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.     

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.     

微结构疏水表面上液滴的表观接触角

虽然微结构疏水表面上Cassie-Baxter状态液滴的表观接触角已有理论预测公式,但实验研究发现,由于微结构疏水表面上的瑕疵,以及液滴受到的各种轻微扰动等原因,很容易造成Cassie-Baxter状态液滴局部区域出现Wenzel状态的情况(即混合状态),而有关混合状态液滴表观接触角的研究还比较少.本文通过对比相同微结构疏水表面上Cassie-Baxter状态和混合状态液滴表观接触角的大小,发现将Cassie-Baxter预测公式中的固液接触面积分数F换成最外缘三相接触线处的局部固液接触面积分数F′,则能同时较好地预测上述两种情况下液滴的表观接触角;通过进一步的研究发现,表观接触角的大小仅与最外缘三相接触线处的固液接触状态有关,而与其他处的固液接触状态无关.该结果对于进一步认识微结构疏水表面上液滴的表观接触角以及润湿性质具有重要意义.     

形状记忆聚合物表面微结构及黏附性能的可逆调控

采用模板法在形状记忆聚合物表面构筑了微纳米等级结构,获得了一种具有低黏附性的超疏水表面.在外压作用下,表面微结构发生坍塌,失去超疏水性,同时呈高黏附性.在120℃热处理后,表面微结构恢复到原始状态,同时表面恢复到低黏附状态.通过外压及热处理过程可实现对表面微结构及其黏附性能的可逆调控.研究结果表明,表面不同的微结构状态赋予了表面不同的黏附性能,即在原始表面上,液滴处于低黏附的Cassie态,而在坍塌结构表面上水滴处于高黏附的Wenzel态.     

疏水表面拓扑结构对其润湿状态影响的粗粒化模拟

采用BMW-MARTINI粗粒化分子动力学模拟方法研究了表面的拓扑结构对疏水性表面润湿状态的影响. 模拟结果表明,对于疏水性表面,增大表面的粗糙度对其疏水性影响不大,而主要是影响其润湿状态. 在一定范围内(柱间距不超过4.7 nm),水珠在微柱结构疏水表面的润湿行为受到柱间距(d)和柱高(h)的双重影响. 柱间距一定时,存在一个临界的柱高,可以使得水珠在表面的润湿状态由Wenzel态向Cassie-Baxter态发生转变,并且该临界高度随着柱间距的增大而增大. 进一步分析发现,本文研究范围内,润湿状态的转变和柱间距与柱高的比值d/h有关,当d/h不超过2时,水珠呈Wenzel态,超过2时则由Wenzel态向Cassie-Baxter态发生转变. 通过能量分析,发现润湿状态的转变主要取决于水珠与表面之间的范德华作用. 本文研究结果可以为开发具有特定功能的疏水性材料提供参考.     

形状记忆聚合物表面水下油黏附性的可逆调控

采用模板法在形状记忆聚合物表面获得一种具有形状记忆特征的表面微结构, 在氧等离子作用下, 表面呈现低黏附的水下超疏油特性. 在外压作用下, 表面微结构发生坍塌, 失去水下超疏油性, 同时表面对油滴呈高黏附特征. 在120 ℃热处理后, 表面微结构恢复到了原始状态, 在等离子进一步作用下, 表面即可恢复到最初的低黏附水下超疏油状态. 因此通过外压、 热处理及等离子作用即可实现对表面微结构及其水下油黏附性能的可逆调控. 研究表明, 表面不同的微结构状态赋予表面不同的黏附性能, 在原始表面液滴处于低黏附的Cassie态, 而在坍塌结构表面水滴处于高黏附的Wenzel态.     

图案化基底上液滴的挥发行为

采用液滴形状分析仪, 在线跟踪了液滴在图案化基底上的挥发过程. 结果表明, 与平滑基底结果相比, 图案化基底上的挥发过程明显不同. 首先, 接触角减小; 另外, 由于发生了Cassie态到Wenzel态转变, 所得接触直径在减小过程中产生了一个突然变大的阶段. 在该挥发过程中, 突起部分的面积分数扮演了十分重要的角色.     

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.     

