the frictional force on the contact line motion of the drop. Friction coefficients are calculated on the basis of continuum hydrodynamic (HD) and molecular kinetic theory (MKT) theories. The dependence of the friction coefficients on the surface properties depicts the strong dominance of the liquid-substrate friction on the drop’s perimeter motion based on molecular kinetic theory.

中文翻译：

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纳米液滴在可切换的亲水-疏水表面上的润湿转变

摘要 在可切换的亲水-疏水表面上动态调节接触角是设计小型化光学器件的重要因素之一。使用原子分子模拟研究了放置在覆盖两个平面电极层并与导电尖端垂直接触的绝缘体表面上的纳米水滴的扩散和保留。液滴形状的变化是通过改变绝缘体原子和水分子之间的相互作用能而引发的。接触角分析显示出对底层基材性质的独特依赖性。基于液滴质心高度随时间变化的计算的时间相关函数分析估计了受表面亲水性和疏水性条件影响的润湿-去湿行为的弛豫率。对于亲水情况，发现液滴的弛豫速率比亲水的慢两倍。通过考虑摩擦力对液滴接触线运动的影响来分析松弛率的差异。摩擦系数是根据连续流体力学 (HD) 和分子动力学理论 (MKT) 理论计算的。摩擦系数对表面特性的依赖性描述了基于分子动力学理论的液体-基材摩擦对液滴周边运动的强烈支配。