【作者】 王凯；

【导师】 马学虎；

【作者基本信息】 大连理工大学 ， 化学工程， 2018， 博士

【摘要】 超疏水表面具有很低的表面自由能,液滴能在表面上形成完美球状,运动性强。当液滴在超疏水表面上合并时,由于合并过程中表面自由能的释放,合并后的液滴会由于过剩的动能而跳离表面,即发生由于合并诱导的液滴弹跳现象。液滴弹跳现象的一个显著特点是,不需要添加外力(场)就能将液滴从超疏水表面移除,在自清洁、防结冰和过程强化等方面具有很好的应用前景。本文结合实验观测、理论分析和数值模拟,系统地研究了超疏水表面上液滴弹跳现象及其影响因素,揭示了液滴尺寸效应和初始液滴排布对液滴弹跳的影响规律;在上述获得的规律基础上,提出利用改变液桥撞击壁面位置的方法来调控液滴弹跳过程,明晰了液桥撞击壁面位置与液滴弹跳速度之间的内在关联。本文主要研究内容如下:利用高速显微技术对超疏水表面上液滴合并诱导弹跳过程进行了可视化研究。根据图片序列分析,可以将液滴弹跳过程分为四个阶段,即液桥生长、液桥撞击、三相线收缩和液滴弹跳。研究发现液桥半径与合并时间的平方根成正比,而由于液桥生长过程液面曲率发生转变,比例系数由1.18减小为0.83。在实验考察的液滴半径范围内,液滴的合并过程为惯性力占主导,三相线的收缩动力学是相似的,液滴的弹跳速度则满足毛细力与惯性力相互作用导致的相似律,即无量纲化液滴弹跳速度近似为0.2。利用格子Boltzmann方法研究了不同初始液滴半径下液滴弹跳速度的演化规律,获得了超疏水表面上液滴合并机制转变的临界液滴半径(1.3μm)。当初始液滴半径大于1.3μm时,液滴弹跳速度则满足毛细力与惯性力相互作用导致的相似律,即无量纲化液滴弹跳速度近似为0.2;反之,合并过程中粘性力逐渐占据主导,液滴弹跳速度迅速减小,不再遵循上述相似律。结合可视化实验和格子Boltzmann方法,考察了初始液滴半径比(小液滴半径与大液滴半径的比值)对液滴弹跳速度的影响规律,获得了超疏水表面上液滴合并后发生弹跳的临界半径比。当液滴合并过程由惯性力占主导时,该临界半径比为0.56;而随着液滴合并过程中粘性力逐渐占据主导,该临界半径比增大,直到等径液滴合并也不发生弹跳。以超疏水表面上三液滴合并诱导弹跳现象为例,利用格子Boltzmann方法研究了初始液滴排布对多液滴合并诱导弹跳现象的影响规律。对多液滴弹跳而言,可以将初始液滴排布大致分成两大类:密集型排布和离散型排布。当液滴合并过程由惯性力占主导时,对于液滴密集型排布构型而言,液滴弹跳速度随着排布角度的增加先减小后增大;随着液滴合并过程中粘性力逐渐占据主导,初始液滴排布对液滴弹跳速度的影响可以忽略。相比于液滴密集型排布,液滴离散型排布不利于液滴弹跳的,因为这种排布构型下液滴弹跳速度被削弱。结合模型分析、格子Boltzmann模拟和可视化实验,从界面结构效应入手,考察了液桥撞击壁面位置对液滴弹跳速度的影响规律。当液滴在凸起结构上方合并时,凸起结构能够改变液滴在合并过程中的流体动力学,加速液滴收缩其底部固液接触面积,提高动能转化率,促使液滴合并后更快地跳离壁面;当液滴在凹槽结构上方合并时,由于液桥生长阶段延长,延滞了液桥撞击壁面的时间,降低了动能转化率,减小了液滴弹跳速度。在上述基础上,以三棱柱结构为例,研究了三棱柱结构参数与液滴弹跳速度之间的关联。研究结果表明,通过在表面构造三棱柱结构能够增大液滴弹跳速度,是一种有效的液滴弹跳强化手段。液滴弹跳过程中的初始动能转化率则取决于三棱柱的顶角以及高度。当三棱柱的顶角一定时,初始动能转化率随着三棱柱高度的增大而增大;当三棱柱的高度一定时,初始动能转化率随着三棱柱顶角的减小而增大。更多还原

