【摘要】 超疏水表面上的液滴合并诱导弹跳现象在冷凝传热、除霜防冰、自清洁等领域具有广阔应用前景.先前研究表明,液滴弹跳速度遵循毛细-惯性标度定律,无量纲弹跳速度v_j~*≤0.23,自弹跳过程效率低下,能量转化率η≤4%.为研究微尺度表面结构对液滴弹跳的影响,突破跳跃速度限制并诱导液滴扫掠,提高液滴清除效率,设计制备了棱角β=135°和β=90°的V型槽超疏水表面.利用高速显微技术记录了两种V型槽表面上液滴聚合过程中的形态演化.实验结果表明,两种V型槽表面都突破了平壁表面的毛细-惯性标度定律,无量纲最大弹跳速度分别为v_j~*=0.51和v_j~*=0.61,跳跃方向与表面法线最大夹角分别为36°和60°.与β=135°相比,β=90°的V型槽具有更高的面内速度,有利于提高液滴扫掠效率,清除更多凝结液滴.使用格子玻耳兹曼方法数值模拟合并液滴动力学,包括合并期间液滴速度和基底驱动力的演变,以及液滴内部压力分布和瞬时速度矢量.此外,利用仿真数据计算液滴合并过程中的不同方向液体动量,以及液滴总动能、弹跳动能和振荡动能。仿真结果表明,V型槽的β=90°槽角诱导液滴在收缩过程中的两侧液体回流方向,从几乎相向改变为趋近合并液滴弹跳的同一方向.β=90°槽角抑制了水平方向的液滴振荡,降低了黏性耗散,使更多过剩表面能转化为弹跳动能.更多还原

【Abstract】 The phenomenon of coalescence-induced droplet jumping on superhydrophobic surfaces has promising applications in condensation heat transfer,anti-icing and defrosting,self-cleaning. Previous studies demonstrated that the droplet jumping speed follows the inertial-capillary scaling law with a dimensionless jumping velocity v_j~*≤0.23. The self-jumping process is inefficient,with an energy conversion efficiency η≤4%. To study the effect of microscale surface structure on droplet bounce,break through the jumping speed limit,and induce droplet sweeping to improve the droplet removal efficiency,superhydrophobic surfaces of V-groove with edge angle β=135° and β=90° were designed and prepared. High-speed microscopy was used to record the morphological evolution of droplets on two V-grooves surfaces during the merger process. The experimental result shows that both V-groove surfaces break through the capillary-inertial scaling law of flat surface. The maximum dimensionless jumping velocity is v_j~*=0.51 and v_j~*=0.61,and the maximum angle between the jumping direction and the surface normal is 36° and 60°,respectively. Compared with the β=135°,the surface of V-groove with β=90° has higher in-plane velocity,which is beneficial to improve droplet sweeping efficiency and remove condensed droplets. The lattice Boltzmann method was used to numerically simulate coalescence droplet dynamics,including the evolution of the droplet velocity and substrate driving force during coalescence and the internal pressure distribution instantaneous velocity vector of the droplet. We also used simulation data to calcu-late liquid momentum in different directions during coalescence,as well as total kinetic energy,jumping kinetic energy,and oscillation kinetic energy. The simulation results show that the edge angle β=90° of the V-groove induces the liquid backflow direction on both sides of the droplet during the contraction process,changing from almost opposite to approach the same direction of the merged droplet jump. The edge angle β=90° suppresses the horizontal droplet oscillation,reduces the viscous dissipation,and converts more excess surface energy into jumping kinetic energy.更多还原