双重V型结构在凝液核化生长与聚集排除过程中的分级调控机理

The intensification of droplet nucleation, growth and condensate drainage efficiency provide important techniques to the improvement of steam condensation heat and mass transfer. In the present proposal, a double-V structured surface is constructed with V-shaped structures in both micro-nano and millimeter scales. The structured surface is designed to realize the hierarchical improvement of droplet nucleation and drainage efficiency within different spatial scales. Experimental investigation and molecular dynamics simulations are conducted to reveal the effect of nano-scale V-shaped structures to the nucleation of initial condensate droplets, and discuss the effect of structural parameters to the droplet wetting mode. Using the force balance of local contact line and the interfacial free energy minimization theory, theoretic models are proposed to describe the relationship between the double-V structural parameters and the droplet morphology. The effect of double-V structured surface on the local contact line movement are investigated and the driving mechanism for the directional movement of droplets within the double-V structured surface are discussed as well. By comparing the interfacial effect of V-shaped structures within different spatial scales, the overall effect of double-V structured surface on the intensification of droplet nucleation, growth and condensate drainage efficiency are presented. Finally, optimized structured surfaces and design principles are obtained based on model analysis and comparative experiments. The resultant interfacial effect and the double-V structure actuated droplet deformation and directional movement also provide an advantageous solution for the intensification of interfacial heat and mass transfer processes, including the liquid transport, droplet manipulation, wetting and dewetting processes, and various advanced applications where structural and interfacial effect matters.

液滴在冷凝表面上形成和聚集、排除过程的双重优化是提高蒸汽冷凝表现的重要手段。本项目拟构建微纳尺度和毫米尺度下的双重V型结构，实现冷凝液滴核化生长速率和凝液聚集、排除效率的分级调控和协同强化。通过实验研究和分子模拟，揭示微纳尺度V型结构强化液滴成核的物理机制，阐明结构参数对液滴生长过程中润湿形态变化的影响；以局部受力平衡和界面自由能为桥梁，建立描述不同结构参数下凝液局部接触线受力情况的数理模型，揭示表面结构对凝液增长时接触线移动的限制和引导作用，阐明毫米级V型结构非对称界面力对液滴定向迁移的驱动机制；通过不同尺度V型结构界面作用的综合对比，揭示毫米尺度下液滴定向迁移和微纳尺度下液滴核化更新的协同效应，在此基础上获得典型的利于液滴快速成核生长、凝液快速排除或有效聚集的结构表面及相应的设计准则。形成的双重界面结构驱动液体迁移机制可为界面润湿、液体输运、液滴调控相关的热质传递过程强化提供新思路。

本项目通过分子动力学模拟、界面能模型分析、结构表面构建、微观冷凝实验和冷凝全过程模拟，在不同尺度下研究了V型结构和表面润湿性能对液滴形成、生长、聚集和排除过程的影响，并构建了双重V型结构用于蒸汽冷凝和液滴输运。微观尺度下，通过成核自由能垒分析、MD模拟和微观实验，探讨了以V型截面几何结构为代表的微观结构对冷凝成核位点分布和液滴润湿形态的影响。结果显示，和平表面相比，沟穴结构、沟槽结构，以及冷凝表面上因物理加工、材料自身特性等存在的缺陷、位错、台阶等均表现出显著的促进冷凝成核特性。结合材料自身较高的本征接触角和较小的V型截面夹角，可以促使液滴脱离V型沟槽底部，并最终生成Cassie模式的大液滴，获得较大的表观接触角和动态迁移特性。近宏观尺度下，对比研究了矩形沟槽和V型沟槽内的凝液聚集形态和演化趋势。在传统翅片管相邻翅片间形成的矩形沟槽内，液体演化时受到沟槽顶部的限制作用而难以迅速排除，而是局限在沟槽内横向发展，逐渐聚集形成液条、液环，不利于冷凝液体的快速排除。利用具备非对称结构的V型沟槽，并将超疏水表面与V型结构结合，可以有效地改变液体在V型结构内的聚集形态和演化规律，驱动液滴在V型结构内发生由内向外的定向迁移。研究表明，为驱动液滴定向迁移，需要液滴脱离V型沟槽底部，且V型沟槽内表面应具备较小的接触角滞后和壁面粘滞阻力，因此需要对沟槽内表面进行超疏水处理。将微纳级别的V型结构与宏观尺寸（毫米级，与液滴尺寸同一量级）的V型结构结合，构建双重V型结构，可以有效地改变液体的聚集形态，并促使液滴在体积增长的过程中自发的向外迁移，加速凝液排除过程。研究中同时开展了滴状冷凝的全过程模拟，揭示了超疏水表面表观接触角对液滴空间分布、微液滴多次核化，以及液滴弹跳和其扫除效应对冷凝液滴尺寸分布和传热性能的影响，定量阐述了通过表面性能和界面结构驱动液滴快速脱落提高冷凝传热性能的强化效率。