Behavioral patterns of drop impingement onto rigid substrates with a wide range of wettability and different surface temperatures

This article concerns behavioral patterns of droplet impingement onto solid substrates covering a wide range of wettability from hydrophilic to superhydrophobic surfaces heated at different temperatures. For droplet impingement onto partial hydrophobic surfaces (mirror‐polished Cu substrate), the maximum heights of receding droplet undergoing a consecutive increment with surface temperature can be explained taking account of Marangoni flow. Also, the relation to predict the increment of droplet heights with surface temperature was manifested in the light of lubrication approximation combined with energy conservation. However, this relation is only valid for droplet impacts onto partial hydrophobic surface, because the recoiling droplet height was observed to be independent of surface temperature for both hydrophilic and superhydrophobic targets. This phenomenon was attributed to inherent wettability accompanying larger contact angle hysteresis for the hydrophilic substrate and to the presence of an adiabatic gas layer between the composite surface and impacting droplet, for the superhydrophobic target. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

质子交换膜燃料电池内扩散层/流道表面处液态水滴的运动特征

针对燃料电池流场内任意选取的特征单元建立二维模型,分析在剪切流作用下附着在扩散层（GDL）/流道（GFC）表面液滴的运动特征,认为液滴运动时迟滞角（液滴运动过程中前进角与后退角间的差值）与液滴高度、气体流速存在着一定关系;而与所选取的特征单元长度无关。模型还有效耦合了实验总结的液滴开始运动的条件关系式,真实反映了液滴开始运动、不断加速追赶及聚并的运动趋势。结果表明：选取矮流道,增大气体流速,采用疏水的GDL增大液滴的不稳定性,同时增加流道表面的亲水性,有利于液滴获得较大加速度,并以拐角流的形式快速从电池内排除。     

梯度表面能材料上液滴运动机理

通过显微测量技术获得了梯度表面能材料表面的微观结构,分析了经扩散控制的硅烷化处理的硅基材料表面上表面能梯度形成的机制和影响因素。通过可视化实验,并从能量转换关系和液滴受力分析上探讨了液滴在梯度表面能材料上的快速运动机理,研究了液滴运动过程,及薄膜润滑对液滴运动速度的影响。     

倾斜微结构疏水表面液滴的滞后特性

倾斜微结构疏水表面液滴的滞后特性包括接触角滞后和滚动角。目前,具有较高精度的微结构疏水表面滚动角模型是以理想液滴形状为计算基础,忽略了重力、接触角滞后以及能垒引起的变形。本文以聚二甲基硅氧烷（PDMS）为基底,制备了方柱状微结构疏水表面,考虑疏水表面微观结构以及液滴大小两方面的因素,研究了倾斜微结构疏水表面液滴的滞后特性。从力和能量的角度对其影响机理进行了分析,通过滚动角理论值与实际值的比较发现,微方柱间距较大时,接触角滞后和能垒对滚动角影响显著,证实了该分析的合理性,为研究更加精确的滚动角模型奠定了理论基础。     

液滴撞击亲疏水间隔条纹表面

在高温高压环境下,将线切割成特殊形状的铝片镶嵌在树脂表面,再通过CuCl₂溶液对铝表面腐蚀,利用硬脂酸无水乙醇溶液的恒温水浴,进行表面修饰,适当处理得到亲水区域与疏水区域共存的复合表面。采用高速相机记录了液滴撞击水平与倾斜表面的动力过程,实验结果表明：液滴撞击水平放置的亲疏水复合表面时,其铺展过程与液滴撞击单一润湿性表面相似,但收缩行为差异化明显。液滴在亲疏水间隔表面收缩时,液膜会因亲疏水性并存而产生高度差。液滴撞击倾斜亲疏水复合表面时,液滴撞击的速度越大,其在斜面进行的位移量越大,速度过大的情况下液滴不会被宽度为20mm的斜面捕获;当速度为定值,Oh数越小,液滴跨越的亲疏水段越多,Oh数过小的情况下液滴不会被斜面截留;其他参数不变,比值系数H越小,液滴更易被斜面捕获。     

Driving liquid droplets on microstructured gradient surface by mechanical vibration

