Smoothed particle hydrodynamics study of the roughness effect on contact angle and droplet flow.
Smoothed particle hydrodynamics study of the roughness effect on contact angle and droplet flow.
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DOI：
10.1103/physreve.96.033115
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发表时间：
2017-09
期刊：
影响因子：
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通讯作者：
E. Shigorina;J. Kordilla;A. Tartakovsky
中科院分区：
文献类型：
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作者：
E. Shigorina;J. Kordilla;A. Tartakovsky
我们采用成对力平滑粒子流体动力学（PF-SPH）模型来模拟粗糙的疏水和亲水表面上的固着和瞬态液滴。PF-SPH允许自由表面流动的建模，而不离散的空气相，这是通过施加表面张力和动态接触角与成对的相互作用力。我们使用PF-SPH模型来研究表面粗糙度和微观接触角对有效接触角和液滴动力学的影响。在这项工作的第一部分，我们调查静态接触角的固着液滴在不同类型的粗糙表面。我们发现Cassie和Wenzel液滴在粗糙表面上的有效静态接触角大于相应的微观静态接触角。因此，微尺度疏水粗糙表面也表现出有效的疏水行为。另一方面，微观亲水表面可以是宏观亲水或疏水的，这取决于粗糙度的类型。我们研究了Cassie和Wenzel状态之间的过渡对粗糙度和液滴尺寸的依赖性，这可以与给定的流体-衬底组合的临界压力相联系。我们观察到良好的模拟和理论预测之间的协议。最后，我们研究了粗糙度方向的影响（即，各向异性粗糙度）和表面倾角对液滴流速的影响。模拟结果表明，液滴流动速度较低，如果表面粗糙度的取向垂直于流动方向。如果表面粗糙度的主要元素与流动方向对齐，则与光滑表面相比，流速增加，这可以归因于类似于莲花效应的流体-固体接触面积的减小。我们表明，经典的线性缩放关系的债券和毛细数的液滴流动在平坦的表面上也持有粗糙表面上的流动。
We employ a pairwise force smoothed particle hydrodynamics (PF-SPH) model to simulate sessile and transient droplets on rough hydrophobic and hydrophilic surfaces. PF-SPH allows modeling of free-surface flows without discretizing the air phase, which is achieved by imposing the surface tension and dynamic contact angles with pairwise interaction forces. We use the PF-SPH model to study the effect of surface roughness and microscopic contact angle on the effective contact angle and droplet dynamics. In the first part of this work, we investigate static contact angles of sessile droplets on different types of rough surfaces. We find that the effective static contact angles of Cassie and Wenzel droplets on a rough surface are greater than the corresponding microscale static contact angles. As a result, microscale hydrophobic rough surfaces also show effective hydrophobic behavior. On the other hand, microscale hydrophilic surfaces may be macroscopically hydrophilic or hydrophobic, depending on the type of roughness. We study the dependence of the transition between Cassie and Wenzel states on roughness and droplet size, which can be linked to the critical pressure for the given fluid-substrate combination. We observe good agreement between simulations and theoretical predictions. Finally, we study the impact of the roughness orientation (i.e., an anisotropic roughness) and surface inclination on droplet flow velocities. Simulations show that droplet flow velocities are lower if the surface roughness is oriented perpendicular to the flow direction. If the predominant elements of surface roughness are in alignment with the flow direction, the flow velocities increase compared to smooth surfaces, which can be attributed to the decrease in fluid-solid contact area similar to the lotus effect. We demonstrate that classical linear scaling relationships between Bond and capillary numbers for droplet flow on flat surfaces also hold for flow on rough surfaces.