超疏水表面上液滴撞击动力学的研究

利用高速摄像技术研究了超疏水表面上液滴撞击过程的动力学。根据能量守恒观点理论研究了撞击液滴的最大铺展直径的关系式,理论推导过程不仅考虑了撞击过程中的表面能的变化,同时考虑了微结构缝隙中流体的粘性耗散作用,最终的理论关系式与实验结果相吻合。     

液滴撞击高温梯度表面的动态行为特性

以铜片为基底制备了微方孔结构浸润梯度表面,利用高速摄像技术对液滴撞击高温梯度表面的动态行为特性进行了研究。结果表明：在不同表面温度和韦伯数（We）下,液滴撞击在梯度表面上会出现5种不同的撞击模式,即润湿模式、接触沸腾模式、过渡模式、碎裂模式和反弹模式;当表面温度达到Leidenfrost温度时,液滴进入反弹模式,反弹液滴会沿着梯度能方向发生多次连续弹跳行为,且由于弹跳过程中能量的不断衰减,反弹高度逐渐减小直至趋于零。基于表面物理化学理论分析了液滴的定向弹跳行为,并利用图像处理技术,分析了弹跳液滴的动态特征。进一步地,通过液滴以相同We数撞击不同表面温度实验,研究了液滴在弹跳运动过程中反弹高度、铺展因子和运动的水平加速度变化特征,发现三者随反弹次数的增加具有相似的变化过程,即呈现先快速减小、后逐渐稳定的特征。水平加速度的与铺展因子的变化趋势具有一致性,从而验证了理论分析的合理性。     

倾斜微结构疏水表面液滴的滞后特性

倾斜微结构疏水表面液滴的滞后特性包括接触角滞后和滚动角。目前,具有较高精度的微结构疏水表面滚动角模型是以理想液滴形状为计算基础,忽略了重力、接触角滞后以及能垒引起的变形。本文以聚二甲基硅氧烷（PDMS）为基底,制备了方柱状微结构疏水表面,考虑疏水表面微观结构以及液滴大小两方面的因素,研究了倾斜微结构疏水表面液滴的滞后特性。从力和能量的角度对其影响机理进行了分析,通过滚动角理论值与实际值的比较发现,微方柱间距较大时,接触角滞后和能垒对滚动角影响显著,证实了该分析的合理性,为研究更加精确的滚动角模型奠定了理论基础。     

液滴撞击疏水/超疏水表面防结冰技术研究进展及未来展望

抑制撞击液滴的结冰在实际工程应用中具有重要意义。寻求经济、高效的防/除冰方法是结冰领域的研究热点。针对撞击液滴的结冰问题,首先从动力学特性、结冰特性以及撞击液滴结冰的理论研究等方面进行综述,并对目前关于撞击液滴结冰中存在的一些问题进行分析。然后,基于撞击液滴的反弹特性可从源头上抑制结冰这种思路,提出降低接触时间、增加成核再辉时间的新方法。这些方法的提出有助于从源头上解决撞击液滴的结冰问题,将使“利用撞击液滴的反弹特性抑制结冰”的应用范围极大扩展,积极推动防结冰技术的发展。最后,对利用液滴的反弹特性进行抑制结冰的研究进行展望。     

速度对液滴撞击超疏水壁面行为特性的影响

通过可视化实验研究了直径为2.58 mm的液滴在不同速度下撞击静态接触角为156°超疏水壁面后的运动特性,其液滴Weber数在8~310之间。实验利用光学原理在同一画面上同时记录了液滴撞击壁面过程的正面及底面图像。实验结果表明:液滴撞击壁面的速度对液滴的前进角、后退角,三相接触线的速度以及液滴反弹后的空中运动特性都有较大的影响;液滴高速撞击超疏水壁面后会产生明显的液指及多组卫星液滴,回缩阶段相邻的液指发生聚并直至液滴完全回缩。     

改性液滴撞击荷叶表面沉积特性对比

利用高速摄像机拍摄去离子水液滴、3种相对分子质量的聚氧化乙烯(polyethylene oxide,PEO)液滴、表面活性剂硫酸钠琥珀酸二辛酯[bis(2-ethylhexyl)sulfosuccinate,AOT]液滴和两种助剂组合液滴撞击荷叶表面,对比分析不同特性液滴在叶面上的沉积特性。实验结果表明：PEO助剂能够明显抑制液滴反弹和增加液滴在叶面停留时间,阻止向外发射卫星液滴,高相对分子质量PEO(5×10⁵、2×10⁶)抑制效果更加明显,但会降低液滴在叶面上的铺展面积;低质量分数(0.005%、0.02%)的AOT助剂能够增加液滴在叶面上的铺展面积,减小液滴弹起高度,润湿局部撞击区域,但是在撞击速度较高(v>1.66m/s)时,液滴会在收缩时破碎多个卫星液滴而弹离叶面;增加AOT质量分数为0.1%时,液滴能够在铺展阶段润湿叶面,并在短时间内扩散到更大面积;采用PEO和AOT两种助剂的混合液滴结合了聚合物和表面活性剂的优点,能够在局部润湿区域形成牵扯液滴的液丝,增强了液滴在叶面上的有效沉积;通过对实验数据整理分析,发现液滴在降低雷诺...     

