【作者】 李伟；

【导师】 陈蓉；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2023， 博士

【摘要】 开放式液滴操控平台被广泛应用于细胞培养、生化分析、快速诊断等领域。操控液滴输运、分裂、聚合等是开放式液滴操控技术的核心内容和关键环节。液滴在固体表面上的接触角滞后是阻碍液滴操控的关键因素。高能表面因其对双组份液滴的无接触角滞后特性,以极小的驱动力(～μN)即可实现双组份液滴操控,受到广泛关注。目前,高能表面上液滴操控技术存在着液滴操控功能单一,难以实现程序化液滴操控等局限。近年来,将光热效应引入到开放表面液滴操控催生了一种新的开放式光热操控液滴技术,具有响应迅速、灵活可控、时空分辨率高等优点,并可实现液滴温度场及界面行为等的精确操控。本论文结合高能表面上双组份液滴的无接触角滞后特性以及光热操控优点,提出利用局部光热效应操控高能表面上双组份液滴的可控输运、振荡、高通量分裂等。通过本文的研究,旨在揭示光热操控高能表面上双组份液滴过程中的相变热质传递特性与界面行为,获得光热效应致双组份液滴操控规律。相关研究成果可为基于光热效应的开放式液滴操控平台的设计和开发提供理论依据。本文首先研究了光热效应致开放表面上双组份钉扎液滴的相变和热质传递特性,获得了局部热源作用下双组份液滴非均匀温度场分布和相变热质传递特性及界面演化规律;其次,利用双组份液滴在高能表面上的无接触角滞后特性,通过激光与高能基底局部光热转换构造非对称力场来驱动双组份液滴,实现双组份液滴避光运动,建立了液滴避光运动理论模型;然后,探究了固定激光通过光热转换直接加热高能基底上双组份液滴的界面行为,探明了光热致双组份液滴振荡物理机制;进一步提出了利用移动激光与双组份液滴的光热转换实现高能表面上双组份液滴趋光运动模式,探究了液滴趋光运动特性以及关键参数的影响规律,实现了可编程液滴操控;最后,探究了光热效应致双组份液滴分裂机制以及子液滴同步输运机制,提出了光热致液滴高通量分裂和输运策略。本文获得的主要结论如下:(1)研究了光热效应致开放表面上丙二醇-水双组份钉扎液滴相变热质传递特性和界面演化规律。结果表明,光热诱导双组份液滴蒸发先经历以水蒸发主导的混合蒸发模式,随后经历丙二醇蒸发主导的恒定接触半径蒸发模式;液滴初始浓度主要影响混合蒸发模式持续时间和三相接触线回缩程度,两者都与液滴初始含水量呈正相关;随着激光功率增加,液滴整体蒸发速率提高,混合蒸发模式持续时间缩短,三相接触线回缩减弱。(2)提出了利用激光与高能基底的光热转换构造非对称力场来驱动双组份液滴,实现双组份液滴的避光运动。结果表明,激光局部加热效应在高能基底表面形成彗星状温度分布,在邻近热源的双组份液滴气液界面上产生表面张力梯度,为双组份液滴避光运动提供驱动力;建立了双组份液滴避光运动理论模型。研究发现,避光运动模式下双组份液滴具有自适应调节能力,液滴能自发地调节其与激光之间距离来保持驱动力与粘滞阻力平衡,实现液滴的稳定运动。通过实时调节激光移动路径和速度,可以精确控制液滴运动速度和轨迹,实现了多液滴序列化聚合、逆重力运动等多功能操控。(3)研究了固定激光直接加热高能表面上双组份液滴时的液滴振荡行为。研究发现,与基底局部光热转换操控液滴运动不同,固定激光直接加热高能表面上双组份液滴时,双组份液滴一开始会被激光吸引并往温度更高一侧运动,其受力方向取决于光热作用下液滴边界上表面张力梯度分布,液滴所受驱动力方向始终指向激光。在惯性力、粘性力和驱动力共同作用下,液滴发生振荡。在液滴振荡过程中,液滴振荡频率会增加,并且激光越偏离液滴,驱动力越大,液滴速度越小;此外,随着激光功率的提高,液滴振荡主频率增加;激光功率过高时,液滴振荡行为失效。(4)研究了局部光热效应直接作用于高能表面上双组份液滴所导致的液滴趋光运动特性。利用激光对高能表面上双组份液滴的吸引作用,通过移动激光可实现双组份液滴的趋光运动。研究发现,高激光功率或者低激光移动速度下,液滴稳定运动时的液滴变形更弱,激光与液滴之间距离也更小;激光与液滴形心之间距离可以自适应调节来确保力平衡,使得液滴稳定运动。趋光运动模式可实现液滴沿正弦曲线和对数螺线轨迹运动等可编程液滴操控。(5)研究了光热效应致高能表面上双组份液滴的分裂以及分裂形成的子液滴的自输运行为。研究表明,局部光热效应诱导的Marangoni流动以及粘性效应导致的液滴底部向内流动受限是导致液滴分裂的主要原因。基底温度梯度可以使分裂后的子液滴拥有较大运动初速度,液滴脱离核心加热区域后,惯性和双组份液滴在高能表面上的无接触角滞后特性是子液滴连续运动的主要原因。提出了在高能表面上构造疏水条纹实现液滴高通量分裂和输运的方法,子液滴数量以及体积可精确调控,并实现了含生物样本双组份液滴的高通量分裂及自输运以及并行微化学反应。更多还原

【Abstract】 Open droplet manipulation platform has been widely applied in a broad spectrum of fields,such as cell culture,biochemical analysis,rapid diagnosis,etc.Versatile manipulations of droplet on open surfaces,such as droplet transportation,splitting and coalescence,are crucial in open droplet manipulation techniques.The contact angle hysteresis on solid surfaces is regarded as one of the critical challenges hindering droplet manipulation.Currently,high energy surfaces have attracted extensive attention due to the hysteresis-free characteristic with binary droplets,which allows for the manipulation of the binary droplet on high energy surface even with a small propulsion force(～μN).However,traditional droplet manipulation techniques on high energy surfaces suffer from limitations of limited functionality and difficulty in achieving programmable droplet manipulation.Recently,the incorporation of the photothermal effect into the droplet manipulation on open surfaces results in a new technique of the photothermal effect based open droplet microfluidics,which offer numerous merits including fast response,flexibility,prominent spatial and temporal resolution.This technique also enables precise manipulation of droplet temperature and interface dynamics.In this thesis,by leveraging the advantages of the hysteresis-free characteristic of binary droplets on high energy surfaces and photothermal manipulation,the local photothermal effect manipulating the binary droplets on high energy surfaces is proposed for controllable droplet transportation,oscillation,and