微尺度下锥形微操作探针表面液滴形成的研究（英文）

基于液滴的转移方法可实现微操作任务中微对象的拾取,锥形操作探针则常作为一种毛细力微操作执行工具。主要研究在空气冷凝模式下锥形探针端面的液滴形成。建立了微液滴形成的数学模型,主要包括初始液滴的形成、液滴的合并和液滴的移动,研究了影响操作液滴的关键参数,分析表明:过冷度决定最小液滴半径。对单液滴的生长机制进行理论分析,并通过数值求解的方法模拟了锥形操作探针端面的液滴形成。搭建实验测试平台,实验研究了微尺度下锥形微操作探针端面的液滴形成。实验结果表明:在空气冷凝模式下,操作探针端面能够形成微液滴。经过初始液滴的形成,液滴的合并和移动等过程最终可形成稳定的微液滴,且不同锥顶角下液滴的形成呈现多样化。     

单指式微执行器端面冷凝液滴的迁移特性

操作液滴是液体介质微操作机器人的先决条件,研究单指微执行器端面冷凝液滴的迁移特性,对操作液滴的稳定获取具有重要指导意义.首先分析基于冷凝液滴的微构件柔顺操作原理,推导作用于微球上的液桥力方程.建立单指微执行器端面的冷凝液滴生长模型,包括单液滴生长、液滴合并、温度诱导和边缘滞后,分析影响端面冷凝液滴迁移的因素.直径130—400μm单指微执行器的端面冷凝实验表明,冷凝液滴经过生长合并后,在端面形成单个液滴,温度梯度和边缘效应会影响所形成液滴的稳定性.在制冷片表面温度–5℃,环境温度24℃,湿度37%的条件下,直径400μm的疏水执行器末端最终获取5.5 nL的液滴,且固着在端面.相对于未处理执行器,疏水处理后的微执行器末端的冷凝液滴更稳定.实验结果验证了理论分析的有效性.     

低压蒸汽滴状冷凝过程中液滴生长特性

研究了低压条件对滴状冷凝过程液滴生长特性的影响。首先,研究了超疏水表面上空气环境和蒸汽环境中附着液滴的接触角,发现蒸汽环境中的接触角比空气环境中的小,而蒸汽压力对接触角没有显著影响。第二,实验研究了冷凝过程中的液滴的生长周期和脱落尺寸,液滴的脱落半径随压力的降低而增大,生长周期也随之延长。第三,实验研究了液滴合并生长速率,并结合理论分析直接冷凝长大的生长速率,直接冷凝生长速率随压力的减小而减小,并随过冷度的减小而下降,而实验范围内合并生长速率不受压力影响。第四,根据滴状冷凝液滴分布的时间序列模型,分析了不同压力下液滴生长的临界尺寸,随着压力的降低,液滴生长方式的临界尺寸增大。     

超疏水表面冷凝液滴行为与生长机制

本文制备了具有微-纳二级结构的超疏水表面,并在湿空气自然对流条件下进行了水平表面冷凝实验。实验结果表明,冷凝液滴行为可分为液滴合并、液滴弹跳和液滴扫掠三类。其中液滴弹跳与液滴扫掠行为均是由液滴合并触发的自推进液滴行为。液滴扫掠与液滴弹跳的区别是,目标液滴未能跳离表面,而是沿表面运动,并吞并沿途的液滴,留下狭长的运动轨迹,且轨迹尽头停留一个大液滴。本文亦通过统计方法,获得了超疏水表面上冷凝液滴的粒径分布、平均直径、表面覆盖率等,研究了液滴种群和单个液滴的生长机制。     

