IMECH-IR  > 非线性力学国家重点实验室
微观力调控的一类功能化表面的仿真设计
英文题名Simulation design for the functional surfaces tuned by micro-forces
王帅
导师陈少华
2018-05-20
学位授予单位中国科学院大学
学位授予地点北京
学位类别博士
学位专业固体力学
关键词功能化表面 微观力 输运 自发集水 表面除湿
摘要

以微纳机械、微纳电子和光电器件为主要内涵的信息技术革命,开辟了以纳米科学为标志的新领域。在纳米尺度下,宏观尺寸可以忽略的微观力将占主导作用,力学工作者亦被带入从宏观到纳观的世界,激起了研究者们对微观力及微观世界的深入探索。本文针对范德华力、表面张力、毛细力等微观力作用下的多个纳尺度系统,应用分子动力学方法,研究了不同微观力引起的纳米颗粒和液滴输运现象,进而提出了不同微观力驱动下,可以实现纳米颗粒输运及筛选、自发集水、表面除湿的功能化表面设计方法,为实现一类功能化表面提供了新概念。论文的主要工作包括:

(1)建立了纳米颗粒-基底-滑块系统的分子动力学模型,提出了一种基于范德华力驱动纳米颗粒输运的新机制及纳米筛设计新方案,考虑了颗粒和滑块尺寸、滑块运动速度、基底预应变等因素对纳米颗粒输运行为的影响。研究表明:当滑块在基底下方滑动时,滑块与纳米颗粒之间的范德华力可以作为驱动力使纳米颗粒运动;基底的预拉伸应变有利于促进纳米颗粒的输运;颗粒尺寸、滑块速度或粘性阻力越小,颗粒越易被驱动。基于该输运新机制,进一步设计一种新型纳米筛,能够实现多颗粒系统的筛选和分离;亦可以用于碳纳米管中杂质的清除。

(2)在疏水性表面,设计了楔状的亲水区域,建立了液滴在含楔状亲水区表面的输运模型,分析了液滴体积、楔角等对液滴输运速度和输运距离的影响。数值结果表明:随着液滴体积减小或楔角增加,液滴运动速度增加但运动距离减小;进一步建立了相应的理论模型,揭示了液滴在该表面的输运机理。基于该输运机制,设计了多种功能化表面,可以实现液滴的钉扎、加速以及沿任意路径的定向输运,对于实现液滴的精确操控具有一定指导意义。

(3)基于液滴在含楔状亲水区表面的输运机制,数值仿真了液滴在含多级分支亲水区表面的自发汇聚现象。在疏水性基底上,设计树状的多级楔状亲水区,研究液滴在该表面自发地沿着分支向主干的定向汇聚,考虑了分支楔角、分支方向以及分支对称性等因素对液滴定向汇聚的影响。结果表明:分支楔角越小、分支与主干夹角越小且两侧分支不对称时表面上的液滴更容易发生定向汇聚;主干缺陷较大时会阻碍液滴的定向汇聚。基于此,进一步分析了表面随机分布液滴的定向汇聚,为高效集水功能表面提供了设计依据。

(4)利用分子动力学模拟,研究了在潮湿环境下,水分子在纳米柱阵列表面底部逐渐凝聚成水滴,随着液滴体积增大自发集结于微结构表面的行为。考虑了液滴体积、纳米柱高度、直径、间距和阵列表面浸润性等因素的影响。研究表明:对于特定形状及尺寸的纳米柱阵列表面,仅当液滴的相对体积超过相应的临界值时,液滴才会自发地沿纳米柱上升;液滴的相对临界体积随纳米柱的增高、纳米柱直径的增大和纳米柱间距的减小而减小,随纳米柱阵列表面亲水性的增强而增大;在纳米柱表面凝结的液滴更容易沿着纳米柱上升。水滴在微结构表面浸润状态的自发转变为设计具有表面除湿作用的功能表面提供了理论基础。

