超疏水表面微通道内水的流动特性

在铝制微通道内壁上制造出超疏水表面，水滴在其表面上的接触角达到153°。对水在内径同为0.60 mm的超疏水微通道和超亲水微通道中流动的压降进行实验测定与对比，得出水在超疏水微通道内的流动阻力降有明显降低，降低的最大值可达25%。研究了水在超疏水微通道内的流动特性，发现水由层流向湍流转变发生在Reynolds数为2500左右，且在层流范围内fRe值基本保持不变。通过计算得出了不同流量下水在超疏水表面微通道壁面处的滑移速度和滑移长度，结果显示滑移速度和滑移长度均随流量的增大而增大。     

超疏水表面微通道内水的传热特性

微通道传热效率高但流动阻力大,超疏水表面因其与水具有滑移边界而表现出低流阻的特征,在微过程中具有应用前景。利用化学刻蚀法制备出具有微纳米阶层结构的铝基超疏水表面微通道(内径为0.68mm)。在超疏/亲水微通道内进行了水的流动传热实验研究,并将结果进行对比。研究发现存在于超疏水表面微纳米结构里的气泡层减小了水的流动阻力,也降低了表面传热系数,但降低程度明显小于流动阻力的降低,传热系数高于考虑纳米气泡层计算的传热系数。因此认为在水的滑移速度作用下,凹穴中微纳米级气泡内产生了气体的涡旋流动,一定程度上增强了传热效果。     

入口角度及壁面性质对蛇形微通道内弹状流流动特性影响的格子Boltzmann模拟

在Y型汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同壁面性质的蛇形微通道内弹状流流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。通过验证实验后,模拟计算了气相速度,Y型夹角和壁面性质对气泡长度的影响,以及Y型夹角对微通道内弹状流压降和流动阻力的影响;探讨了粗糙度与壁面润湿性对流动阻力的影响;同时,针对蛇形微通道弯管部分,分析了角度和壁面性质对弹状流流动的影响。通过计算,发现当壁面接触角及Y型夹角为90?时,气泡长度最大;当直微通道为亲水性光滑壁面,回转弯道为粗糙度较大的疏水壁面时,Po数较小。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

硅基正弦波纹微通道内的流动阻力特性

硅基正弦波纹微通道在微尺度强化换热、微流体器件中均有重要运用,而目前尚未见有关于其内部流动特性的实验报道。利用标准微机电系统（MEMS）工艺在硅基芯片上加工制成一系列具有不同相位差和周期、当量直径为160 μm的正弦波纹微通道,通过实验研究了其内部流动阻力特性。研究表明：正弦波纹微通道较平直微通道阻力均有增加,且增加幅度与波纹微通道两侧壁的相位差和周期有关。对于周期相同的波纹微通道,两侧壁相位差越大,阻力也越大。对于相位差相同的波纹微通道,周期对阻力的影响则较为复杂：当相位差为0时,呈现出周期越大、阻力越小的趋势（除最小周期通道外）;当相位差为π时,呈现出周期越大、阻力越大的趋势。研究还发现：随着周期减小,相位差对阻力的影响减小,当周期缩短至0.5 mm时,相位差为0和π的波纹微通道内的阻力特性曲线几乎重合。     

液滴滴浸微通道入口段的动力学特性分析

采用流体体积(Volume of Fluid, VOF)函数捕捉气液相界面,研究液滴滴浸微通道入口段的运动,通过改变微通道入口段的截面宽度、润湿特性及液滴雷诺数(Re)和韦伯数(We)研究滴浸过程的动力学特性。结果表明,微通道入口段的截面宽度对液滴浸入微通道时的撞击过程影响最明显,随截面宽度减小,液滴撞击通道入口后通过微通道的难度增加,整个过程液滴所受阻力逐渐增大;当微通道截面宽度减至0.2 mm时,壁面润湿性效应凸显,表现为壁面静态接触角越大,液滴滴浸微通道时所受的阻力也越大。表面接触角较大时,为使液体通过微通道入口段,可适当增大液滴的Re,液体在通道内的浸润长度随Re增加成比例增大,当Re增至4000时,通道内开始出现射流现象。We减小,表面张力效应变得明显,通道内的流动阻力变大,液体流过微通道入口段的难度增大。     

