纳米结构表面上冷凝液滴的生长模式及部分润湿液滴的形成机制

分析并计算了纳米结构表面上冷凝液滴按照不同途径长大的过程中液滴能量的增加速率, 并以能量增加最小为判据来确定液滴的生长途径. 结果表明, 纳米结构内形成的冷凝液斑在初期按接触角(CA)增加的模式生长时, 其能量增加速率远低于其它模式, 于是, 初始液斑先按增大接触角、并保持底面积不变的模式生长, 直至液滴达到前进角状态. 此后, 沿接触角增加的模式长大所导致的能量增加速率开始远高于其它生长模式, 于是液滴三相线开始移动, 底面积开始增加, 但接触角保持不变. 液滴所增加的底面积可以呈润湿或复合两种状态, 分别形成Wenzel 液滴及部分润湿液滴, 前者的表观接触角一般小于160°, 而后者则明显大于160°. 液滴的生长模式及其润湿状态均与纳米结构参数密切相关, 仅当纳米柱具有一定高度、且间距较小时, 冷凝液滴才能呈现部分润湿状态. 最后, 本模型对纳米结构表面上冷凝液滴润湿状态的计算结果与绝大部分实测结果相一致, 准确率达到91.9%, 明显高于已有公式的计算准确率.     

纳米二氧化硅粒子对聚氯乙烯膜的表面结构及浸润性能的调控作用

报道了一种简便的调控聚合物材料表面结构及浸润性能的方法.利用流延成膜和纳米二氧化硅粒子的印迹修饰作用,制备出3种具有不同表面结构的聚氯乙烯(PVC)膜,膜的浸润性能表现为与水的接触角从103°的疏水性变为65°的亲水性,再改变至130°的疏水性.扫描电镜结果表明印迹修饰后的PVC膜具有纳米和微米尺寸的凹凸表面结构.通过对比实验证实了溶剂氯仿和NaOH溶液并不影响膜表面的疏水性能.     

各向异性浸润表面在液体操控中的应用

在自然界中,各种生物表面,如水稻叶、蝴蝶翅膀、沙漠甲虫鞘翅、蜘蛛丝、仙人掌刺及猪笼草口缘等,都存在着各向异性浸润性.当液体接触线遇到固体表面的物理不对称性或化学不均匀性时,就会发生各向异性浸润现象,表现为表面在特定而非随机的方向上展现出不同的液滴接触角和滑动角,并伴随着液体的各向异性扩散和各向异性运动行为.近年来,各向异性浸润表面的理论研究和实际应用引起了人们广泛的关注.液体操控,尤其是可控定向液体输送,作为一种极具发展前景的重要智能液体操纵形式,在许多领域发挥着重要作用.各向异性浸润表面因其独特的物理化学性质,在液体操控领域的应用获得了快速发展.本文结合本课题组的研究工作,提出了各向同性浸润和各向异性浸润的有关观点,总结了以微/纳米结构为主的人造各向异性浸润表面在液体操控领域的最新进展,包括可控浸润性、微流控及液体运输和水/雾收集.最后讨论了这一快速发展领域目前面临的挑战和未来的发展前景.     

超疏水表面微纳二级结构对冷凝液滴最终状态的影响

从超疏水表面(SHS)上初始冷凝液核长大、合并、形成初始液斑开始,分析计算了冷凝液斑变形成为Wenzel或Cassie液滴过程中界面能量的变化,并以界面能曲线降低、是否取最小值为判据,确定冷凝液滴的最终稳定状态.计算结果表明:在只有微米尺度的粗糙结构表面上,冷凝液滴的界面能曲线一般都是先降低再升高,呈现Wenzel状态;而当表面具有微纳米二级粗糙结构,且纳米结构的表面空气面积分率较高时,冷凝液滴的能量曲线持续降低,直至界面能最小的Cassie状态,因此可以自发地形成Cassie液滴.还计算了文献中具有不同结构参数的SHS上冷凝液滴的状态和接触角,并与实验结果进行了比较,结果表明,计算的冷凝液滴状态与实验观察结果完全吻合.因此,微纳二级结构是保持冷凝液滴在SHS上呈现Cassie状态的重要因素.     