仿生超疏水表面的抗冷凝失效研究进展

在存在一定过冷度或蒸汽过饱和度的条件下,水蒸汽可在固体表面凝结成核.随着过冷度增大,液滴成核半径将随之减小,冷凝液滴的生长融合将无法避免地发生在超疏水表面不可或缺的微/纳米结构内.若液滴不能及时排出,则会滞留在表面结构内并挤出空气,形成局部浸润,导致材料表面的超疏水性能下降或失效,甚至引起泛洪.本文首先总结了表面因冷凝...     

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.     

Studies on the Binary Coalescence Model

The jumping coalescence phenomenon between two separate water drops under microgravitation was observed for the first time. Two suspended water drops separated by quite a distance (0.558 mm) coalesce in a certain time (158 s) without any extra forces. The coalescence driving force within the water drops should be responsible for the jumping coalescence phenomenon. Copyright 2001 Academic Press.     

Fog deposition and accumulation on smooth and textured hydrophobic surfaces

We investigated the deposition and accumulation of droplets on both smooth substrates and substrates textured with square pillars, which were tens of micrometers in size. After being coated with a hydrophobic monolayer, substrates were placed in an air flow with a sedimenting suspension of micrometer-sized water droplets (i.e., fog). We imaged the accumulation of water and measured the evolution of the mean drop size. On smooth substrates, the deposition process was qualitatively similar to condensation, but differences in length scale revealed a transient regime not reported in condensation experiments. Based on previous simulation results, we defined a time-scale characterizing the transition to steady-state behavior. On textured substrates, square pillars promoted spatial ordering of accumulated drops. Furthermore, texture regulated drop growth: first enhancing coalescence when the mean drop size was smaller than the pillar, and then inhibiting coalescence when drops were comparable to the pillar size. This inhibition led to a monodisperse drop regime, in which drop sizes varied by less than 5%. When these monodisperse drops grew sufficiently large, they coalesced and could either remain suspended on pillars (i.e., Cassie-Baxter state) or wet the substrate (i.e., Wenzel state).     

Limiting conditions for applying the spherical section assumption in contact angle estimation

The shape of liquid drops on solid surfaces deviates from the spherical as tension decreases and gravity effects start affecting the drop shape. This paper attempts to define this deviation and estimates the dimensionless Eotvos number limits above which the deviation becomes "significant." The use of these limiting values can facilitate estimation of contact angle in the following manner. It is well known that the equilibrium contact angle made by a liquid drop on a solid surface can be estimated from measurements of two drop parameters. These parameters can be any two chosen from the drop volume, height, and wetted radius. In case the effect of gravity on the drop shape is negligible, simple algebraic relations derived from the spherical section assumption exist, from which the contact angle can be estimated. In systems where the "spherical section" assumption is invalid, the Laplace equation for the drop shape has been solved numerically with any two of the above parameters as the constraints, to obtain the contact angle. In this paper, Eotvos numbers at which the deviation of the drop profile from the spherical is significant enough to result in contact angle deviation of 1 degrees are estimated. The limiting values of Eotvos number, expressed as a function of the original contact angle made by the spherical profile, are obtained by solving the Laplace equation for the drop shape with the drop volume and wetted radius constraints for decreasing values of Interfacial tension. These limiting values are also estimated for different drop sizes and for cases where the drop phase is heavier (sessile) and lighter (buoyant) than the surrounding fluid. The independence of the Eotvos number estimates from the sign of the density difference as well as the drop size is shown. These Eotvos number limits can be used to check if the spherical section assumption, with the resulting simple algebraic relations, can be used for contact angle estimation and other shape-related analysis for a system.     

A theory of a curved vapor-liquid separation boundary: The lattice gas model

Molecular theory of curved vapor-liquid interphase boundaries was considered in terms of the lattice gas model. The theory uses the quasi-thermodynamic concept of curved layers of a separation boundary with a large radius. The transition from a rectangular lattice to such layers is performed by the introduction of a variable number of the nearest neighbors. The problems (1) of the transition from distributed molecular models to layer models reflecting macroscopic symmetry of the interphase boundary and (2) of a minimum linear size of the surface region to which thermodynamic approaches are applicable were considered. Equations for the quasi-equilibrium distribution of molecules at the vapor-liquid boundary in a metastable system were constructed in the quasi-chemical approximation taking into account direct correlations between the nearest interacting molecules. A metastable state is maintained by a pressure jump described by the macro-scopic Laplace equation on a separation surface inside the interphase region. Equations for local mean pressure values and normal and tangential pressure tensor components inside the interphase region were constructed. These equations were used to obtain microscopic difference mechanical equilibrium equations for curved boundaries of spherical and cylindrical drops in the metastable state. The relation between the micro-scopic difference mechanical equilibrium equations and similar differential equations and the macroscopic Laplace equation, which described pressure jump in a metastable system, was considered. Various definitions of surface tension are discussed.     