壁面润湿性对蛇形微通道内两相流流动特性影响的格子Boltzmann模拟

在90°Y形汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同接触角的蛇形微通道内气液两相流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。验证模型合理性后,根据模拟计算结果,以气液相流速为坐标绘制了不同接触角下的流型图并分析其差异性及原因;同时深入研究了液相黏度和接触角对于弹状流流体力学性质的综合影响;比较了具有不同接触角壁面的蛇形微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力的分布规律,并讨论了蛇形微通道内气液两相流动的影响因素。研究表明疏水壁面即接触角大于90°时,微通道内两相流压降、摩擦因子、壁面摩擦系数和剪切应力均低于亲水壁面微通道内相关参数,更利于流体流动。     

基于抛物线形气-液界面的超疏水微通道减阻特性

针对超疏水表面微通道中的流动减阻特性,基于抛物线形气-液界面假设,采用VOF模型模拟了微通道中的二维层流流动,分析了流动和结构参数对减阻效果的影响。结果表明,含矩形微坑的超疏水表面微通道具有显著减阻作用,fRe随Reynolds数增大而略有提高,量纲1压降比随入口速度增大而略有下降。当增大微坑面积比或减小微通道高度时,fRe减小,量纲1压降比增大;且微通道高度越小,微坑面积比对fRe的影响越显著。随抛物线形高度增加,压降比和滑移长度均线性减小,而fRe则线性增加。当微坑深度大于其宽度的40%时,压降比和滑移长度趋于定值。微坑形状对减阻效果的影响依次是燕尾形、矩形、梯形和三角形。     

粗糙表面微通道电渗流的数值模拟

电渗流（EOF）广泛应用于微流控芯片中的流体的传输与混合。针对带有粗糙表面的平行板微通道,建立了描述EOF的控制方程,基于有限元分析方法对具有不同粗糙度和EDL厚度的微通道内的EOF进行了数值模拟。结果表明,当EDL厚度接近0.3倍粗糙度大小时,粗糙度对EDL的影响较大,EOF受到粗糙度的阻力作用较为明显,而当EDL厚度相对粗糙度较大和较小时,EOF速度受到粗糙度的影响相对较小;粗糙表面微通道中部EOF速度与相对EDL厚度的关系呈“V”型曲线,EOF平均速度呈“L”型曲线。研究结论对于微通道表面的优化设计以及微流控芯片中流体的精确操控具有一定的参考意义。     

Gas-liquid two-phase flow in serpentine microchannel with different wall wettability

Gas-liquid flow in serpentine microchannel with different surface properties exhibits drastically different flow behavior.With water and air as working fluids,the method of numerical simulation was adopted in this paper based on CLSVOF (coupled level set and volume of fluid method) multiphase model.After verifing the reasonability of the model through experiment,by changing wall properties and Re number (Re ＜ 1500),the influences of contact angle and surface roughness on flow regime and Po number were discussed.Moreover,the difference of pressure drop between curve and straight microchannel was also calculated.Beyond that,the combined effect of curve channel and wall properties on flow resistance was analyzed.This paper finds that wall properties have great influence on gasliquid flow in microchannels not only on flow regime but also flow characteristics.Meanwhile,the pressure drop in curve microchannels is larger than straight.It is more beneficial for fluid flowing when the straight part of microchannel is hydrophilic smooth wall and curve part is hydrophobic with large roughness.     

Capillary Filling Flows inside Patterned‐Surface Microchannels

Capillary flows inside microchannels with patterned‐surfaces are investigated theoretically and numerically. The surface energy method is used to derive an equivalent contact angle (ECA) model for small capillary number flows. The SIMPLE algorithm using a volume of fluid (VOF) method is adopted to investigate the flows in those microchannels. The flow characteristics such as the liquid front shapes and the evolution of the liquid lengths are obtained. The numerical results reveal that capillary flows in a patterned‐surface microchannel still follow the traditional capillary theories. The ECA model is confirmed by the numerical results. It indicates that the capillary flows inside the patterned‐surface microchannels can be estimated by means of the homogeneous‐surface microchannels with the equivalent contact angle. The ECA model provides a good criterion for the total wettability of a patterned‐surface microchannel, as well.     

