几何构型对流动聚焦生成微液滴的影响

流动聚焦型微流控装置能够方便、高效地生成均一度好且大小精确可调的微液滴(气泡),故被广泛应用于颗粒材料合成、药物封装、细胞培养等诸多领域. 进一步优化通道结构有助于实现对合成微粒粒径、均一度和尺寸范围的精确调控. 本文数值研究了通道深度、缩颈段长度以及两相夹角等几何构型因素对流动聚焦生成微液滴直径及其生成周期各个阶段的影响. 控制液滴生成方式为滴流式,发现液滴直径随通道深度d 的增加近似呈线性增大,且当通道深度小于30 μm 时,随着通道深度的下降,微液滴生成周期在毛细力的强烈作用下出现骤升,通道深度超过80 μm 时,微液滴的生成周期基本接近恒定. 连续相和离散相的夹角θ接近90°时,液滴直径及其生成周期最短,夹角太大或太小均不利于生成均一度好且粒径微小可控的液滴. 调整缩颈段长度l引起液滴直径及其生成周期的变化幅度仅为其平均值的3%~5% 左右. 此外,缩颈段宽度也是影响流动聚焦生成微液滴直径及其生成周期的重要因素,在通道深度固定时,缩颈段越宽,微液滴直径及其生成周期越大.      

微流体驱动与控制技术研究进展

随着微流体系统,尤其是生物芯片和缩微芯片实验室(lab-on-a-chip)技术的发展,微米乃至纳米尺度构件中流体的驱动与控制技术越来越引起人们的注意.由于微流体流动的影响因素众多,它的驱动和控制技术,与宏观流体相比,更为复杂和多样化.本文首先结合流体的驱动和控制技术,并着眼于微观与宏观的不同,对微流体的流动特性进行了分析,然后对目前微流体驱动与控制技术的研究进展进行了总结,对各种微流体的驱动和控制技术进行了对比,并对他们各自的优缺点进行了分析和讨论.      

微纳米结构壁面对流场及动压润滑的影响研究

固体边界具有的微纳米结构将影响流体在近壁面处的流动行为,进而由于尺度效应改变流体在整个微间隙的流动或润滑规律.将壁面可渗透微纳米结构等效为多孔介质薄膜,采用Brinkman方程来描述流体在近壁面边界渗透层内的流动,并将其与自由流动区域的不可压缩流体Navier-Stokes控制方程耦合,在界面处的连续边界条件下求解和分析了速度分布规律和压力变化规律.针对恒定法向承载力的油膜润滑条件,进一步讨论了静止表面或运动表面的微纳米结构对近壁面流动行为的影响;并揭示了考虑壁面微纳米结构的流体动压润滑的油膜厚度和摩擦系数的变化规律.论文结果为具有可渗透微结构表面的微间隙流动与润滑提供了理论参考.     

基于小型电磁动力驱动器的微流道混合控制研究

对于许多微流体应用系统来说,流体的混合是至关重要的.本文研究了基于电磁流体动力混合器的微流道主动混合控制方法,建立了该微流道混合系统的理论模型并进行了数值模拟.在交变Lorentz驱动下,流经混合器流道的流体及其分界面在混合器流道内往复运动,流体界面的反复折叠与流体局部流动使流体混合,流体的横向往复运动使流体分界面反复折叠从而使流体间的接触面积大大增加.对流体混合过程进行了讨论并给出了漉体混合程度评价方法.     

组装式微流控系统制备双重包裹微液滴的方法研究

介绍了一种组装式微流控系统制备单、双重包裹微液滴的方法。微系统中用三通接头构成T型微流体通道,使得分散相在连续相强烈的剪切力和压力差作用下断裂形成单个微米级液滴。在制备单个微液滴基础上,用毛细管将两个三通接头串联,通过调控三相流量,可产生双重包裹液滴。结合实验结果,分析了流体粘度比对液滴大小的影响,并得出液滴的尺寸与流量比之间的关系式,为制备不同尺寸的液滴提供了参考依据。对制备的样品进行统计分析,结果显示,液滴的多分散性指数均小于3.2%,表明微液滴的高度均匀性。此外,通过调节三相液体的流量不仅可以控制内外层液滴的大小,还可以调节内层包裹液滴的个数。本文提出的制备方法,设备组装拆卸简便,不需表面亲疏水性处理,装置利用率高,产生的单、双重包裹微液滴可满足高通量的测量分析要求。     