特殊浸润性表面的液滴凝结

以铝片为基底, 经电化学腐蚀和沸水处理制备了多级微纳米结构; 通过气相沉积和涂油分别制备了超疏水表面、 疏水超润滑(slippery)表面和亲水slippery表面; 探究了表面不同的特殊浸润性(超亲水、 超疏水、 疏水slippery和亲水slippery)对液滴凝结的影响. 结果表明, 超亲水表面的液滴凝结属于膜状冷凝, 超疏水表面和slippery表面的液滴凝结均属于滴状冷凝. 超疏水表面液滴合并时, 合并的液滴会不定向弹离表面. 疏水slippery表面和亲水slippery表面由于表面浸润性的不同导致液滴成核密度和液滴合并的差异, 亲水slippery表面凝结液滴的最大体积远大于疏水slippery表面凝结液滴的最大体积. 4种表面的雾气收集效率由大到小依次为亲水slippery表面>疏水slippery表面>超亲水表面>超疏水表面.     

Spreading of liquid drops over dry porous layers: complete wetting case

Spreading of small liquid drops over thin dry porous layers is investigated from both theoretical and experimental points of view. Drop motion over a porous layer is caused by an interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in an expanding of the drop base; (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and an expanding of the wetted region inside the porous layer. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations is derived to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters: one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments are used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters are determined. Experiments were carried out on the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer are monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to our theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in our system. These conclusions again are in good agreement with our experimental observations.     

Effects of Applied Electric Fields on Drop—Interface and Drop—Drop Coalescence*

In this work, coalescence of a single organic or aqueous drop with its homophase at a horizontal liquid interface was investigated under applied electric fields. The coalescence time was found to decrease for aqueous drops as the applied voltage was increased, regardless of the polarity of the voltage. For organic drops, the coalescence time increased with increasing applied voltage of positive polarity and decreased with increasing applied voltage of negative polarity. Under an electric field, the coalescence time of aqueous drops decreases due to polarization of both the drop and the flat interface. The dependency of organic drop-interface coalescence on the polarity of the electric field may be a result of the negatively charged organic surface in the aqueous phase. Due to the formation of a double layer, organic drops are subjected to an electrostatic force under an electric field, which, depending on the field polarity, can be attractive or repulsive. Pair-drop coalescence of aqueous drops in the organic phase was also studied. Aqueous drop-drop coalescence is facilitated by polarization and drop deformation under applied electric fields. Without applied electric fields, drop deformation increases the drainage time of the liquid film between two approaching drops. Therefore, a decrease in the interfacial tension, which causes drop deformation, accelerates drop-drop coalescence under an electric field and inhibits drop coalescence in the absence of an electric field.     