液滴撞击平板的铺展特征

采用高速摄像仪研究了液滴在不同韦伯数和撞击角下撞击干燥平板时的铺展行为.结果表明,当撞击角相同时,韦伯数越小,液滴最大横向铺展直径也越小;韦伯数相同,撞击角越大,液滴横向铺展直径越大;在铺展和回缩过程中,环形液层外圈相对于内圈在达到横向最大铺展直径时具有滞后现象.     

单液滴撞击超疏水冷表面的反弹及破碎行为

对直径2.8 mm的液滴撞击冷表面的动态行为进行快速可视化观测,对比研究单液滴撞击普通冷表面以及超疏水冷表面的动力学特性,同时对初始撞击速度以及冷表面温度对液滴动态演化行为的影响进行了对比分析。实验结果表明：与液滴撞击普通冷表面（温度-25~-5℃）发生瞬时冻结沉积相比,液滴撞击超疏水冷表面时均未发生冻结,而且伴随铺展、回缩、反弹以及破碎行为;撞击速度越大,普通冷表面上液滴铺展因子越大,而且液滴越易冻结。液滴低速（We≤76）撞击超疏水冷表面会发生反弹现象,但速度对液滴最大铺展时间无影响;液滴高速（We≥115）撞击超疏水冷表面后会产生明显液指,而且破碎为多组卫星液滴。此外,冷表面温度仅影响液滴反弹高度,对液滴最大铺展因子以及液滴铺展时间影响较小。结果表明超疏水表面可显著抑制液滴撞击冷表面的瞬时冻结沉积。     

纳微结构疏水表面结霜过程及抑霜特性

采用静电纺丝法,通过调节电纺母液浓度,制备出7种具有典型微米颗粒结构、微米颗粒/纳米纤维混合结构和纳米纤维结构的疏水表面,实现了纳微结构的表面可控;对表面微观结构进行了表征,测试了其疏水性能;通过表面结霜过程和抑霜性能实验比较,结合表面分形维数分析,研究了纳微结构疏水表面的抑霜特性。结果表明,虽然7种纳微结构疏水表面接触角均在139°~152°之间,具有明显的疏水特性,但其抑霜性能与疏水性能并不呈一致关系;抑霜性能与表面结构密切相关,随着表面结构分形维数的增大而减小。     

振动诱导微结构粗糙表面水滴Wenzel-Cassie状态转变特性

以聚二甲基硅氧烷（PDMS）基底采用光刻蚀技术制备了微方柱结构粗糙表面。采用高速摄影对液滴在垂直振动作用下的动态浸润状态进行了图像采集。通过对水滴振动过程中的动态浸润特性分析,研究了粗糙表面水滴的Wenzel-Cassie浸润状态转变特征。结果表明,对于一定尺寸的Wenzel状态水滴,只有当施加的振动能量超过某一阈值时,微方柱粗糙表面Wenzel状态液滴才可以发生向Cassie状态的完全转变,且存在发生Wenzel-Cassie浸润转变的阈值范围;此外,当外加振动频率和液滴固有频率一致时,即在共振频率时,液滴发生Wenzel-Cassie状态转变需要的能量最小。外加振动频率偏离液滴固有频率越远,发生Wenzel-Cassie状态转变需要的能量最大。基于表面化学和振动力学理论,建立了液滴发生Wenzel-Cassie转变时的物理模型。     

Collective behavior of evaporating droplets on superhydrophobic surfaces

We study the evaporation dynamics of multiple water droplets deposited in ordered arrays or randomly distributed (sprayed) on superhydrophobic substrates (SHP) and smooth silicon wafers (SW). The evaluation of mass of the droplets as a function of time shows a power-law behavior with exponent 3/2, and from the prefactor of the power-law an evaporation rate can be determined. We find that the evaporation rate on a SHP surface is slower than a normal surface for both single droplet and collection of droplets. By dividing a large droplet into more smaller ones, the evaporation rate increases and the difference between the evaporation rates on SHP and SW surfaces becomes higher. The evaporation rates depend also on the distance of the droplets which increase with increasing this distance.     