Dynamic transportation of water droplet on microstructured hydrophobic silicon substrate with a contact angle gradient is studied in this article. We propose a new type of substrate designed with microridges on a silicon wafer fabricated by photolithograph and subsequent coating with octadecyltrichlorosilane (OTS). When horizontal vibration is applied on the substrate, the water droplet can move to the direction with larger solid–liquid contact area fraction. It is found that the dynamic contact angle of the water droplet varied with the vibration direction and the speed of the substrate. The contact angle difference at the left and the right edge of the water droplet on the vibrated surface is obviously magnified compared to the contact angle difference of the droplet on the static surfaces, resulting in the increasing driving force. When the vibration amplitude of the exciter source (20 Hz) increases from 0.14 to 0.43 mm, the average velocity of 10 μL water droplet increases from 10 to 23 mm/s. The internal flow pattern of the water droplet moving on the microstructured hydrophobic surfaces is also obtained using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques. Both rolling and slipping motions are observed for the water droplet during its movement in the vibrated substrate.     

恒热流条件下亲疏水表面液滴蒸发特性

以恒热流方式结合高速摄影技术同步观察记录3 μl的小液滴在不同亲疏水表面的蒸发过程。通过一系列的对比实验观察记录了不同亲疏水表面液滴蒸发时接触角、接触直径、蒸发时间等的动态特性。从实验分析中可以看出亲水表面液滴蒸发速率比疏水表面上液滴蒸发速率快,并且随着热通量的增大,液滴蒸发速率增大。在恒热流条件下亲疏水表面液滴蒸发以CCR模式为主,在蒸发后期呈现混合蒸发模式,全程未出现CCA模式。     

表面特性对结霜和融霜排液的影响

对亲水表面、裸铝表面和疏水表面上结霜和融霜排液过程进行实验研究,分析了表面特性对冷凝水珠冻结、霜层生长和融霜排液的影响。结果表明：疏水表面上冷凝水珠呈规则球缺状、冻结较晚,而亲水表面和裸铝表面上冷凝水珠形状不规则;相比于亲水表面和裸铝表面上平整霜层,疏水表面上霜层不平整,有凹穴和凸起;疏水表面上霜层平均厚度增长较亲水表面和裸铝表面缓慢;在湿空气温度和冷面温度较低的情况下,表面特性对霜层生长的影响减弱;亲水表面具有较好的排液效果,其循环再结霜量最小。     

Maximum spread of droplets on chemically striped surfaces

Chemically striped surfaces have a strong ability to control the shape of droplets and even to split a droplet, which can be applied in microfluidic, printing fields and spray cooling fields. The droplet splitting behavior is affected by the number of hydrophobic stripes spanned by the droplet at its maximum spread moment, which is determined by the maximum spreading factor. In this study, an area ratio is adopted to analyze a droplet's spread on the hydrophilic and hydrophobic regions at maximum spread moment. Coupled with an energy conservation equation, an analytical model is presented to predict the maximum spread factor of a droplet on a chemically striped surface.     

组合表面调控液滴特性强化蒸汽冷凝传热

制备了具有不同疏水区宽度和面积分率的疏水-亲水间隔规则排列的组合表面。观测常压蒸汽在组合表面上冷凝时疏水区液滴的特性(液滴移除方式和最大液滴半径),利用格子Boltzmann方法模拟组合表面上凝液的运动。考察疏水区、亲水区宽度和表面过冷度对组合表面强化蒸汽冷凝传热的影响。利用滴状-膜状组合传热模型分析组合表面蒸汽冷凝传热性能的影响因素,并与实验结果比较。发现疏水区液滴自发地向亲水区定向迁移,精细设计的组合表面可以实现蒸汽滴状冷凝传热的强化,实验中强化因子可达1.20。疏水区宽度约为0.55 mm时组合表面的传热性能最大。表面过冷度越大,组合表面强化传热的效果越差,模型分析与实验结果吻合良好。     