high-throughput splitting.The objectives of this thesis work are to reveal the characteristics of phase change,heat and mass transfer,as well as interface dynamics during the binary droplet photothermal manipulation process,and to obtain the principle of droplet manipulation.The outcomes are expected to provide a theoretical foundation for the design and development of open droplet manipulation platforms based on photothermal effect.In this thesis work,the photothermally induced phase change and heat and mass transports of pinned binary droplets are first investigated.The characteristics of non-uniform temperature distribution,phase change and heat and mass transfer as well as interface evolution of binary droplet under the localized heating effect are obtained.Subsequently,the combination of hysteresis-free characteristic and the local photothermal effect between the laser beam and substrate induced asymmetric force can drive the binary droplet motion,enabling the photophobic motion of binary droplet.A theoretical model describing the binary droplet transport on the high energy surface actuated by the localized photothermal effect is established.Next,the interfacial behaviors of the binary droplet on high energy surface caused by the local photothermal effect between the laser beam and droplet with a fixed laser beam position is investigated.The mechanism leading to the droplet oscillation is revealed.Furthermore,the direct photothermal conversion between the laser beam and binary droplet is used to realize the light-attracting motion mode of binary droplet via moving the laser beam.The characteristics of light-attracting motion and the influence of key parameters are investigated.Programmable droplet manipulations are achieved by this strategy.Finally,the droplet splitting mechanism by the local photothermal effect and the transport mechanism of the generated microdroplets are studied.The strategy of high-throughput droplet splitting and transport is also proposed.Main conclusions of this thesis are summarized as follows.(1)The photothermally induced phase change as well as the heat and mass transfer of pinned propylene glycol-water binary droplets are studied.It is found that the binary droplet evaporation caused by the localized photothermal effect initially undergoes a mixed evaporation mode dominated by water evaporation,followed by a constant contact radius evaporation mode dominated by propylene glycol evaporation.The initial concentration of the droplet primarily affects the duration of the mixed evaporation mode and the retracting extent of the three-phase contact line,both of which are positively correlated with the initial water content.The overall droplet evaporation rate increases with the increase of laser power.The increase of the laser power also shortens the duration of the mixed evaporation mode and weakens the retraction of the three-phase contact line.(2)A strategy that utilizes the photothermal conversion between laser beam and high-energy substrate to construct an asymmetric force field for driving the binary droplet motion is proposed,realizing the photophobic motion of the binary droplet on the high energy surface.The local heating effect creates a comet-shaped thermal pattern on the surface,generating the surface tension gradient at the droplet interface adjacent to the heat source,which provides the driving force for photophobic motion of binary droplet.A theoretical model describing the photophobic motion of the binary droplet is also developed.It is found that binary droplets can exhibit self-adaptive regulatory capability under