【作者】 李西营；

【导师】 马学虎；

【作者基本信息】 大连理工大学 ， 化学工程， 2010， 博士

【摘要】 撞击液滴同固体壁面相互作用机制与内燃机燃烧、喷淋冷却、农药喷洒及喷墨打印等众多物理及化学过程有关,并且影响液滴撞击过程的固液间作用机制及撞击的初始条件(主要包括Weber、Reynolds、Capillary及Ohnesorge数)决定了以上过程的实施效果。液滴撞击过程中包含的自由界面的流体流动、作为边界条件的动态接触角表达及三相线处不可积分的应力奇异点问题成为研究者持续关注的焦点。本论文利用高速摄像仪实验研究了液滴撞击不同性质的固体壁面后的特殊动态行为,并针对撞击过程的特殊行为及结果进行了相关的理论解释。本文相关的理论结果能够对液滴撞击过程中的撞击行为进行合理的预测。制备了三种不同润湿性表面,它们分别为疏水表面(镜面抛光后的黄铜表面)、亲水表面(亲水SiO2涂层表面)、超疏水表面(氟化处理后的具有微纳米复合结构的氧化铜表面)。通过研究不同润湿性固体表面上在热量传递影响下的液滴撞击行为,揭示了抛光铜表面上液滴收缩高度随壁面温度变化规律。壁面温度升高导致了最大液滴收缩高度增加,该现象可归因于表面张力梯度作用下的液体附加流动,并将该流动简化为具有温度梯度的倾斜平板上的受热液体流动,该状态下的液体流动可以借助润滑近似方程进行求解,相应的理论分析揭示了液滴收缩最大高度与壁温和接触时间的关系,理论结果与实验值相吻合。亲水表面上较小的接触角及后退角致使撞击液滴收缩过程中的三相线处于滞止状态,因此这也抑制了附加流动对液滴收缩高度的影响。此外,超疏水表面上液滴撞击的实验结果也明超疏水表面上的液滴收缩高度同样不受壁面温度变化的影响。考虑到超疏水状态下固液接触状态为复合润湿状态,此时撞击液滴与超疏水表面间捕获的大量空气(液体与气体间接触分率为97%)有效抑制了固液间热量传递,因此超疏水表面上撞击液滴的收缩行为也不受壁面温度变化的影响。论文还通过控制有机蒸汽扩散的方法在洁净的玻璃表面上制备了具有轴对称及径向润湿性梯度分布的表面,并研究了该表面上的液滴撞击过程。实验结果表明撞击液滴铺展过程不受表面润湿性影响,但固体表面的润湿性变化决定了液滴的收缩行为。即使是同一表面上的液滴撞击过程,由于其不同方向上的润湿性差异也会导致液滴在不同方向上的收缩速度差异,最终表面润湿性决定了收缩后液滴的形态。论文利用惯性去湿理论解释了上述不同润湿性梯度分布表面上的液滴收缩运动变化规律。利用光刻与干法刻蚀方法制备了具有正方形排布的微柱体结构表面,微柱体的直径及其间距由预先设计的光掩模控制,微柱体高度(高度为40μm)可以通过控制刻蚀时间获得。结构表面疏水化处理后呈现疏水或超疏水状态。疏水化的结构表面上液滴撞击实验发现液滴收缩过程中在微柱体表面上留下一层液膜,该液膜的存在影响了超疏水表面上的后退角及接触角滞后。液膜出现机理可以理解为液滴收缩过程中的滞后力(单位长度上的滞后力为F=σLG(cosθτ-cosθY))与气液界面不稳定性共同作用下的两相邻微柱体间的液束断裂所致。根据液束断裂导致液膜出现的假定进一步探讨了不同超疏水表面上的撞击液滴与结构表间的接触时间差别。最后对结构超疏水或疏水表面上的撞击液滴最大铺展直径进行了理论预测。考虑了结构表面内流体流动导致的粘性耗散及固液接触分率影响下的理论方程能够很好地反映实验结果。理论预测值与实验结果吻合较好。同样的方法制备出类似结构表面,但微柱体高度控制在20μm,对表面进行疏水化处理后呈现疏水状态。实验研究了这些表面上不同的润湿性特征,并根据变形后的Wenzel方程(结构表面上的粗糙度以接触线长度分率表示)对这些表面上的各向异性润湿特性进行了解释。另外,结构表面的各向润湿性同样反映在液滴撞击实验中。最后对以上结构表面撞击液滴最大铺展直径进行了理论预测,考虑了结构表面内流体流动导致的粘性耗散及粗糙度的影响下的理论方程预测结果与实验相吻合。更多还原