蒸汽冷凝过程中团簇及微液滴尺寸分布演化

将瞬态团簇尺寸分布模型引入蒸汽冷凝体系,通过求解描述团簇增长的动力学方程,模拟研究了特定冷凝条件下团簇及微小液滴尺寸分布的演化特征。模型结果表明,核化过程中,团簇/微液滴尺寸分布迅速从单调递减式分布转变为近似正态分布特征。随着时间延长,尺寸分布的峰值向团簇/微液滴尺寸增加的方向移动。在壁面冷凝过程中,核化生成的最小液滴也具有一定分布,进一步完善了经典冷凝理论中对初始冷凝过程液滴分布和演化过程的描述。同时模拟结果与文献报道的微米级液滴生长演化趋势相似,表明较大尺寸液滴分布特征是核化阶段团簇/微液滴尺寸分布特征进一步演化的结果,揭示了冷凝过程液滴尺寸分布演化的微观物理本质。     

滴状冷凝过程液滴自由表面温度场分析

对于滴状冷凝过程及其传热强化机理, 一般通过分析冷凝壁面上液滴分布和运动规律进行研究, 并且将单个液滴视为稳定的个体, 很少涉及液滴内部运动特征. 本文通过红外热像仪观测了纯蒸气滴状冷凝过程中, 液滴运动时自由表面温度场的演化过程. 发现在疏水壁面上, 液滴由于合并或脱落而发生移动过程中, 其自由表面温度先降低, 而后升高并高于移动前温度. 通过分析疏水表面上液滴移动过程的物理模型, 认为液滴移动时表面液膜发生履带式滚动现象, 或者发生液滴内部与自由表面附近的液体间形成对流和掺混现象. 对液滴运动时表面温度演变规律的分析表明: 触发液滴表面发生持续冷凝可能需要克服一个临界过冷度, 当气液间温差超过该临界值时才诱发冷凝; 液滴合并或脱落等整体运动过程, 导致了液滴内部的运动特征, 并促进了较大尺寸液滴表面发生直接冷凝, 这为强化冷凝传热的研究提供新的思路.     

纳秒激光脉冲诱导硅表面微结构

利用Nd:YAG纳秒激光脉冲,在能量密度为1～10 J/cm2范围内辐照单晶硅,形成了表面锥形微结构,在SF6气氛和空气环境下均形成了锥形尖峰表面微结构。SF6气氛下产生的锥形尖峰顶端都有小球,部分锥形上还有二次尖峰形成,空气中纳秒激光诱导的锥形尖峰微结构顶端和边缘有由液滴固化形成的粒状物质,不同于利用准分子纳秒激光诱导的细长须状结构和飞秒激光辐照下产生的具有表面枝蔓状纳米结构的锥形微结构。实验结果表明,这种尖峰微结构的形成与辐照激光的波长和脉冲持续时间有关。对空气中微构造硅的辐射反射的初步研究表明,在500～2 400 nm范围内的光辐射反射率不高于20%。     

纳米结构表面上部分润湿液滴合并诱导弹跳的理论研究

部分润湿液滴是适宜纳米结构表面上滴状冷凝传热的主要载体,研究纳米结构参数与部分润湿液滴合并弹跳之间的关系有重要意义,本文依据冷凝液滴生长过程中能量增加最小的原理来判断其是否为部分润湿状态,并根据液滴合并前后的体积和界面自由能守恒,确定了合并液滴的初始形状,进而对合并液滴变形过程的动力学方程进行了求解,结果表明:部分润湿冷凝液滴仅在纳米柱具有一定高度、直径间距比较大的表面上形成,而当纳米柱高度过低、直径间距比小于0.1时则形成完全润湿的冷凝液滴;液滴合并后能否弹跳与纳米结构参数紧密相关,仅在纳米柱较高、直径间距比适宜的表面上,部分润湿液滴合并后才能诱发弹跳;液滴尺度及待合并液滴间的尺度比对合并弹跳也有重要影响;多个部分润湿液滴合并后由于具有更多的过剩界面自由能而比两个液滴合并更容易诱发弹跳,本模型对纳米结构表面上冷凝液滴是否合并诱发弹跳的计算结果与绝大部分实测结果相一致,准确率达到95%。     