注压成型纳米结构PP/POE共混物表面的液滴低温冲击行为

以制备的阳极氧化不锈钢模板为模具嵌件, 采用注射压缩成型(ICM)工艺快速成型表面具有纳米丝结构的柔性聚丙烯/乙烯-辛烯共聚物(PP/POE, PP与POE质量比为3∶1)共混物复制物和准刚性PP复制物, 研究常温液滴冲击?10 ℃复制物表面的动态行为. 结果表明, 致密的纳米丝使复制物表面呈现超疏水、 极低黏附的润湿状态. 在低冲击速度范围内, 共混物复制物表面上液滴的接触时间比PP复制物上的短, 可归因于液滴铺展阶段柔性纳米丝储存的弹性势能在回缩阶段转换为液滴的动能; 在高冲击速度范围内, 共混物复制物基板和纳米丝储存的弹性势能被转换为液滴的动能, 进一步缩短了液滴的接触时间. 此结果表明, 共混物复制物的柔性和表面超疏水性使其具有优异的防冻黏性能. 共混物复制物表面上水滴(50 μL)的结冰时间得到明显延长、 冰黏附强度明显降低. 研究结果表明, 可采用ICM快速成型具有优异防冻黏和防冰性能的柔性超疏水高分子材料表面.     

多尺度柔性结构的防覆冰性能

通过软复型和晶体生长的方法制备了具有柔性微米锯齿和纳米棒结构的微纳米复合表面,其具有低温低黏附的特性,达到了优异的防覆冰效果.柔性微纳米结构表面的形变,可以在低温条件下有效去除液滴.研究结果表明,微米结构的弯曲作用改变了液滴在表面的三相线,凹面增大了气/液/固三相线长度,增加了驱动液滴的难度;凸面减小了气/液/固三相线长度,有利于减少液滴与表面之间黏附力,使液滴在重力作用下快速脱除.     

环氧树脂在碳纤维表面的浸润行为

利用场发射环境扫描电境(FESEM)测定了室温下环氧树脂在单纤维表面的接触角,观测并计算了环氧树脂液滴在单根碳纤维表面的接触角随温度的变化,结果表明接触角随温度升高明显降低,说明升高温度有利于改善环氧树脂对碳纤维的浸润性能.用液滴形状分析仪(DSA)在垂直和平行于纤维排列方向上观测了不同温度下单向排列碳纤维集束表面环氧树脂的铺展过程,发现在不同方向上观测到的接触角差别较大,其中垂直于纤维排列方向上观测到的接触角随温度的变化与环氧树脂在单根碳纤维表面的接触角变化基本一致,说明环氧树脂在平行于纤维束方向的接触角真正代表其浸润性能.     

微纳结构超疏水表面的浸润性分析及设计

微纳复合结构超疏水表面在防污、流动减阻、防冰等领域具有广阔的应用前景，超疏水表面主要通过设计表面化学性质和微观几何结构来获得.合理设计保持表面润湿态的稳定性是其性能发挥的关键.以"液滴-超疏水表面"系统为研究对象，基于最小能原理分析了四种稳定润湿形态，指出影响润湿状态的本征接触角和微观结构参数（相对柱距、相对柱高）.推导了本征接触角的计算公式并对常见材料的本征接触角进行了讨论.结合四种润湿态方程，绘制了随着相对柱距和相对柱高的润湿云图，并将润湿云图归纳为"一点三线六区四状态".分析了相对柱距和相对柱高对浸润状态的影响，结果表明较大的本征接触角、较小的相对柱距和较大的相对柱高能够减小浸润状态发生转变的临界参数，从而拓展超疏水表面的区域范围，有利于超疏水表面的稳定性.利用文献数据验证了上述润湿云图能够准确反映出润湿形态.在上述工作基础之上总结提炼了超疏水表面设计的一般思路.研究结果可为超疏水表面的设计提供理论依据和技术基础.     

固着纳米流体液滴颗粒自组装三维模拟

通过理论模拟固着纳米流体液滴在不同的蒸发模式与蒸发条件下会形成不同的沉积结构,有助于揭示纳米流体蒸发自组装机理并推动调控自组装过程方法的发展。建立了适用于小接触角纳米流体液滴蒸发的三维动力学蒙特卡罗(KMC)模型,引入随液滴厚度与相对半径变化的弧形化学势函数,并在其中加入了化学势衰减率,模拟了小接触角纳米流体液滴的蒸发过程,预测了枝状连续结构的形成。同时分别讨论了不同参数(初始化学势、纳米颗粒浓度、纳米颗粒扩散率)对液滴的蒸发过程与沉积结构的影响。模拟结果与两种实验结果进行了定性对比,结果吻合良好。     