Surfactant solutions and porous substrates: spreading and imbibition

In Section 1, spreading of small liquid drops over thin dry porous layers is investigated from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397]. Drop motion over a porous layer is caused by an interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in an expanding of the drop base, and (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and an expanding of the wetted region inside the porous layer. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations has been derived to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters, one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate, and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments were used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters are determined. Experiments were carried out on the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactory for the experimental data. According to theory predictions [1]: (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function, and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation. These conclusions again are in good agreement with experimental observations [V.M. Starov, S.R. Kosvintsev, V.D. Sobolev, M.G. Velarde, S.A. Zhdanov, J. Colloid Interface Sci. 252 (2002) 397]. In Section 2, experimental investigations are reviewed on the spreading of small drops of aqueous SDS solutions over dry thin porous substrates (nitrocellulose membranes) in the case of partial wetting [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]. The time evolution was monitored of the radii of both the drop base and the wetted area inside the porous substrate. The total duration of the spreading process was subdivided into three stages-the first stage: the drop base expands until the maximum value of the drop base is reached; the contact angle rapidly decreases during this stage; the second stage: the radius of the drop base remains constant and the contact angle decreases linearly with time; the third stage: the drop base shrinks and the contact angle remains constant. The wetted area inside the porous substrate expends during the whole spreading process. Appropriate scales were used with a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate, and the dynamic contact angle on the dimensionless time. Experimental data showed [S. Zhdanov, V. Starov, V. Sobolev, M. Velarde, Spreading of aqueous SDS solutions over nitrocellulose membranes. J. Colloid Interface Sci. 264 (2003) 481-489]: the overall time of the spreading of drops of SDS solution over dry thin porous substrates decreases with the increase of surfactant concentration; the difference between advancing and hydrodynamic receding contact angles decreases with the surfactant concentration increase; the constancy of the contact angle during the third stage of spreading has nothing to do with the hysteresis of contact angle, but determined by the hydrodynamic reasons. It is shown using independent spreading experiments of the same drops on nonporous nitrocellulose substrate that the static receding contact angle is equal to zero, which supports the conclusion on the hydrodynamic nature of the hydrodynamic receding contact angle on porous substrates. In Section 3, a theory is developed to describe a spontaneous imbibition of surfactant solutions into hydrophobic capillaries, which takes into account the micelle disintegration and the concentration decreasing close to the moving meniscus as a result of adsorption, as well as the surface diffusion of surfactant molecules [N.V. Churaev, G.A. Martynov, V.M. Starov, Z.M. Zorin, Colloid Polym. Sci. 259 (1981) 747]. The theory predictions are in good agreement with the experimental investigations on the spontaneous imbibition of the nonionic aqueous surfactant solution, Syntamide-5, into hydrophobized quartz capillaries. A theory of the spontaneous capillary rise of surfactant solutions in hydrophobic capillaries is presented, which connects the experimental observations with the adsorption of surfactant molecules in front of the moving meniscus on the bare hydrophobic interface [V.J. Starov, Colloid Interface Sci. 270 (2003)]. In Section 4, capillary imbibition of aqueous surfactant solutions into dry porous substrates is investigated from both theoretical and experimental points of view in the case of partial wetting [V. Straov, S. Zhdanov, M. Velarde, J. Colloid Interface Sci. 273 (2004) 589]. Cylindrical capillaries are used as a model of porous media for theoretical treatment of the problem. It is shown that if an averaged pore size of the porous medium is below a critical value, then the permeability of the porous medium is not influenced by the presence of surfactants at any concentration: the imbibition front moves exactly in the same way as in the case of the imbibition of the pure water. The critical radius is determined by the adsorption of the surfactant molecules on the inner surface of the pores. If an averaged pore size is bigger than the critical value, then the permeability increases with surfactant concentration. These theoretical conclusions are in agreement with experimental observations. In Section 5, the spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view [V.M. Starov, S.R. Kosvintsev, M.G. Velarde, J. Colloid Interface Sci. 227 (2000) 185]. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid-vapor interfacial tension and hydrophilise the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by experimental observations. In Section 6, the process of the spontaneous spreading of a droplet of a polar liquid over solid substrate is analyzed in the case when amphiphilic molecules (or their amphiphilic fragments) of the substrate surface layer are capable of overturning, resulting in a partial hydrophilisation of the surface [V.M. Starov, V.M. Rudoy, V.I. Ivanov, Colloid J. (Russian Academy of Sciences English Transaction) 61 (3) (1999) 374]. Such a situation may take place, for example, during contact of an aqueous droplet with the surface of a polymer whose macromolecules have hydrophilic side groups capable of rotating around the backbone and during the wetting of polymers containing surface-active additives or Langmuir-Blodgett films composed of amphiphilic molecules. It was shown that droplet spreading is possible only if the lateral interaction between neighbouring amphiphilic molecules (or groups) takes place. This interaction results in the tangential transfer of "the overturning state" to some distance in front of the advancing three-phase contact line making it partially hydrophilic. The quantitative theory describing the kinetics of droplet spreading is developed with allowance for this mechanism of self-organization of the surface layer of a substrate in the contact with a droplet.