蒸汽滴状冷凝演变和液滴生长方式的研究

蒸汽的滴状冷凝进程中，液滴生长包括冷凝生长与合并生长。液滴生长作为影响滴状冷凝演变的关键行为，其研究有助于深入理解滴状冷凝演变机制和换热机理。本文基于滴状冷凝全过程的数值模拟，针对不同凝结核密度下的滴状冷凝演变和液滴生长方式开展研究。模拟中采用Cassie模型对液滴的冷凝生长进行描述，耦合邻近搜索算法和守恒定律对液滴的合并生长过程进行模拟。经实验结果和理论模型验证，本文模拟方法具有较高的可靠性。结果表明：最大液滴尺寸主要来源于合并生长，其增速是衡量滴状冷凝演变速度的表征；随着凝结核密度的升高，演变速度呈线性提高，换热能力先增后减；高合并频率的表面能有效提高合并生长对最大液滴尺寸的贡献比例。     

电场下液滴合并自弹跳行为的分子动力学研究

在工程上通常利用滴状冷凝提高冷凝换热效率、进而强化传热。而当冷凝液滴发生合并自弹跳时,冷凝换热系数是传统滴状冷凝的1.3至1.5倍,因此液滴合并自弹跳现象对冷凝传热强化的贡献是非常大的。一些宏观实验和理论研究表明,加入外电场能进一步促进冷凝液滴合并自弹跳的频率和高度,但在纳米尺度下是否仍遵守这一规律还未可知,因此本文使用分子动力学模拟方法,探究了在超疏水表面上电场的方向和强度对纳米纯水液滴合并自弹跳行为的影响,模拟结果表明垂直向上方向电场会抑制液滴合并自弹跳,垂直向下方向存在一个电场促进弹跳的区间,在此区间内电场强度越大,弹跳速度越大.     

An experimental study on the coalescence process of binary droplets in oil under ultrasonic standing waves

The coalescence process of binary droplets in oil under ultrasonic standing waves was investigated with high-speed photography. Three motion models of binary droplets in coalescence process were illustrated: (1) slight translational oscillation; (2) sinusoidal translational oscillation; (3) migration along with acoustic streaming. To reveal the droplets coalescence mechanisms, the influence of main factors (such as acoustic intensity, droplet size, viscosity and interfacial tension, etc) on the motion and coalescence of binary droplets was studied under ultrasonic standing waves. Results indicate that the shortest coalescence time is achieved when binary droplets show sinusoidal translational oscillation. The corresponding acoustic intensity in this case is the optimum acoustic intensity. Under the optimum acoustic intensity, drop size decrease will bring about coalescence time decrease by enhancing the binary droplets oscillation. Moreover, there is an optimum interfacial tension to achieve the shortest coalescence time.     

非对称布置液滴在波纹状基底上的聚并过程

基于润滑理论建立在波纹基底上两个含表面活性剂液滴聚并的演化模型，模拟液滴位于波纹基底波峰和波谷处的聚并过程，分析液滴初始间距和基底高度对聚并的影响，并与对称布置液滴进行比较.结果表明：液滴高度随时间呈现五个阶段变化；活性剂浓度在短时间内完成三个阶段变化；初始间距的增大将延长液滴及活性剂聚并时间；增大基底高度将缩短液滴及活性剂聚并时间；非对称液滴相较于对称液滴聚并时间短，聚并速度快.     

Directional Motion of Droplets in a Conical Tube or on a Conical Fibre

Manipulating the directional movement of liquid droplets is of significance for design and fabrication of some microfluidic devices, An energy-based method is adopted to analyse the directional movement of a droplet deposited in a conical tube or on a conical fibre. We perform an experiment to investigate the directional motion of a droplet in an open conical tube. Our theoretical analysis and experimental observations both demonstrate that surface tension can drive the droplet to move in the conical tube. The critical condition of the liquid moving in the conical tube is presented. We also analyse a droplet on a conical hydrophilic fibre, which can move from the thinner to the thicker end.     