(5)结合课题组其他方面的工作,计算了FCC金属晶体构成的双材料纳米薄板结构的界面能密度,分析了界面晶格结构形貌变化及界面效应对原子势能的影响。结果表明:双材料纳米薄板界面具有周期性褶皱状疏密相间的晶格结构形貌,界面上原子势能亦呈现周期性分布特性,而靠近界面的两侧原子势能与板内原子势能具有明显差异。拉格朗日界面能密度和欧拉界面能密度均随双层薄板厚度的增加而增加,最终趋向于块体双材料结构的界面能密度。

英文摘要

Science has made a great revolution in the field of information technology, micro/nano machine, micro/nano electronics and optoelectronic devices which open the doors to the new dimensions of nanoscience world. The micro-force which is negligible in case of macroscale plays a dominant role in nanoscale and promotes our exploration regarding themicro-force and microcosm. In this paper, we focused on several nanoscale systems affected by micro-force, such as van der Waals' force, surface tension and capillary force. We studied the motion of nanoparticle and nanodroplet triggered by micro-force using molecular dynamics (MD) and further designed several functional surfaces that are capable of driving and separation of nanoparticles, water capturing and dewetting transition. Our work gives a novel idea for the design of new functional surfaces. The main work presented in the paper is given below as follows.

1. A new technique is proposed for the transportation and classification of different size nanoparticles based on van der Waals' forces. The effect of the nanoparticle size, slip velocity of sliding block and pre-tension of the graphene substrate on nanoparticle transportation is investigated, respectively. We found that the nanoparticles can be driven by the van der Waals' force between nanoparticle and sliding block. A pre-tensioned graphene substrate could influence the transportation mechanism and provide easier nanoparticle transportation relatively. The motion of the nanoparticle will be easier if the nanoparticles have small size and thickness, velocity of sliding block is slow along with the small viscous resistance. Based on such a new transportation mechanism, a novel nano-sieve can be designed, through which nanoparticles of different sizes can be screened and classified easily. This transportation technique could also help for the removal of fragments from the inside of carbon nanotubes.

2. The motion of droplet on a wedge-shaped wetting gradient surface was studied using MD simulation and theoretical analysis. The droplet volume and wedge-shaped angle were considered and several functional surfaces were designed. It was found that the droplet having smaller diameter moves faster but covers less distance, while the droplet on a smaller wedge-shaped angle moves smaller but covers long distance. An analytical model was developed to reveal the transport mechanism. Based on this transport mechanism, several functional surfaces were designed. On these surfaces, the droplets can be pinned, accelerated and forced to move on a nonlinear path. The conclusion of this study will provide ideas for manipulation of droplets.

3. Based on the wedge-shaped gradient surface, a multiple wedge-shaped gradient surface was designed to explore the spontaneous aggregation of droplets on solid surface. We designed the multiple wedge-shaped hydrophilic surface imbedded on a hydrophobic surface and studied the aggregation of droplets on the surface. The angle, symmetry and direction of the lateral branch were considered during study. It was found that aggregation of droplets will be easy if the angle of wedge-shaped lateral braches is small, the lateral branches on both side of main track are not symmetrical while the lateral branches and main track have same direction respectively. Based on this study, a multiple wedge-shaped gradient surfaces having large area could be designed for the aggregation of droplets on the surface.

4. The dewetting transportation of condensed droplet on a pillared surface is studied using MD simulation. In a moist ambient environment, droplets condense on the bottom of nanopillars and grow smoothly. A sudden dewetting transition phenomenon occurs while the droplet volume is large. We considered the effects of pillar height, diameter spacing and wettability of nanopillars. It was found that the dewetting transition can only be triggered if the relative volume of the droplet is larger than the critical volume of droplet. The relative critical volume decreases with the increase in pillar height and diameter, but increases with the increase of pillar spacing and wettability.

5. Combining with the work in our group, the interface energy densities of several face-centered cubic (FCC) metallic bilayers are also computed using MD simulation, the morphology and potential energy of atom in the interface is depicted. We found the periodic wrinkled atoms in the interface having distributed potential energy which is different from atoms inside the material. The Lagrange interface energy density and Euler interface energy density increases as the thickness of the bilayer increase, and both of them tend to the interface energy density of body material.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73124
专题非线性力学国家重点实验室
推荐引用方式
GB/T 7714
王帅. 微观力调控的一类功能化表面的仿真设计[D]. 北京. 中国科学院大学,2018.
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