染料敏化的TiO2纳米管薄膜的光电浸润性

采用阳极氧化法在钛箔表面制备TiO2纳米管阵列, 并在其表面修饰N3染料(Ruthenium dye)作敏化剂, 用氟硅烷来提高表面疏水性, 获得超疏水薄膜. SEM测定结果表明, 纳米管薄膜具有各向异性浸润结构, 同时阳极氧化的非均匀性增加了表面的粗糙度. UV-Vis吸收光谱及电化学阻抗谱结果表明, 薄膜具有优异的光电性能. 通过施加超过一定阈值的电压, 液滴在薄膜表面由超疏水状态转变为亲水状态. 利用光电协同激励作用时, 阈值电压比单独使用电激励时降低了10 V, 这是使用高效的N3染料光电敏化层的结果.     

A micropump controlled by EWOD: wetting line energy and velocity effects

A Laplace pressure gradient between a droplet and a liquid meniscus was utilized to create an on-demand constant flow rate capillary pump. Electrowetting on dielectric was implemented to induce the pressure gradient in the microchannel. For an initial droplet volume of 0.3 μL and a power of 12 nW a constant flow rate of 0.02 μL s(-1) was demonstrated. The effects of the wetting line energy on the static contact angle and the wetting line velocity on the dynamic contact angle in the pump operation were studied. Sample loading on-demand could be achieved by regulating an electric potential.     

Liquid nanodroplets spreading on chemically patterned surfaces

Controlling the spatial distribution of liquid droplets on surfaces via surface energy patterning can be used to deliver material to specified regions via selective liquid/solid wetting. Although studies of the equilibrium shape of liquid droplets on heterogeneous substrates exist, much less is known about the corresponding wetting kinetics. Here we present large-scale atomistic simulations of liquid nanodroplets spreading on chemically patterned surfaces. Results are presented for lines of polymer liquid (droplets) on substrates consisting of alternating strips of wetting (equilibrium contact angle theta0 = 0 degrees) and nonwetting (theta0 approximately 90 degrees) material. Droplet spreading is compared for different wavelength lambda of the pattern and strength of surface interaction on the wetting strips. For small lambda, droplets partially spread on both the wetting and nonwetting regions of the substrate to attain a finite contact angle less than 90 degrees. In this case, the extent of spreading depends on the interaction strength in the wetting regions. A transition is observed such that, for large lambda, the droplet spreads only on the wetting region of the substrate by pulling material from nonwetting regions. In most cases, a precursor film spreads on the wetting portion of the substrate at a rate strongly dependent on the width of the wetting region.     

Droplets wetting on filament rails: Surface energy and morphology transition

The morphology of liquid droplets wetting on filaments depends on the filament configuration, droplet volume, and contact angle. A stable morphology is the one that minimizes the potential energy of the droplet–filament system, while morphology transition may happen when an intermediate state exists which corresponds to a higher potential energy. This paper aims to explore such morphology transition of droplet wetting on filament rails made of two parallel identical microfilaments. Detailed numerical simulations were performed to extract the surface energy of the droplet–filament system at varying filament spacings, droplet volumes, and contact angles. Critical conditions of the morphology transition between two symmetrical wetting morphologies (i.e., liquid droplet bridge and barrel-shaped droplet) were determined. A family of characteristic curves in terms of the dimensionless droplet volume vs the filament spacing at varying contact angles was obtained, which can be used as a universal law to govern the morphology transition for such droplet–filament rail systems. The results and concepts presented in this work can be extended to broad wetting systems and utilized for the analysis and design of microfluidic devices and testers based on droplet–filament systems.     

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.     

Patterned nonadhesive surfaces: superhydrophobicity and wetting regime transitions

Nonadhesive and water-repellent surfaces are required for many tribological applications. We study mechanisms of wetting of patterned superhydrophobic Si surfaces, including the transition between various wetting regimes during microdroplet evaporation in environmental scanning electron microscopy (ESEM) and for contact angle and contact angle hysteresis measurements. Wetting involves interactions at different scale levels: macroscale (water droplet size), microscale (surface texture size), and nanoscale (molecular size). We propose a generalized formulation of the Wenzel and Cassie equations that is consistent with the broad range of experimental data. We show that the contact angle hysteresis involves two different mechanisms and how the transition from the metastable partially wetted (Cassie) state to the homogeneously wetted (Wenzel) state depends upon droplet size and surface pattern parameters.     