微/纳流控系统电渗流研究进展

简要介绍了微/纳流控系统中双电层和电渗流的基本原理、当前研究热点以及最新进展.从胶体界面理论出发,基于流体连续性模型和微纳米尺度流动的多物理场数值分析法,介绍了电渗流特性, 如焦耳热效应、 反离子效应、 表面电场调控双电层、双流体输运和周期电渗输运等电渗流控制方法.最后对电渗流测量作简要介绍.      

纳米流体液滴撞击壁面铺展动力学特性研究

纳米流体液滴撞击固体壁面的铺展动力学特性是基于液滴沉积实现高效传热传质过程的关键因素,然而由于纳米流体的非牛顿流变特性及液滴内微流动与纳米颗粒的耦合作用,目前对纳米流体液滴撞击固体壁面的铺展动力学行为缺乏足够的认识.本研究利用了两步法分别配制了分散有3种纳米颗粒的均匀稳定纳米流体(碳纳米管、石墨烯、纳米石墨粉),并对流体的流变特性进行了测量分析.利用显微高速数码摄像技术捕捉了液滴撞击固体壁面的动态过程,通过图像处理技术分析铺展过程中液滴的无量纲高度、铺展因子及动态接触角,探究了液滴在韦伯数约为200及800时撞击壁面后铺展沉积形态的演变规律.研究表明,3种不同纳米颗粒的加入均使基液表现出明显的剪切变稀特性,在液滴撞击壁面的铺展过程中,流体的剪切黏度起重要作用,液滴的无量纲高度和铺展因子的变化幅度随着纳米流体剪切黏度的增大而减小.纳米流体液滴撞击疏水表面时能更快的达到平衡状态,液滴的惯性力主导着液滴的初始铺展阶段,液滴的铺展范围和速度随撞击速度的增大而增大.开展该研究能够为基于液滴沉积的增益冷却技术以及微型高导热及导电材料的制造提供理论依据和技术指导.      

微纳米尺度效应作用下充流微通道系统波动特性研究

结合非局部弹性应力/应变梯度耦合本构关系和流体非局部应力关系式,基于Euler梁理论,建立了充流微通道流固耦合波传导模型;根据耦合固体非局部应力/应变梯度弹性效应以及流体非局部效应,分别模拟了微通道和管腔内流体的尺度效应,推导得出了充流微通道在微纳米尺度的波动控制方程和边界条件。通过对控制方程的求解,分析了不同类型尺度效应对微通道的波动和振动特性的影响。结果显示,各类尺度效应对系统的动力学特性影响不同。微通道非局部弹性效应对波动产生阻尼,特别是对波长较短的波传导;而应变梯度弹性效应对波传导有促进作用,且该效应对波动的影响与波长无关;非局部效应和应变梯度效应对微通道刚度产生不同影响,非局部效应降低刚度,应变梯度效应增加刚度。     

表面应力调制的微流控技术的驱动原理

发展和优化对薄膜、液滴和气泡进行流动控制操作的多功能装置, 要求深入了解界面现象和微流体动力学流动.表面积/ 体积的大比值和低雷诺数流动是此类系统的特点.毛细数和Bond数强烈地受边界效应影响, 因而可以通过各种表面处理和表面力 来进行控制.本文综述了运用调制法向或切向应力, 对均匀的、带化学处理条纹及拓扑结构纹理表面上的微滴和液膜进行驱动 的常用技术的基本原理.     

胶体微泵的耗散粒子动力学模拟

应用耗散粒子动力学方法研究了胶体微泵.每个胶体小球按照既定的运动规律相继运动,从而可驱动流体.首先利用耗散粒子动力学方法计算了泊肃叶流动,验证了模拟的正确性.然后模拟了由六个胶体小球组成的周期性胶体微泵的工作过程.胶体颗粒与周围流体粒子之间采用了弹性碰撞模型;模拟中选择了合适的参数,从而可提高流体的粘度并保证DPD流体的不可压缩性.模拟结果与他人的实验数据进行了对比,两者很好吻合.模拟结果显示,胶体微泵的无量纲流量的绝对值随着小球运动ω的变小而增大;而随着ω的减小,无量纲流量的振幅也相应变大.     

