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蒸发液滴中的胶体颗粒沉积机制与空间实验研究
英文题名Mechanisms of colloidal particle deposition in an evaporating droplet and the experimental research in space
李伟斌1,2
导师王育人
2018-05-26
学位授予单位中国科学院大学
学位授予地点北京
培养单位中国科学院力学研究所
学位专业一般力学与力学基础
关键词胶体液滴 蒸发 颗粒沉积 图案化表面 空间微重力
英文摘要

液滴的蒸发现象在自然界中普遍存在。在过去的二十几年里,液滴蒸发中的微流动和颗粒沉积引起了科研人员极大的研究兴趣,这既因为蒸发过程中蕴含着丰富的物理学现象,例如常见的咖啡环效应,又由于其在微流控制、光子晶体制备、疾病诊断等领域展现出广阔的应用前景。然而,关于蒸发液滴中的颗粒组装动力学理论仍然不完善,这大大限制了其应用。

本文的主要工作之一是参与研制了实践十号卫星有效载荷分系统-胶体材料箱。载荷由结构单元、注液管理单元、样品管理单元、光学观察单元、驱动控制单元五部分构成,其主要功能包括在空间实现复杂流体的均匀分散、管理控制、加热蒸发和光学观察。我们对载荷开展了大量的地面测试及试验验证,表明胶体材料箱满足空间微重力环境下的液滴蒸发及胶体组装实验要求。我们最终顺利完成了空间在轨实验。

基于胶体材料箱空间实验平台,利用表面亲疏水图案化技术在空间成功开展了液滴操控实验,并通过液滴蒸发研究了图案化基片的浸润和控制机理。结果表明,基片的浸润行为呈现三种机制:常疏水角模式、常接触面积模式、常亲水角模式,其中常接触面积模式是基片能够实现液滴控制的关键。液滴在常重力和微重力下的形状分别可以通过椭球缺和球缺模型来描述,与常重力相比,微重力环境下能够实现更大体积液滴的控制。表面图案化技术具有操作简单,控制能力强等优点,因此在未来空间流体管理、生物传感器、空间制药等领域具有广阔的应用前景。

利用微分干涉显微技术,研究了液滴蒸发最后阶段薄液膜的破裂和颗粒的二次组装过程。揭示了咖啡环内部二维网络状图案的形成机制,网络状图案由大量干区组成,原位显微观察表明单个干区的形成可分为三个阶段:破裂启动、干区出现并扩展、残液干燥。通过研究发现,颗粒的聚集和液膜的去浸润是两个相互耦合的过程:颗粒不均匀分布造成的抽吸效应,导致干区在颗粒稀疏区域首先出现,加速了液膜的破裂过程;液膜的去浸润(干区扩展)控制了颗粒的聚集,通过表面张力主导了颗粒的组装过程,决定了最终的沉积图案。这项工作拓展了Deegan提出的咖啡环效应,完善了蒸发液滴中的胶体颗粒沉积理论。

开展了空间微重力和地面常重力下胶体液滴的蒸发实验。通过对蒸发液滴侧视轮廓和内部颗粒运动的分析,提出了微重力环境下咖啡环效应的弱化机制,阐明了重力在颗粒组装过程中的影响。通过进一步研究包含不同粒径颗粒的正置液滴和倒置液滴的沉积图案,表明重力沉降效应、界面捕获效应和咖啡环效应在蒸发不同阶段分别主导了颗粒的沉积。对于正置液滴和倒置液滴,提出颗粒和气液界面之间的相对运动分别存在着追击机制和相遇机制。这两种机制的提出进一步丰富了蒸发液滴中的颗粒沉积理论,对于预测和调控沉积图案的形貌,具有很大的指导意义。

基于蒸发液滴中的颗粒沉积理论,本文最后对沉积图案的调控开展了初步的探索研究。通过基底加热,一方面增大了颗粒的沉积速率,提高了胶体晶体的制备效率,另一方面得到高质量的密排结构。通过控制初始液滴的体积、颗粒浓度及颗粒粒径,可以实现对沉积形貌的调节。利用表面改性的方法及蒸发液滴的特定物理效应,实现了液滴边界以及内部沉积形貌的调控。通过倒置液滴中的重力沉降、液面捕获及咖啡环效应的竞争关系,成功实现了二元胶体液滴沉积图案中的大小颗粒分离。沉积图案的调控研究,有助于在未来制备和发展特定结构和功能的微纳尺度材料。

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The phenomenon of droplet evaporation occurs commonly in nature. Over the last two decades, there has been a surge of scientific interest in the microflow and deposition of particles in an evaporating droplet. This is not only because the evaporation process involves multiple physical phenomenon such as the common coffee ring effect, but also because these phenomena influence a wide range of applications such as microfluidic, photonic crystal formation, disease diagnosis, etc. However, the self-assembling dynamic theory of particles in drying droplet is still incomplete, which has greatly limited its application.