【Abstract】 The surface free energy of superhydrophobic surfaces is rather low.Droplet shows a perfect spherical "pearl" and can roll easily on superhydrophobic surfaces.When two small droplets coalesce on superhydrophobic surfaces,they will jump away from the surface due to the release of excess surface free energy,which is called the coalescence-induced droplet jumping.An outstanding feature of droplet jumping is that the droplets can be removed without external force or field.The coalescence-induced droplet jumping has showed its great potential in surface self-cleaning,anti-icing,process intensification,etc.In this dissertation,the coalescence-induced droplet jumping and its effect factors were investigated by experimental test,theoretical analysis and numerical simulation.The effects of initial droplet size(droplet radius and droplet radius ratio)and droplet arrangements on droplet jumping were revealed.And the interfacial structure effects were used to regulate droplet jumping process.The inherent relations between the impact position of liquid bridge on the substrate and droplet jumping velocity were understood.A set of experiment apparatus to trigger droplet jumping was designed and time-lapse images of coalescence-induced droplet jumping on transparent superhydrophobic surfaces were took by high-speed cameras.Generally,the overall jumping process can be divided into four stages,i.e.,the growth of liquid bridge,the impact of liquid bridge,the retraction of three-phase contact line,and the jumping of coalesced droplet.The liquid bridge radius was proportional to the square root of time,whereas the dimensionless prefactor was decreased from 1.18 to 0.83 due to the transition of liquid interface curvature.As the droplet coalescence process on superhydrophobic surfaces was inertia-dominated in the tested droplet radius range,the retraction dynamics of the three-phase contact line was self-similar,i.e.,the retraction velocity of the contact line showed limited dependence on initial droplet radii.The coalesced droplet finally jumped away from the substrate with a dimensionless jumping velocity of around 0.2,which followed an inertial-capillary scaling.The evolution of droplet jumping velocities at different initial droplet radii were investigated by lattice Boltzmann method,the critical droplet radius for the transition of coalescence regime on superhydrophobic surfaces was found to be 1.3μm.If the initial droplet radius was larger than 1.3μm,the droplet jumping velocity followed an inertial-capillary scaling with a dimensionless jumping velocity of around 0.2.And conversely,the droplet jumping velocity decreased steeply due to the increased viscous effects in droplet coalescence process,the aforementioned scaling collapsed.The effects of droplet radius ratio(the radius of large droplet to that of the small one)on droplet jumping velocity were explored by experimental observation and lattice Boltzmann method.The critical droplet radius ratio for droplet jumping on superhydrophobic surfaces was found and this value was about 0.56 when the droplet coalescence process was inertia-dominated.The critical droplet radius ratio would increase if the effect of viscosity played a key role in the coalescence process.Taking the coalescence of three equal-sized droplets as an example,the effects of initial droplet arrangements on droplet jumping were studied by lattice Boltzmann method.Depending on the initial position of droplets on the surface,the droplet coalescence behaviors were classified into two types:concentrated configurations and spaced configurations.When the droplet coalescence process on superhydrophobic surfaces was dominated by inertia,the jumping velocity decreased with increasing distribution angle firstly,and then it increased with distribution angle.As the droplet coalescence process was dominated by viscosity,the droplet jumping velocities no longer depend strongly on initial droplet arrangements.It was observed that spaced multidroplet coalescence results in drastically reduced jumping velocities compared with that for concentrated multidroplet coalescence,which showed the negative effect on jumping velocity for spaced configurations.Starting from the interfacial structure effect,the effects of liquid bridge impact on droplet jumping velocity were investigated by model analysis,lattice Boltzmann method and experimental observation.For droplets coalesce on superhydrophobic surfaces with raised structures,the liquid mass was redistributed and the retraction of droplet base areas was accelerated with the assistance of raised structures.The initial kinetic energy conversion efficiency was increased leading to a larger jumping velocity.For droplets coalesce on superhydrophobic surfaces with grooves,the impact of liquid bridge on the substrate was delayed and the growth of liquid bridge was extended due to large gaps between structures.The initial kinetic energy conversion efficiency was decreased leading to a smaller jumping velocity.Taking triangular prism as an example,the effect of structural parameters of triangular prism on droplet jumping velocity was studied.The results indicated that droplet jumping can be dramatically enhanced by exploiting structure effects,which was a promising reinforcement method.The initial kinetic energy conversion efficiency was determined by the apex angle and the height of triangular prism,which increased with the height of triangular prism for a fixed apex angle.Also,the initial kinetic energy conversion efficiency decreased with increasing apex angle for a fixed height.更多还原