Pumping of a fluid through a microchannel by means of a bubble driven by a light beam

A new method of pumping a fluid through a microchannel device using a gas bubble-piston, set in motion by the thermocapillary force induced by a light beam, is proposed. To demonstrate the method, a model micropump has been assembled. The model consists of two reservoirs connected by two channels with a bubble-piston driven by a light beam. The pumping rate and the volume per piston stroke are evaluated experimentally. The method proposed is compared with known microfluid pumping methods. Some advantages of the new method are indicated.     

Surface wettability effect on flow pattern and pressure drop in adiabatic two-phase flows in rectangular microchannels with T-junction mixer

Wettability is an important parameter in micro-scale flow patterns. Previous research has usually been conducted in conventional microtubes due to limitations of visualizing flow patterns and fabricating microchannels. However, most microchannels in practical applications have rectangular shape. Furthermore, pressure drop is closely related with flow pattern. Hence, we studied water liquid and nitrogen gas flows in rectangular microchannels with different wettabilities. The rectangular glass microchannels were fabricated from photosensitive glass, whose surface is hydrophilic. The surface of one was silanized using octadecyl-trichloro-silane (OTS) to prepare a hydrophobic microchannel. The two-phase flow pattern was visualized with a high-speed camera and a long distance microscope. The frictional pressure drop in the microchannel was measured directly with embedded pressure ports. The flow pattern and pressure drop in the hydrophobic microchannel were totally different from those in the hydrophilic microchannel. Finally, the two-phase frictional pressure drop was analyzed based on the flow patterns of different wettabilities.     

Velocity field measurements of cavitating flows

A particle Image Velocimetry (PIV) system has been developed to study the microfluid mechanics of cavitating flows. Planar PIV was used to examine the non-cavitating flow in the thin boundary layer near a hydrofoil surface for the cases of a naturally developing boundary layer and a boundary layer stimulated to turbulence by roughness near the foil leading edge. PIV was also used to examine the flow near the surface of individual cavitation bubbles and incipient attached cavitation. A system was devised to create a single nucleus in the flow upstream of a hydrofoil, and planar PIV was used to study the flow around the resulting traveling cavitation bubble. Velocity vectors were determined close to the solid surfaces and the gas/liquid interfaces of the bubbles. Seeding of the flow with particles did not result in the addition of active cavitation nuclei.     

Viscous oil–water flows in a microchannel initially saturated with oil: Flow patterns and pressure drop characteristics

Immiscible viscous liquid–liquid two-phase flow patterns and pressure drop characteristics in a circular microchannel have been investigated. Water and silicone oil with a dynamic viscosity of 863 mPa s were injected into a fused silica microchannel with an inner diameter of 250 μm. As the microchannel was initially filled with the silicone oil, an oil film was found to always form and remain on the microchannel wall. Different flow patterns were observed and classified over a wide range of water and oil flow rates. A flow pattern map is presented in terms of Re, Ca, and We numbers. Two-phase pressure drop data have also been collected and analyzed to develop a simple correlation for slug, annular and annular-droplet flow patterns in terms of superficial water and oil velocities.     

Bubble dynamics in microchannel: An overview of the state-of-the-art

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

     

3-D numerical simulation of contact angle hysteresis for microscale two phase flow

Understanding the physics of microscale two-phase flow is important for a broad variety of engineering applications including compact PEM fuel cells and heat exchangers. The low Bond number and confined geometry make it critical to consider both the surface tension at the liquid–gas interfaces and the surface forces acting at the channel boundaries. Within the framework of a numerical volume of fluid (VOF) approach, the present work proposes a model to account for surface adhesion forces by considering the effects of contact angle hysteresis. A transient model is developed by correcting boundary force balances through specification of the local contact angle and instantaneously updating the local angle values based on the variation of the volume fraction from previous time steps. The model compares very well with new data provided here for droplets on a rotating disk and liquid slug flow in microchannel. The simulation reveals that the contact angle distribution along the slug profile in the microchannel flow can be approximated using a piecewise linear function. This study indicates that the asymmetric distribution of the contact angle might be responsible for several phenomena observed in the microchannel experiments, including slug instability.     

