液滴在梯度微结构表面上的铺展动力学分析

Spreading dynamics of liquid droplet on gradient micro-structured surfaces

本文通过改变肋柱宽度和间距,构造了二级和多级梯度微结构表面,采用格子-Boltzmann方法对液滴在两种梯度表面上的铺展过程进行了研究,探析液滴运动的机理和调控方法.结果表明,在改变肋柱间距的二级梯度表面上,当液滴处于Cassie态时,接触角滞后大小与粗糙度梯度成正比关系;当液滴从Cassie态转换为Wenzel态或介于两者之间的不稳定态时,这一正比关系不再遵循.在改变肋柱宽度的二级梯度表面上,接触角滞后大小与粗糙度梯度始终成正比关系.在多级梯度表面上,随液滴初始半径增大,接触角滞后减小,但液滴平衡位置相较于初始位置偏离增大.对梯度微结构表面上液滴运动和接触角滞后的定量分析,可为实现梯度微结构表面液滴运动调控提供理论依据.

Designed microtextured surfaces have shown promising applications in tuning the wettability of a liquid droplet on the surfaces and attracted great attention over the past decade; unfortunately, the effect of surface geometry on wetting properties is still poorly understood. In this work, two- and multi-stage pillar microtextures are designed to construct gradient surfaces by altering pillar width and spacing. Then, the multi-phase lattice-Boltzmann method（LBM） is used to investigate the wetting dynamics of a liquid droplet on the gradient surface. Results show that for the two-stage gradient surface with variable pillar spacing, the contact angle hysteresis is found to be proportional to the roughness gradient when droplet/surface system is in the Cassie-Baxter state. However, this proportional relation is no longer correct when the system is in the transition state between the Wenzel and Cassie-Baxter states. For the two-stage gradient surface with variable pillar spacing, the contact angle hysteresis always increases linearly with increasing roughness gradient.Results also show that when a larger droplet is placed on the multi-stage gradient surface, stronger droplet motion is observed due to the smaller contact angle hysteresis. The present LBM simulations provide a guideline for the design and manufacture of the microtextured surfaces to tune the droplet wettability and motion.