【作者】 司廷；

【导师】 尹协振；

【作者基本信息】 中国科学技术大学 ， 流体力学， 2009， 博士

【摘要】 流动聚焦（Flow Focusing,FF）是一种毛细流动现象,描述为从毛细管流出的流体由另一种高速运动的流体驱动,经小孔聚焦后形成稳定的锥形,在锥的顶端产生一股微射流穿过小孔,并在小孔外一定距离处射流破碎成单分散性的微滴。该技术稳定、易操作、没有苛刻的环境条件,可以制备微米量级甚至数百纳米量级的液滴和颗粒,在化学、医学、生物、工农业等许多领域都有重要的应用价值。本文系统研究了液-气流动聚焦的现象和规律,在实验中得到了六种流动聚焦流动模式,重点对流动聚焦进行了时间、时空和空间线性稳定性分析,揭示了其内在的物理机理,并进行了实验和理论结果的比较,主要内容描述如下:1、自行设计加工了便于观察和控制的流动聚焦装置,并在搭建的实验系统上研究了物理参数对流动聚焦中锥形、射流和液滴形貌的影响,测量了小孔出口处射流的直径d₁、射流破碎长度L和射流表面的扰动波长λ随液体流量Q₁和气体压力差Δp_g的变化规律。2、实验中首次得到了流动聚焦中的六种流动模式及其工作区域,分析了不同模式的破碎机理。六种模式是:锥振动模式、锥粘连模式、螺旋射流模式、共存射流模式、轴对称射流模式和滴模式。其中锥振动模式和锥粘连模式主要与锥形的不稳定性有关,其他四种模式与射流的不稳定性有关。射流不稳定性中,滴模式与绝对不稳定性有关,轴对称射流模式和螺旋射流模式分别是由轴对称扰动和非轴对称扰动引起的,而共存射流模式是轴对称和非轴对称扰动共同作用的结果。3、建立了用于研究流动聚焦稳定性分析的理论模型。该模型由半径为R₁的圆柱液体射流和外径为R₂的环形气体射流组成,液体和气体均为粘性、不可压缩的牛顿流体,忽略温度和重力的影响。影响现象的主要无量纲参数有,雷诺数（Re）、韦伯数（We）、密度比（Q）、粘性比（N）和半径比（a）。两种流体的基本速度型采用满足边界条件的双曲正切型,控制参数为界面上的速度Us和界面上液体速度剖面的斜率K。4、利用正则模方法和Chebyshev谱配置法,针对轴对称（n=0）和第一类非轴对称（n=1）扰动进行了时间、时空和空间线性稳定性分析,研究了各个参数对不稳定模的影响,结果表明较小（较大）的K和中等的Us对应着较小（较大）的We数和中等的Re数,属于长波（短波）不稳定区域,而密度比（Q）和粘性比（N）主要反映了外部气体对不稳定性的影响,半径比（a）大于一定值时对不稳定性没有影响。5、在轴对称时间稳定性分析中得到的最不稳定扰动的波长与实验测量的结果一致;在轴对称时空稳定性分析中首次得到绝对/对流不稳定性的转换边界,并与实验的滴/射流边界一致。6、在轴对称（n=0）和第一类非轴对称（n=1）空间稳定性分析中,发现在低We数下,轴对称模式的扰动增长要快于非轴对称模式的扰动,随着We数增大,非轴对称扰动逐步占优,而Re数几乎不引起不稳定性的转换,首次解释了实验中发现的轴对称射流模式向非轴对称射流模式的转变。更多还原

【Abstract】 Flow focusing（FF） is one of capillary flows characterized by the formation of a steady meniscus in the core of an extensional high-speed fluid focused by a small hole when a fluid is injected through a capillary needle.A very thin jet is issued from the vertex of the meniscus,passes through the hole and breaks up into monodisperse droplets at a certain distance outside the hole.The FF technique is steady,controllable and does not require rigorous conditions in producing droplets and particles down to the micrometer dimension and below with important applications in fields of chemistry,medicine,biology,industry and agriculture and so on.This dissertation is devoted to the investigation on the phenomena and rules in the liquid-gas FF.Six flow modes are distinguished in the FF experiments,the linear temporal,spatio-temporal and spatial stability analyses are carried out,the physical mechanisms in FF are explored and the comparisons between experimental and theoretical results are performed.The main contents are described as follows:1.A FF device which is convenient to observating and controlling has been developed.The effects of experimental parameters on the meniscus,jet and droplet are investigated using the developed experimental setup.Values of the jet diameter d₁ at the hole exit,the breakup length of the jet L and the disturbance wavelengthλat the jet surface are measured as the liquid flow rate Q₁ and gas pressure dropΔp_g vary.2.The flow modes and their corresponding domains are distinguished and the breakup mechanisms of them are investigated for the first time.Six flow modes obtained are respectively the cone-shaking mode,cone-adhering mode,helical jetting mode,coexisting jetting mode,axisymmetric jetting mode and dripping mode.The cone-shaking mode and the cone-adhering mode are related to the instability of the meniscus,whereas the other modes to the instability of the jet.The dripping mode is associated with absolute instability of the jet.The axisymmetric jetting mode and helical jetting mode are caused by axisymmetric and non-axisymmetric disturbances, respectively.Moreover,the coexisting jetting mode results from both axisymmetric and non-axisymmetric disturbances.3.The theoretical model in terms of linear stability analysis of FF is established. The physical model consists of a cylinder liquid jet of radius R₁ surrounded by an annular gas stream of radius R₂.Both the liquid and the gas are assumed to be viscous, incompressible and Newtonian.The effects of temperature and gravity are neglected. The main dimensionless parameters governing the phenomenon of FF include the Reynolds number Re,the Weber number We,the density ratio Q,the viscosity ratio N, and the radius ratio a.The velocity profile of hyperbolic-tangent function in two fluids satisfying all the boundary conditions is utilized,with two control parameters of the velocity at the interface Us and the slope of the liquid velocity profile at the interface K.4.Using the normal mode method and the Chebyshev spectral collocation method, the linear temporal,spatio-temporal and spatial stability analyses considering both the axisymmetric（n=0） and first non-axisymmetric（n=1） disturbances are performed,and the effects of parameters are studied.The result indicates that small（or large） We and moderate Re,as well as velocity profiles with small（or large） K and slightly moderate Us,favor the instability of long（or short） waves;the density ratio Q and the viscosity ratio N reflect the effects of outer gas on the jet instability;the radius ratio a has no influence on the jet instability when its value is slightly high.5.The most unstable disturbance wavelength obtained in axisymmetric temporal stability analysis is in good agreement with experimental measurement.The transition boundary between absolute and convective instability in axisymmetric spatiotemporal stability analysis is obtained for the first time and is in accordance with the experimental boundary between dripping and jetting.6.The spatial stability analysis considering both n=0 and n=1 shows that axisymmetric disturbances grow faster than non-axisymmetrie ones for low Weber number,while the non-axisymmetric disturbances become predominant as the Weber number increases.The Reynolds number hardly induces the change of the instability mode.The transition from the axisymmetric to non-axisymmetrie jetting mode in FF experiments is illuminated for the first time.更多还原