Spread and recoiling of liquid droplets impacting solid surfaces

The impact of water droplets on solid surfaces is studied experimentally and theoretically. Theoretical equations based on energy conservation are developed. In our theoretical study, the droplet is modeled as a ring‐like shape, which matches the dynamic shape of droplets observed from our experimental tests. In the analysis of energy conservation, the nonuniform distribution of pressure inside the deformed droplet is taken into account by introducing a flow potential energy term in the theoretical equations. To derive viscous dissipation for recoiling, a viscous layer coefficient is introduced. Its values for the tests using smooth surfaces are found to be within a small range. Both theoretical predictions and experimental data show significant influence of surface wettability on maximum spread and recoiling process. With the increase of advancing contact angle, surface energy shows a decreasing trend, whereas flow potential energy shows an increasing trend and becomes significant for hydrophobic surfaces. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2683–2691, 2014     

Capillary pumping between droplets on superhydrophobic surfaces

The famous two-balloon experiment involves two identical balloons filled up with air and connected via a hollow tube, and upon onsetting the experiment one of the balloons shrinks and the other expands. Here, we present the liquid version of that experiment. We use superhydrophobic (SHP) substrates to form spherical droplets and connect them with a capillary channel. Different droplet sizes, substrates of different hydrophobicities, and various channel pathways are investigated, and morphometric parameters of the droplets are measured through image processing. In the case of SHP substrates, the pumping is from the smaller droplet to the larger one, similar to the two-balloon experiment. However, if one or both of the droplets are positioned on a normal substrate the curvature radius will indicate the direction of pumping. We interpret the results by considering the Laplace pressures and the surface tension applied by the channel at the connecting points.     

Maximum spread of droplets on chemically striped surfaces

Chemically striped surfaces have a strong ability to control the shape of droplets and even to split a droplet, which can be applied in microfluidic, printing fields and spray cooling fields. The droplet splitting behavior is affected by the number of hydrophobic stripes spanned by the droplet at its maximum spread moment, which is determined by the maximum spreading factor. In this study, an area ratio is adopted to analyze a droplet's spread on the hydrophilic and hydrophobic regions at maximum spread moment. Coupled with an energy conservation equation, an analytical model is presented to predict the maximum spread factor of a droplet on a chemically striped surface.     

Rebound dynamics of two droplets simultaneously impacting a flat superhydrophobic surface

In this letter, we investigate the rebound dynamics of two equally sized droplets simultaneously impacting a superhydrophobic surface via lattice Boltzmann method (LBM) simulations. We discover three rebound regimes depending on the center-to-center distance between the two droplets: a complete-coalescence-rebound (CCR) regime, a partial-coalescence-rebound (PCR) regime, and a no-coalescence-rebound (NCR) regime. We demonstrate that all the rebound regimes are closely associated with dynamic behaviors of the formed liquid ridge or bridge between the two droplets. We also present the contact time in the three regimes. Intriguingly, although partial coalescence takes place, the contact time is still dramatically shortened in the PCR regime, which is even smaller than that of single-droplet impact. These findings provide new insights into the contact time of multiple-droplet impact and thereby offering useful guidance for some applications such as anti-icing, self-cleaning, and so forth.     

疏水表面结霜初期液滴生长的理论分析

研究了自然对流条件下疏水表面结霜初期冷凝液滴的生长过程, 建立了考虑不凝气影响的液滴传热及生长模型, 分析了表面接触角和冷面温度对液滴生长的影响。结果表明, 液滴生长过程中的主要热阻为液滴内部导热热阻和相界面热阻, 随着表面接触角的增大, 这两个主要热阻均增大, 因此表面疏水性越好, 液滴生长越缓慢;而由于冷凝传热温差随冷面温度降低而增大, 因此冷面温度越低, 液滴生长越快。     

润湿性图案表面上的液滴侧向弹跳行为

表面润湿性不均匀会影响液滴撞击表面后的运动行为。通过液滴撞击润湿性图案表面,利用疏水表面上的亲水条纹可以实现液滴的侧向弹跳。实验过程中探究了不同的表面性质与撞击条件对液滴侧向弹跳运动行为的影响,为实现液滴定向弹跳提供了新的思路。实验结果表明,润湿性图案主要影响液滴撞击表面后的回缩过程,并且图案尺寸、液滴速度、液滴撞击位置均会对液滴撞击表面后的分裂以及侧向弹跳产生影响。通过实验获得了上述参数对液滴侧向弹跳的质量和距离的影响规律。