纳米结构超疏水表面冷凝现象的三维格子玻尔兹曼方法模拟

采用三维多相流格子玻尔兹曼方法 (lattice Boltzmann method, LBM),对纳米结构超疏水表面液滴的冷凝行为进行模拟研究.通过Laplace定律和光滑表面的本征接触角理论对三维LBM模型进行定量验证.模拟分析了超疏水表面纳米阵列的几何尺寸和润湿性的局部不均匀性对冷凝液滴形核位置和最终润湿状态的影响规律.结果表明,较高的纳米阵列使液滴在纳米结构间隙的上部侧面和底部优先形核长大,通过采用上下不均匀的间隙可避免液滴在底部形核长大,而在上部侧面形核的冷凝液滴在生长过程中向上运动,其润湿状态由Wenzel态转变为Cassie态;较低的纳米阵列使液滴在纳米结构底部优先形核长大,液滴的最终润湿状态为Wenzel态;润湿性不均匀的纳米结构表面使液滴在阵列顶端亲水位置处优先形核长大,成为Cassie态.冷凝液滴在不同几何尺寸的纳米结构表面上的最终润湿状态的模拟结果与文献报道的实验结果符合良好.通过模拟还发现,冷凝液滴在生长过程中的运动行为与液滴统计平均作用力的变化有关.本文的LBM模拟再现了三维空间中液滴的形核、长大和润湿状态转变等物理现象.

Three-dimensional lattice Boltzmann modeling of droplet condensation on superhydrophobic nanostructured surfaces

Superhydrophobic surfaces resulting from nanoarrays have good performance in anti-condensation. However, the study of droplet nucleation during water vapor condensation is a challenge because of the limitation of observation on a nanoscale, and therefore the fundamental understanding of the influence of geometrical parameters of nanoarrays on the condensation behavior is still less clear. In this work a three-dimensional (3D) multiphase lattice Boltzmann (LB) model is employed to simulate the phenomenon of droplet condensation on the superhydrophobic nanostructured surface. The model validation is carried out through the comparison of the simulations with the results from the Laplace's law and the intrinsic contact angle theory. The LB simulations accord well with the results from Laplace's law. The relative deviation between the simulated intrinsic contact angle and the theoretical value is less than 0.14%, demonstrating the validity of the LB model. Then, the 3D LB model is used to simulate the different preferential nucleation positions and final wetting states of condensate droplets by changing the geometrical parameters, including interpost space, post height and post width, and local wettability of the nanoarrays on superhydrophobic surfaces. It is found that for the nanostructured surfaces patterned with tall posts, the droplets nucleate in the upside interpost space and at the bottom of nanostructures simultaneously. By designing wider and thinner interpost spaces at the downside and upside of the tall nanostructures, respectively, the phenomenon of droplet nucleation at the bottom can be avoided. The simulation results show that the condensate droplets nucleated in the upside interpost space of tall nanostructures migrate upwards during growth, producing a Wenzel-to-Cassie wetting state transition. On the other hand, the condensate droplets nucleated at the bottom of nanostructured surface patterned with short posts produce the Wenzel state. However, by setting non-uniform hydrophilic and hydrophobic regions on the top of the short nanostructures, the condensate droplets are found to nucleate on the hydrophilic top and generate a Cassie state. The simulated final wetting states of condensate droplets on the nanostructures, having various geometrical parameters, compare reasonably well with the experimental observations reported in the literature. It is demonstrated that the migration of condensate droplets is correlated with the evolution of the statistical average force. If the direction of the statistical average force acting on the droplet is upward, the condensate droplets nucleated in the upside interpost space move upward during growth. The 3D LB simulations provide an insight into the physical mechanism of droplet nucleation, growth and wetting state transitions on superhydrophobic nanostructured surfaces.