Cellular Behavior of Human Adipose-Derived Stem Cells on Wettable Gradient Polyethylene Surfaces

Appropriate surface wettability and roughness of biomaterials is an important factor in cell attachment and proliferation. In this study, we investigated the correlation between surface wettability and roughness, and biological response in human adipose-derived stem cells (hADSCs). We prepared wettable and rough gradient polyethylene (PE) surfaces by increasing the power of a radio frequency corona discharge apparatus with knife-type electrodes over a moving sample bed. The PE changed gradually from hydrophobic and smooth surfaces to hydrophilic (water contact angle, 90º to ~50º) and rough (80 to ~120 nm) surfaces as the power increased. We found that hADSCs adhered better to highly hydrophilic and rough surfaces and showed broadly stretched morphology compared with that on hydrophobic and smooth surfaces. The proliferation of hADSCs on hydrophilic and rough surfaces was also higher than that on hydrophobic and smooth surfaces. Furthermore, integrin beta 1 gene expression, an indicator of attachment, and heat shock protein 70 gene expression were high on hydrophobic and smooth surfaces. These results indicate that the cellular behavior of hADSCs on gradient surface depends on surface properties, wettability and roughness.     

Layered liquid-liquid flow in microchannels having selectively modified hydrophilic and hydrophobic walls

When two immiscible liquids make contact in a microchannel, the flow pattern is affected by the affinity between channel walls and liquids. In this study, microchannels (200 μm in width and 200 μm in depth) having a T-shaped bifurcation point were fabricated on PMMA plates. The inner walls of the microchannels were modified in a zone-selective manner to be either hydrophilic or hydrophobic, based on verification accomplished via a laser interference fringe technique. The microchannel was placed horizontally, and water and octane were introduced into the upper-side channel (hydrophilic) and into the lower-side channel (hydrophobic), respectively. The experimental results showed that water and octane formed a stable layered flow, and the two liquids were virtually completely separated at the T-shaped section, even when static pressure was intentionally applied to the outlets. CFD simulation, using FLUENT 6.3 software, was performed to explain the role of zone-selective modification of microchannel walls.     

Effects of roughness elements on laminar flow and heat transfer in microchannels

A model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively.     

粗糙疏水表面冷凝形核特性研究

为了研究疏水基底粗糙度对形核特性的影响规律,采用腐蚀及修饰的方法得到具有不同粗糙度的疏水基底,通过对基底表面粗糙度因子的计算和表观润湿角的测量,考察了基底粗糙度对基底表面水的表观润湿角的定量关系;在制备的粗糙基底上进行了冷凝蒸汽形核实验,利用统计方法得到基底粗糙度因子与冷凝液滴数量的关系。结果表明:基底微观形貌对水在基底表面的表观润湿性和形核特性具有显著影响,对于疏水基底,随着基底粗糙度的增加,水滴在其表面的表观润湿角增大;相同的基底过冷度下,越粗糙的基底表面蒸汽冷凝形核点越少。分析认为,基底微观形貌通过影响液胚在其表面的表观润湿角,进而改变异质形核功,造成了粗糙基底表面形核特性的改变。实验现象与基于Wenzel模型的粗糙基底异质形核理论取得了一致。     

壁面润湿性对微细通道内R141b流动沸腾不稳定性的影响

为了探究壁面润湿性对制冷剂R141b流动沸腾不稳定性的影响，设计微细通道流动沸腾实验平台，制备3种不同润湿性的矩形微细通道，其壁面接触角分别为62.3°、接近0°和158.7°。以R141b为实验工质，在截面宽×高为1mm×2mm的矩形微细通道内进行流动沸腾换热实验，研究了沿程测点压力波动情况以及影响进出口总压降波动的因素，最后对总压降波动信号进行Hurst指数分析，结果表明：微细通道沿程测点波动方差最大的位置正处于沸腾起始点（ONB）附近，热流密度的减小以及质量通量的增大均会使沸腾起始点推后；进出口总压降波动受热流密度、质量通量和壁面润湿性的影响，相同工况下，热流密度增大和质量通量的减小都会引起系统不稳定性增强，超疏水表面微细通道的总压降波动方差均比其他两种表面的大，是波动方差最小的超亲水表面的1.35～1.84倍；利用Hurst指数分析，表明系统具有混沌现象，超疏水表面微细通道的Hurst指数最大，表现出更强烈的不稳定性。