In this doctoral dissertation, I have participated in the design of the colloidal material box that is a subsystem and payload of SJ-10 satellite. The payload is composed of five parts: a structure unit, an injection management unit, a sample management unit, an optical observation unit, and a drive control unit. Its main functions include uniform dispersion, management and control, heating and evaporation, and optical observation of complex fluids in space. We have further launched a lot of ground testing and test validation, which show that the colloid material box could satisfy the demands of both droplet evaporation and colloidal assembling experiments in space microgravity environment. We have successfully completed the in-orbit experiments in space. 

Based on space experimental platform (colloid material box), we have carried out the drop manipulation experiment in space successfully by using the hydrophilic/hydrophobic patterned surface technique, and the mechanism of the drop wetting and control on the patterned substrate during drop evaporation have been further studied. The results show that the wetting behavior of the patterned substrate has three regimes: the constant hydrophobic angle mode, the constant contact area mode and the constant hydrophilic angle mode. The contact area regime is responsible for the drop control. The drop shape in normal gravity and microgravity can be described as ellipsoidal cap and spherical cap model, and the patterned substrate could confine aqueous drops with larger volume under microgravity than in normal gravity. With advantages of simple operation and strong capability to control large drops, this technique exhibite the wide application prospect in the fields of fluid management, bio-sensing, pharmacy in microgravity condition in the future.

The rupture of liquid film and rearrangement of particles in the final stage of droplet evaporation has been investigated by utilizing differential interference contrast microscopy. We have revealed the formation mechanism of a network pattern inside a coffee ring, which is consist of a large number of dry patches. Real-time bottom-view images show that the evolution of a typical dry patch can be divided into three stages: rupture initiation, dry patch formation and expansion, and drying of the residual liquid. It could be shown that particles aggregation and liquid film dewetting are mutual influenced by each other. The uneven distribution of particles lead to the suction effect, which promotes the rupture of the liquid film and the formation of the dry patch at the particle-poor region; the particle-assembling process is totally controlled by the liquid film dewetting and dominated by the surface tension of the liquid film, which eventually determine the ultimate deposition patterns. This work expands the coffee ring effect proposed by Deegan, and enriches the theories of particles deposition in an evaporating droplet.

Experiments of colloidal droplet evaporation has been conducted in both space microgravity and terrestrial normal gravity conditions. By analysis of the profile change and particles motion in an evaporating droplet, we have illuminated the gravity effect on the particles deposition process and the mechanism of the weakened coffee ring effect. We report on the interplay of the interface shrinkage, the gravitational sedimentation and the outward capillary flow in drying droplets. This interaction effect is the inference we draw from deposition patterns of both sessile and pendant droplets, which contain particles in different sizes, evaporating on a patterned substrate. We have proposed two different regimes for the relative motion between the particles and the interface: the pursuit regime (sessile droplet) and the meeting regime (pendant droplet), which further the theory of particles deposition in drying droplet. It is significant to predict and regulate the morphology of the final deposition patterns.

This thesis has finally conducted a preliminary exploration research of the deposition patterns control based on the theories of particle deposition in an evaporating droplet. Through the base heating, we could increase the efficiency of the colloidal crystal preparation and obtain high quality closed packed structure. By controlling the initial droplet volume, particle concentration and particle sizes, the deposition morphology can be regulated. Using the method of surface modification and specific physical effects in an evaporating droplet, the deposition patterns near the edge and the center of the droplet have been modified. By using the interplay of the interface shrinkage, the gravitational sedimentation and the outward capillary flow in a drying pendant droplet, we have successfully realized the separation of colloidal particles with different sizes. The manipulation of deposition patterns will help to develop new functional microstructures and the related materials in the future.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73181
专题国家微重力实验室
作者单位1.中国科学院力学研究所
2.中国科学院大学
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李伟斌. 蒸发液滴中的胶体颗粒沉积机制与空间实验研究[D]. 北京. 中国科学院大学,2018.
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