超疏水表面上纳米气泡与气液界面的演化及稳定性研究

With the development in microfabrication techniques, the superhydrophobic surfaces have attracted much attention of many researches recently, due to their capacity of reducing fluid resistance by surface slip. However, the method of drag reduction by superhydrophobic surface still cannot enter the stage of the practical engineering. The gas-liquid interface is the key factor to produce a large velocity slip on the superhydrophobic surface. However, the gas-liquid interfaces often occur instability under large Reynolds number flow. It will cause the drag reduction effect failure, and even increase the resistance to flow. This project is going to be carried out from two aspects, experiments and calculations. First, the formation and evolution of nanobubbles on superhydrophobic material will be studied. The internal mechanism of why nanobubbles can exist stability will be explored. Then, the correlation between nanobubbles and gas-liquid interfaces on superhydrophobic surface will be investigated. The effect factors on the stability of the gas-liquid interface and gas-liquid interface evolution process will be explored. Especially, the physical mechanism of recovery process, that is overcoming the energy barrier for transition from the Wenzel state to the Cassie-Baxter state, will be investigated further. The surface topography characteristics and wettability of the surfaces helping maintain the gas-liquid interface will be studied. The theoretical basis for the surface with the gas-liquid interface stability will be setup. The surfaces with good gas-liquid interface stability will be developed to reach stable and effective flow drag reductions under the larger flow Reynolds number flow.

近年来虽然利用超疏水表面的速度滑移进行流动减阻研究方兴未艾，然而却迟迟进入不了工程实用阶段，其主要原因是产生超疏水表面较大速度滑移的关键因素——超疏水材料表面上的气液界面经常在较大流动雷诺数下发生失稳，使得超疏水表面的减阻效果失效、甚至增加流动阻力。本研究计划从实验和计算两个方面开展，首先研究超疏水材料表面上纳米气泡的发生和演化的物理规律，探寻纳米气泡可以稳定存在的内在机理；进而进行超疏水表面上纳米气泡和气液界面的关联性研究，探索影响气液界面稳定性的影响因素和气液界面演化过程的规律性，特别是揭示浸润状态从Wenzel 到 Cassie-Baxter 状态——这一克服能量势垒的恢复过程产生的内在机制，探讨有利于气液界面保持的表面形貌特征及浸润性特征，为设计出具有良好气液界面保持性的表面奠定理论基础，以实现超疏水表面在较大流动雷诺数下保持稳定有效的减阻效果。

鉴于超疏水表面减阻的关键因素——超疏水材料表面上的气液界面，其经常在较大雷诺数下发生失稳，使得超疏水表面的减阻效果失效、甚至增加流动阻力。本项目从实验和计算两个方面开展，通过分子动力学模拟研究了超疏水材料表面上纳米气泡的发生和演化的物理规律，探寻了纳米气泡可以稳定存在的内在机理，发现疏水壁面对气体分子的吸引要大于对水分子的吸引力，使得疏水壁面特别是纳米坑的角落处气体分子易于聚焦，揭示表面纳米泡的形成机制；所形成的纳米泡在其界面上氮气密度增加，水分子的排列在气液界面附近具有取向性，这两点均增强了纳米泡的稳定性，进一步揭示了纳米泡可以在超疏水表面稳定存在的机理。深入研究了表面微结构及浸润性对气液界面演化的影响，发现通过改变两平板与液滴表面的接触角，平面间液滴表面聚焦射流存在板间射流和壁面射流两种模式，通过数值模拟和理论分析揭示了这两种平面聚焦射流模式存在的物理机制，建立了基于上下板液滴接触角的板间射流位置偏移和聚焦效率的理论预测公式。通过沟槽与超疏水表面相结合的方法制备了气液界面更稳定的复合表面，获得远大于普通表面的减阻效果，发现复合表面上发夹涡包倾角减少、涡间距减少以及旋转强度减弱，使得边界层内雷诺应力减小，揭示了复合表面的减阻机理，为水下航行器和管道输运等节能和减阻提供新策略。探索影响气液界面稳定性的影响因素和气液界面演化过程的规律性，特别是揭示浸润状态从Wenzel 到 Cassie-Baxter 状态-这一克服能量势垒的恢复过程产生的内在机制，探讨有利于气液界面保持的表面形貌特征及浸润性特征，为设计出具有良好气液界面保持性的表面奠定理论基础，并采用侧面双内凹结构和空间微立柱相结合的方式实现了亲水表面、细观尺度结构上气液界面的稳定保持，揭示了气液界面失稳机理，为微槽道流动减阻提供了新方案。