【Abstract】 The interaction between solid surface and impacting droplet is involved in many practical processes such as internal combustion, spray cooling, deposition of pesticide and ink-jet printing, and the impinging results are directly related to the wettability of solid surface and dimensional parameters (Weber, Reynolds, Capillary and Ohnesorge numbers). Also, because of its fundamental points in free surface flow, the expression for dynamic contact angle as a boundary condition as well as non-integrated shear force singularity in the vicinity of triple line, of particular interest is the study on droplet impact on various surfaces. In this paper, the experimental and theoretical study on droplet impact was carried out on different solid surfaces which were fabricated referring to the public literature.Three kinds of surfaces were fabricated, which are polished Cu surface, hydrophilic SiO2 coatings surface and superhydrophobic surface, respectively. Dynamic behavior was examined by the collision of water droplet on the polished copper surface with a wider range of wettability and heated at different temperatures. The experimental results showed the maximum height of the retracting droplet increased with the surface temperature increasing. Moreover, the differences in maximum heights of recoiling droplet were ascribed to surface tension gradient and analysed in the context of lubrication approximation. However, the same law is not ubiquitously accessible for all solid surfaces. In the case of water droplet impinging on hydrophilic surface, the pinned triple line considerably suppressed the additional flow resulting from surface tension gradient and the maximum height of receding droplet is free of heat transfer. For impacting events on superhydrophobic surface, the entrapped air between rigid surface and water droplet effectively prevented heat transfer from the heated surface to the impacting droplet. As a consequence, the maximum receding height of impacting droplet on superhydrophobic surface is also independent of the surface temperature.We fabricated a series of surfaces with wettability gradient in radial/axisymmetric direction by controlling vapor phase diffusion of n-Octyltrichlorosilane (OTS) on common glass slides. The interaction between wettability gradient surfaces and water droplet released at a given height was investigated. Experimental results revealed the maximum spreading diameter was independent of surface wettability. However, surface wettability had important bearings on the receding motion of triple line. In this regard, the receding velocity of triple line responded to wettability gradient distribution. As a consequence, the wettability distribution was responsible for the eventual profile of water droplet after impingement. The dynamic of receding triple line conforms to inertial dewetting.By combination of photolithographic and dry etching, we have fabricated a series of textured silicon surfaces decorated by square arrays of pillars whose radius and pitch can be adjusted independently. The height of the pillars was set at 40μm by controlling etching time. These surfaces displayed a hydrophobic/superhydrophobic property after silanization reaction. The dynamic behavior of water droplet impacting on these structured surfaces was examined using a high-speed camera. Experimental results illustrated that a remaining liquid film on the pillars top gave rise to a wet surface instead of a dry surface as water droplet began to recede off textured surfaces. The remaining liquid film can account for the receding contact angle and contact angle hystersis. The synergistic effect of hysteresis force per unit length (F=σLG (cosθr-cosθY) and the gas-liquid interface instability can be responsible for the occurrence of liquid film. Also, experimental results demonstrated the difference in contact time was ascribed to the solid fraction defined as the ratio of the actual area contacting with liquid to its projected area on textured surface. Since the mechanism by which residual liquid film emerges on the pillars top is essentially ascribed to the pinch-off of the liquid threads, we further addressed the changes in contact time in terms of characteristic time of pinch-off of an imaginary liquid cylinder whose radius is related to solid fraction and the maximum contact area. The agreement of the theoretical analysis and the experimental results substantiates the assumption that the pinch-off of liquid thread should be responsible for the difference in contact time.Likewise, the textured surfaces were fabricated with square arrays of microposts whose height was set at 20μm. After silanization reaction, these textured surfaces displayed hydrophobic state and tended to undergo a wetting transition from Cassie regime to Wenzel regime. The axisymmetric wetting behavior was analysed according to the modified Wenzel equation taking account of the surface roughness in terms of line fraction instead of contact area. Furthermore, the dynamic behavior of droplet impacting on these textured surfaces was also examined. The theoretical analysis, taking consideration of the viscous dissipation and surface roughness, is in a good agreement with experimental results.更多还原