the photophobic motion mode,allowing the droplets to spontaneously adjust the distance between the laser beam and droplet to balance the driving force and viscous resistance and thereby realizing stable photophobic motion.By real-time adjusting moving path and velocity of laser beam,the binary droplet transport with dynamically tunable velocity and arbitrary trajectory can be achieved.This photophobic motion also allows for multiple functional manipulations of binary droplets,including sequential coalescence of multiple droplets and anti-gravity motion.(3)The droplet oscillation behaviors of binary droplets are investigated when a fixed laser is directly irradiated to the binary droplets on high energy surfaces.When the fixed laser heating directly acts on a binary droplet,unlike the droplet motion manipulation by the photothermal conversion of the substrate,the droplet is initially attracted by the laser beam and moves towards the side with higher temperature.The direction of the driving force exerted on the binary droplets is determined by the distribution of the surface tension gradient along the droplet boundary,which is always directed to the laser beam.Under the action of the inertia force,viscous force and driving force,the droplet starts to oscillate.During the binary droplet oscillation process,the larger the deviation of the laser beam from the droplet center,the greater the driving force and the slower the droplet velocity.The oscillation frequency also increases.Besides,as the laser power increases,the main oscillation frequency of the binary droplet increases.However,the droplet oscillation will fail when the laser power exceeds a certain threshold.(4)The light-attracting motion of binary droplets on high energy surfaces caused by the direct photothermal conversion between the laser beam and droplet is studied.Unlike the droplet motion manipulation by the photothermal conversion of the substrate,the use of the light attraction along with the laser beam movement can realize the light-attracting motion of binary droplets on the high energy surface.For the light-attracting motion mode,higher laser power or smaller moving velocity of laser beam can lead to smaller droplet deformation,and reduce the distance between the laser beam and the droplet under stable motion state.It is found that the distance between the laser beam and droplet can adaptively self-adjust to ensure the force balance,enabling stable droplet motion.Based on the light-attracting motion mode,programmable droplet motion manipulation has been achieved,such as sinusoidal and logarithmic spiral trajectories.(5)Photothermally induced binary droplet splitting and transport on the high energy substrate are studied.It is found that the localized photothermal effect induced Marangoni flow and restricted inward flow at droplet bottom due to viscous effect enables droplet splitting.The temperature gradient on the high energy surface allows for the generated microdroplets to have a larger initial moving velocity.After the generated microdroplets depart from the core laser heating zone,the inertia and the hysteresis-free characteristics of the binary droplet on the high energy surfaces are responsible for the successive transport of the generated droplets.A strategy is proposed to achieve high-throughput droplet splitting and transport by creating hydrophobic stripes on the high energy surfaces,which also enables precise control of the number and volume of the generated droplets.High-throughput splitting and transport of bio-samples and parallelized micro-reactions are also demonstrated.更多还原