Hydrodynamic binary coalescence of droplets under air flow in a hydrophobic microchannel

Based on the volume of fluid(VOF) method, we conduct a numerical simulation to study the hydrodynamic binary coalescence of droplets under air flow in a hydrophobic rectangular microchannel. Two distinct regimes, coalescence followed by sliding motion and that followed by detaching motion, are identified and discussed. Additionally, the detailed hydrodynamic information behind the binary coalescence is provided, based on which a dynamic mechanical analysis is conducted to reveal the hydrodynamic mechanisms underlying these two regimes. The simulation results indicate that the sliding motion of droplets is driven by the drag force and restrained by the adhesion force induced by the interfacial tension along the main flow direction. The detachment(i.e., upward motion) of the droplet is driven by the lift force associated with an aerodynamic lifting pressure difference imposed on the coalescent droplet, and also restrained by the adhesion force perpendicular to the main flow direction. Especially, the lift force is mainly induced by an aerodynamic lifting pressure difference imposed on the coalescent droplet. Two typical regimes can be quantitatively recognized by a regime diagram depending on Re and We. The higher Re and We respectively lead to relatively larger lift forces and smaller adhesion forces acting on the droplet, both of which are helpful to detachment of the coalesced droplet.     

Coalescence in low-viscosity liquids

The expected universal dynamics associated with the initial stage of droplet coalescence are difficult to study visually due to the rapid motion of the liquid and the awkward viewing geometry. Here we employ an electrical method to study the coalescence of two low-viscosity droplets at early times. We measure the growth dynamics of the bridge connecting the two droplets and observe a new asymptotic regime inconsistent with previous theoretical predictions. The measurements are consistent with a model in which the two liquids coalesce with a slightly deformed interface.     

Decompressing emulsion droplets favors coalescence

The destabilization process of an emulsion under flow is investigated in a microfluidic device. The experimental approach enables us to generate a periodic train of droplet pairs, and thus to isolate and analyze the basic step of the destabilization, namely, the coalescence of two droplets which collide. We demonstrate a counterintuitive phenomenon: coalescence occurs during the separation phase and not during the impact. Separation induces the formation of two facing nipples in the contact area that hastens the connection of the interfaces prior to fusion. Moreover, droplet pairs initially stabilized by surfactants can be destabilized by forcing the separation. Finally, we note that the fusion mechanism is responsible for a cascade of coalescence events in a compact system of droplets where the separation is driven by surface tension.     

The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.     

分形理论结合相变动力学的冷表面结霜过程模拟

运用分形理论并结合相变动力学模拟冷表面上结霜过程.在相变动力学基础上成功模拟了结霜初始阶段水蒸气在冷表面上凝结、液滴生长及冻结过程,随后运用分形理论的有限制的扩散凝聚(diffusion limited aggregation,DLA) 模型模拟了霜晶在冻结液滴表面上的形成生长过程.模拟结果与实验结果取得良好的一致,模拟过程中凝结液滴出现及冻结的时间与实验结果几乎完全符合;液滴冻结之前其表面接触半径随时间变化的模拟结果与实验结果基本一致,同时模拟霜层厚度与实验测得霜层厚度也非常接近.研究结果对于探讨分形理 关键词： 分形 相变动力学 结霜 模拟     

Coalescence of spreading droplets on a wettable substrate

We investigate experimentally and theoretically the coalescence dynamics of two spreading droplets on a highly wettable substrate. Upon contact, surface tension drives a rapid motion perpendicular to the line of centers that joins the drops and lowers the total surface area. We find that the width of the growing meniscus bridge between the two droplets exhibits power-law behavior, growing at early times as t1/2. Moreover, the growth rate is highly sensitive to both the radii and heights of the droplets at contact, scaling as ho3/2/Ro. This size dependence differs significantly from the behavior of freely suspended droplets, in which the coalescence growth rate depends only weakly on the droplet size. We demonstrate that the scaling behavior is consistent with a model in which the growth of the meniscus bridge is governed by the viscously hindered flux from the droplets.