Macroscopic-wetting anisotropy on the line-patterned surface of fluoroalkylsilane monolayers

Micropatterned fluoroalkylsilane monolayer surfaces with liquidphilic/liquidphobic area (line width 1-20 microm) were prepared with few defects by vacuum ultraviolet (VUV) photolithography. The anisotropic wetting of a macroscopic droplet with a 0.5-5 mm diameter on the micropatterned surfaces was investigated. The strong anisotropy of the contact angle and the sliding angle and droplet distortion for fluoroalkylsilane/silanol patterned surfaces was attributed to the difference in the energy barrier of wetting between parallel and orthogonal lines. The wetting anisotropy decreased with decreases in the liquidphilic area. Fluoroalkylsilane/alkylsilane patterned surfaces with small differences in the surface free energies of the components showed anisotropic wetting only for the low-surface-tension liquids.     

Wetting transitions

When a liquid droplet is put onto a surface, two situations distinguishable by the contact angle may result. If the contact angle is zero, the droplet spreads across the surface, a situation referred to as complete wetting. If the contact angle is between zero and 180°, the droplet does not spread, a situation called partial wetting. A wetting transition is a surface phase transition from partial to complete wetting. The wetting transition is generally first-order (discontinuous), implying a discontinuity in the first derivative of the surface free energy. As a consequence, at the transition a discontinuous jump in film thickness occurs from a molecularly thin to a thick film. We show here that the first-order nature of the transition can lead to the observation of metastable surface states and an accompanying hysteresis. The second part of this review deals with the exceptions to the first-order nature of the wetting transition. Two different types of continuous or critical wetting transitions have been reported, for which a discontinuity in a higher derivative of the surface free energy occurs. This consequently leads to a continuous divergence of the film thickness. The first type is long-range critical wetting, due to the long-range van der Waals forces. We show that this transition is preceded by the usual first-order wetting transition, which, however, is not achieved completely. This leads to the existence of a new intermediate wetting state, in which droplets coexist with a mesoscopic film: frustrated complete wetting. The film thickness diverges continuously from this mesoscopic film to a thick film. The second type of continuous transition is short-range critical wetting, for which the layer thickness diverges continuously all the way from a microscopic to a macroscopically thick film. This transition is interesting, as renormalization-group studies predict non-universal behaviour for the critical exponents characterizing the wetting transition. The experimental results, however, show mean field behaviour, the reason for which remains unclear.     

Controlled Formation of Low-Volume Liquid Pillars between Plates with a Lattice of Wetting Patches by Use of a Second Immiscible Fluid

We describe a method for forming an array of microdroplets between two plates, at least one of which is patterned with a lattice of wetting patches, using a second immiscible fluid to control droplet formation. The method may be useful for performing multiple, small-volume biochemical reactions in parallel. We analyze the forces responsible for droplet formation, describe results of a computer simulation using Surface Evolver, and derive an analytic criterion for droplet formation in terms of the contact angles of the droplet:second fluid interface on the wetting patches and surrounding surface, the diameter of the wetting patches, the distance between wetting patches, and the distance between the plates. Copyright 1999 Academic Press.     

Theoretical consideration of wetting on a cylindrical pillar defect: pinning energy and penetrating phenomena

Wetting on a cylindrical pillar defect is discussed in terms of the free-energy difference ΔG. Wetting is divided into wetting on a flat surface, a pinning effect at the apex of the defect, and wetting on a pillar wall. First, we confirmed that ΔG between before and after ideal wetting on a flat surface can be derived as a function of the contact angle θ in which the free-energy minimum is obtained as the equilibrium contact angle θ(eq) described by Young's and Wenzel's laws. Second, the pinning effect at the apex in the cross section of the pillar defect is discussed in ΔG, where the pinning effect is shown to originate from the energy barrier by an increase in the air-liquid interfacial area of a pinned droplet induced by deformation. Next, the ΔG profiles of wetting on the pillar wall are drawn based on the theory of Carroll (Carroll, B. J. J. Colloid Interface Sci.1976, 57, 488-495) to better understand the ΔG profile during penetration. Differences in the manner of wetting between the wetting state on a flat surface and the pillar wall are reflected in ΔG. Finally, penetration of a droplet into a pillar defect is comprehensively discussed on the basis of wetting on a flat surface and a pillar wall. If we consider a simple manner of penetration, another type of energy barrier resulting from an anomalous deformation of the air-liquid interface of the penetrating droplet can be theoretically suggested. Consequently, two types of energy barrier are found. These energy barriers should play a significant role in the hysteresis of wetting, the liquid-repellent Cassie-Baxter state (CB), and the CB-Wenzel wetting transition on a microtextured surface.