太阳能加热液滴在亲疏水表面"黏-滑"蒸发

实验研究了亲水和疏水表面上太阳能加热去离子水及金纳米流体液滴三相接触线动力学。在亲水和疏水表面滴加2μL去离子水和纳米流体液滴,用一定功率太阳能模拟器照射液滴使其蒸发,期间采用高速摄像机实时记录液滴在不同表面上的蒸发过程。由MATLAB程序处理图像得到液滴在不同表面上蒸发过程中接触角和接触圆直径的动态变化过程。发现液滴接触线在不同亲疏水表面上存在不同运动特性。去离子水液滴在亲水表面上常接触面积模式和常接触角模式依次控制蒸发过程。去离子水液滴在疏水表面上都呈现出“黏-滑”蒸发特性,即液滴先以常接触面积模式蒸发,之后接触线快速滑动,接触线固定后再以常接触面积模式蒸发,依次往复。纳米流体液滴在亲水表面上主要以常接触面积蒸发模式为主,在疏水表面上同样呈现“黏-滑”蒸发特性。从液滴表面能角度出发,对液滴接触线“钉扎”和“去钉扎”过程进行详尽分析,得出基底润湿性和纳米颗粒沉积是影响液滴接触线在表面上运动的重要因素。     

Effect of Polymer Dispersant Structure on Electrosteric Interaction and Dense Alumina Suspension Behavior

The present paper reports on research on the effect of molecular structure of polymer dispersants on the relationship between the electrosteric interaction of dispersants on solid surfaces and the viscosity of suspensions. Ammonium polyacrylate with different hydrophilic to hydrophobic ratios ( m:n ) was prepared and added to dense Al₂O₃ suspensions (40 vol%). The steric interactions and adsorbed structures of dispersants on Al₂O₃ powders were examined under an atomic force microscope (AFM). An optimum hydrophilic to hydrophobic group ratio, which was obtained from the maximum repulsive force and the minimum viscosity of suspension, was determined at m :n = 3:7. The changing mechanism of the adsorbed structure and the steric interaction of dispersants and the suspension viscosity by the hydrophilic to hydrophobic molecular ratio were discussed by comparing the experimental force curve and DLVO theory.     

Tracking the hydrophobicity recovery of PDMS compounds using the adhesive force determined by AFM force distance measurements

Polydimethylsiloxane (PDMS) materials show the unique phenomenon that when exposed to electrical discharge, such as corona discharge, their hydrophobic surface becomes hydrophilic. However, after a certain relaxation time they gradually regain their hydrophobicity. In this study the adhesive force obtained by AFM force distance measurements using a hydrophilic Si₃N₄ probe is used to track the recovery of the hydrophobicity. The time constant of the recovery can be determined by measuring the adhesive force as a function of the recovery time after corona exposure. It is shown how these time constants can be used to monitor the recovery rate as a function of corona treatment time for both filled and unfilled PDMS compounds.     

Rheology and morphology of nanosilica‐containing polypropylene and polypropylene/liquid crystalline polymer blend

We studied the rheology and morphology of hydrophilic and hydrophobic silica containing polypropylene (PP) and PP/liquid crystalline polymer (LCP) blend. It was found that hydrophilic silica has higher tendency for aggregation and forms bigger aggregate size. The lower percolation threshold and the higher fractal dimension for hydrophilic silica are due to its stronger particle–particle interaction compared with the hydrophobic one. Although the hydrophobic silica has lower thickening capability and lower coalescence hindrance by aggregates, its presence at the interface of PP/LCP blend results in smaller droplet size and higher elasticity of hybrid samples in comparison to hydrophilic silica. These results confirm that the hydrophobic silica has a compatibilization capability for PP/LCP blend, whereas the hydrophilic silica mostly works as a thickening agent and suppresses the coalescence. We suggested that for comparison of different particulate compatibilizers, the elasticity of filled blend sample against filled matrix phase can be used in high and low frequency ranges. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A selective drug-release system consisting of surface-modified electrospun carbon fibers by oxy/fluorination

A selective drug-release system was prepared for a controlled drug release between hydrophobic and hydrophilic drugs by using a prepared membrane. A membrane was constructed by electrospun carbon fibers. A porous structure was created in the carbon fibers by chemical activation to reduce the initial drug-burst phenomenon by drug storage in the pores. The surface of the activated carbon fibers was modified by the addition of hydrophobic/hydrophilic functional groups by oxyfluorination or fluorination treatments to allow the selective release of mixed hydrophobic and hydrophilic drugs. The in vitro drug permeation was studied under various applied electric voltages. The initial drug-release phenomenon was reduced due to the storage effect by the improved pore structure, and the drug-release rate was controlled by the intensity of the applied electric voltage. In addition, selective drug release was observed with the presence of the hydrophobic or hydrophilic functional groups introduced through oxyfluorination or fluorination treatments.     