Dynamics of a finite-width liquid film in a co-current microchannel gas flow

The velocity fields and the parameters of a finite-width liquid film moving along the bottom of a mini- and a microchannel under the action of a gas flow are calculated. The investigations are performed for different levels of gravity. It is found that the thin liquid film distorts the velocity field in the gaseous phase. In contrast to the minichannel flow, in the microchannel the film surface is not leveled with increase in the gravity force.     

Numerical simulations of droplet formation in a cross‐junction microchannel by the lattice Boltzmann method

An immiscible liquid–liquid multiphase flow in a cross‐junction microchannel was numerically studied using the lattice Boltzmann method. An improved, immiscible lattice BGK model was proposed by introducing surface tension force based on the continuum surface force (CSF) method. Recoloring step was replaced by the anti‐diffusion scheme in the mixed region to reduce the side‐effect and control the thickness of the interface. The present method was tested by the simulation of a static bubble. Laplace's law and spurious velocities were examined. The results show that our model is more advantageous for simulations of immiscible fluids than the existing immiscible lattice BGK models. Computational results of multiphase flow in a cross‐junction microchannel were obtained and analyzed based on dimensionless numbers. It is found that the flow pattern is decided mostly by the capillary number at a small inlet flux. However, at the same capillary number, a large inlet flux will lead to much smaller droplet generation. For this case, the flow is determined by both the capillary number and the Weber number. Copyright © 2007 John Wiley & Sons, Ltd.     

基于MEMS技术的微型流量传感器的研究进展

流量测量是工业生产和科研工作的重要的检测参数.近年来,随着对微电子机械系统(MEMS)的深入研究和取得的进展,传统的工业和流体力学研究的流量传感器向高集成度,微型化,高精度,高可靠性方向发展,同时生命科学的发展大大促进了用于微流体,生物学、医学、卫生、食物等学科研究新型微型流量传感器的研究开发, 微型流量传感器已成为MEMS的重要研究方向.本文对基于MEMS技术的流量传感器技术的原理、分类作了简要介绍,归纳和评述了各种基于MEMS技术的流量传感器(热式型,差压型,升力型,流体振动型, 科里奥利型及仿生型微型流量传感器等)的生产工艺和应用特点,并对基于MEMS技术的微型流量传感器的校正方法做了总结归纳.介绍了国内在微型流量传感器方面的研制工作.最后总结归纳出基于MEMS技术的流量传感器发展不同阶段并阐述了各个阶段的发展特点,并对基于MEMS技术的流量传感器新的发展趋势进行了展望.      

Investigation of saturated critical heat flux in a single, uniformly heated microchannel

A series of tests have been performed to determine the saturated critical heat flux (CHF) in 0.5 and 0.8 mm internal diameter microchannel tubes as a function of refrigerant mass velocity, heated length, saturation temperature and inlet liquid subcooling. The tested refrigerants were R-134a and R-245fa and the heated length of microchannel was varied between 20 and 70 mm. The results show a strong dependence of CHF on mass velocity, heated length and microchannel diameter but no influence of liquid subcooling (2–15 °C) was observed. The experimental results have been compared to the well-known CHF single-channel correlation of Y. Katto and H. Ohno [An improved version of the generalized correlation of critical heat flux for the forced convective boiling in uniformly heated vertical tubes, Int. J. Heat and Mass Transfer 27 (9) (1984) 1641–1648] and the multichannel correlation of W. Qu and I. Mudawar [Measurement and correlation of critical heat flux in two-phase microchannel heat sinks, Int. J. Heat and Mass Transfer 47 (2004) 2045–2059]. The comparison shows that the correlation of Katto–Ohno predicts microchannel data with a mean absolute error of 32.8% with only 41.2% of the data falling within a ±15% error band. The correlation of Qu and Mudawar shows the same trends as the CHF data but significantly overpredicts them. Based on the present experimental data, a new microscale version of the Katto–Ohno correlation for the prediction of CHF during saturated boiling in microchannels has been proposed.