物理化学学报 >> 2012, Vol. 28 >> Issue (05): 1206-1212.doi: 10.3866/PKU.WHXB201202293

软物质 上一篇    下一篇

超疏水表面上冷凝液滴发生弹跳的机制与条件分析

刘天庆, 孙玮, 孙相彧, 艾宏儒   

  1. 大连理工大学化工学院, 辽宁大连 116024
  • 收稿日期:2011-11-28 修回日期:2012-02-09 发布日期:2012-04-26
  • 通讯作者: 刘天庆 E-mail:liutq@dlut.edu.cn
  • 基金资助:

    国家自然科学基金(50876015)资助项目

Mechanism and Condition Analysis of Condensed Drop Jumping on Super-Hydrophobic Surfaces

LIU Tian-Qing, SUN Wei, SUN Xiang-Yu, AI Hong-Ru   

  1. School of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning Province, P. R. China
  • Received:2011-11-28 Revised:2012-02-09 Published:2012-04-26
  • Contact: LIU Tian-Qing E-mail:liutq@dlut.edu.cn
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (50876015).

摘要: 使用液滴合并前后的体积和表面自由能守恒作为两个限制条件, 确定了合并液滴的初始形状, 即为偏离平衡态的亚稳态液滴, 具有缩小其底半径而向平衡态液滴转变的推动力. 进而分析了液滴变形过程中的推动力和三相线(TPCL)上的滞后阻力, 建立了液滴变形的动态方程并进行了差分求解. 如果液滴能够变形至底半径为0 mm的状态, 则根据该状态下液滴重心上移的速度确定液滴的弹跳高度. 不同表面上冷凝液滴合并后的变形行为的计算结果表明, 光滑表面上的液滴合并后, 液滴只能发生有限的变形, 一般都在达到平衡态之前就停止了变形, 因此冷凝液滴不会发生弹跳; 粗糙表面上的Wenzel 态液滴的三相线上的滞后阻力更大, 因而液滴更难以变形和弹跳; 具有微纳二级结构表面上只润湿微米结构, 但不润湿纳米结构的部分Wenzel 态液滴能够变形至Cassie 态, 但没有明显的弹跳; 只有在纳米或微纳二级结构表面上的较小Cassie 态液滴合并后, 液滴易于变形至底半径为0 mm的状态并发生弹跳. 因此, Cassie 态合并液滴处于亚稳态, 并且其三相线上的移动阻力很小, 是导致冷凝液滴弹跳的关键因素.

关键词: 超疏水表面, 纳米, 微纳二级结构, 冷凝, 液滴变形, 弹跳, 机理, 模型

Abstract: The initial shape of a coalesced drop is determined by the conservation of drop volume and the surface free energy before and after two or more condensed drops merge. The coalesced drop is in a metastable state with a driving force to reduce its base radius toward equilibrium state. This driving force and resistance on the three-phase contact line (TPCL) are analyzed during drop transformation. A dynamic equation describing the shape conversion of the drop is proposed and solved. The jumping height of a merged drop is determined by the speed at which the center of gravity moves up when the base radius of the drop reduces to 0 mm on a super-hydrophobic surface. Calculations show that a coalesced drop on a flat surface can transform its shape only in a limited fashion. It will not jump since its transformation stops before it reaches equilibrium. A wetted drop on a rough surface is even more difficult to transform and jump because of the greater TPCL resistance. However, on a two-tier surface, a partially wetted drop impaling only the micro-scale roughness exhibits a shape transition to a Cassie state upon coalescence, but without obvious jumping. Only after the coalescence of two or more small Cassie-state drops on a textured surface, can the merged composite drop easily transform to a 0 mm base radius and jump. It can be concluded that key factors governing condensed-drop jumping are the merged composite drop in a metastable state and a small TPCL resistance on nano or micro-nano two-tier surfaces.

Key words: Super-hydrophobic surface, Nano, Micro-nano two-tier texture, Condensation, Drop transformation, Jumping, Mechanism, Model

MSC2000: 

  • O647