Using Knock-Out Mutants to Investigate the Adhesion of Staphylococcus aureus to Abiotic Surfaces

The adhesion of Staphylococcus aureus to abiotic surfaces is crucial for establishing device-related infections. With a high number of single-cell force spectroscopy measurements with genetically modified S. aureus cells, this study provides insights into the adhesion process of the pathogen to abiotic surfaces of different wettability. Our results show that S. aureus utilizes different cell wall molecules and interaction mechanisms when binding to hydrophobic and hydrophilic surfaces. We found that covalently bound cell wall proteins strongly interact with hydrophobic substrates, while their contribution to the overall adhesion force is smaller on hydrophilic substrates. Teichoic acids promote adhesion to hydrophobic surfaces as well as to hydrophilic surfaces. This, however, is to a lesser extent. An interplay of electrostatic effects of charges and protein composition on bacterial surfaces is predominant on hydrophilic surfaces, while it is overshadowed on hydrophobic surfaces by the influence of the high number of binding proteins. Our results can help to design new models of bacterial adhesion and may be used to interpret the adhesion of other microorganisms with similar surface properties.     

PEMFC中粗糙GDL表面对液态水在GC中传输过程的影响

采用商业软件FLUENT中VOF模型模拟了质子交换膜燃料电池（PEMFC）中液态水在具有粗糙气体扩散层（GDL）表面的气体通道（GC）中的传递过程。考察了GDL表面润湿特性和粗糙度对液态水传输过程的影响。研究结果表明：和亲水GDL表面相比,疏水GDL表面有利于液态水的排出;和光滑疏水GDL表面相比,粗糙疏水GDL表面加快了液滴的排出,减小了液滴覆盖GDL表面的面积;同时,粗糙GDL表面增加了GC相似文献   

不同浸润性冷表面上水滴碰撞结冰的数值模拟

对冷水滴撞击不同表面时的动力学行为和相变过程进行了模拟。通过耦合VOF和Level-set方法追踪气液自由界面,结合焓-孔隙度相变模型,模拟水滴撞击冷表面的动力学行为及相变特征。选取亲水（接触角30°）、疏水（接触角114°）和超疏水（接触角163°）3种典型浸润性的表面,计算了多种壁温条件下的水滴撞击结冰过程。结果表明提高表面疏水性,将减小水滴与冷表面的接触时间和接触面积,降低水滴内的相变速率,延缓水滴结冰的时间。在表面温度高于-15℃时,超疏水表面可以避免冷水滴的冻结黏附,保持表面洁净。将模拟得到的最大铺展直径、回缩速率以及冻结情况,与已有实验结果进行对比验证,表明了模拟方法的有效性和准确性。     

共水解法制备透明超疏水表面

利用硅烷偶联剂与正硅酸乙酯共水解法对SiO2纳米颗粒进行原位疏水化处理,并采用共水解后的溶胶在玻璃基底上浸渍提拉成膜,两次成膜后即可以使玻璃表面呈现良好的超疏水性并保持较好的透明度。论文通过扫描电镜、原子力显微镜、傅立叶变换红外光谱、接触角仪进一步对超疏水表面进行了表征。实验结果显示制备的超疏水表面不仅具有较大的表观接触角（≥150°）,而且该表面有着较小的接触角滞后。通过该方法制备的超疏水表面不需要使用昂贵的全氟烷进行后续疏水化处理,从而简化了超疏水表面的制备工艺。     

Microporous Membrane Solvent Extraction

Abstract

The versatility of the phenomenon of dispersion-free solvent extraction through immobilized aqueous-organic interface in a microporous hydrophobic membrane is demonstrated here. This technique has been used here for an organic-organic interface to extract aromatics from a hydrocarbon feedstock by a polar organic solvent. The phenomenon has been studied further with micropor-ous hydrophilic and composite hydrophobic-hydrophilic membranes. Operational modes and the influence of the membrane and boundary layer resistances for various solvent-solute-membrane systems have been studied for a CELGAKD (hydrophobic) and a CUPROPHAN (hydrophilic) membrane. Limited performance data for microporous hydrophobic CELGARD hollow fibers (X-20) are also presented.