微纳米复合的各向异性结构表面的液滴驱动

通过聚二甲基硅氧烷和Co粉混合物模板复型、外加磁场控制法和氧化锌纳米线水热生长法,制备了微纳米复合的各向异性结构表面.该结构表面规整分布着间距为400μm的倾斜锥状纤维,其底部和顶部直径分别约为200和80μm,长度约为1 mm.锥状纤维的表面覆有氧化锌纳米线,其直径约为80~100 nm,长度约为1μm.该表面呈疏水性,接触角约为142.5°,且具有浸润性各向异性的特征.实验结果表明,液滴在该表面可以定向弹跳或在垂直振动情况下定向滚动,为设计驱动液滴的功能性材料提供了思路.     

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.     

液滴界面的软物质结晶及有序自组织

液滴界面具有可以调节的曲率、表面张力及球面拓扑结构等特点,为结晶研究提供了独特的实验条件.近年来,由液滴界面结晶导致的新颖实验现象层出不穷,如具有多个光学中心的球晶、由亚稳态旋转相驱动的液滴形变、自驱动活性液滴等,引起了研究人员的广泛关注.结合本课题组工作,本文以软物质结晶及有序自组织行为在液滴界面与平面基板的不同为出发点,聚焦于液滴界面软物质相关的新颖实验现象,探讨了液滴界面复杂现象背后的形成机制,同时指出了液滴界面软物质结晶及有序自组织对化学、物理学、生物学等相关领域基础研究的影响.     

电控液滴移动的研究进展

电控液滴移动是一种利用电场作用驱动液滴移动的策略, 因其液滴具有响应速度快、 运动速度快及路径可控等优点而备受关注, 在外场刺激驱动液滴移动等基础研究和智能微流体器件等实际应用中具有重要意义. 本文概述了传统电润湿驱动液滴移动的基本原理和研究进展, 介绍了新型电控液滴移动的代表性成果, 展望了相关研究和应用的发展前景.     

基于微流体脉冲驱动控制技术的电化学微流控芯片制备方法

基于微流体脉冲驱动控制技术搭建了电化学微流控芯片的制备系统.首先将纳米银墨水和甘油溶液分别微喷射到玻璃基底表面形成微电极图形和微流道液体阳模图形;然后分别进行烧结和聚二甲基硅氧烷(PDMS)模塑工艺制得微电极和微流道;最后将微电极和微流道键合形成电化学微流控芯片.研究了系统参量对液滴产生的影响以及液滴直径和重叠率对液滴成线的影响,制得的微电极最小线宽为45 μm、厚度为2.2 μm、电阻率为5.2 μΩ·cm,制得的微流道最小线宽为35 μm,流道表面光滑.采用制得的电化学微流控芯片进行了葡萄糖浓度的电化学流动检测.结果表明,葡萄糖溶液的浓度与响应电流具有较高的线性关系,可对一定浓度范围内的葡萄糖溶液进行定量检测.基于微流体脉冲驱动控制技术的电化学微流控芯片制备方法具有微喷射精度高、重复性好,制备系统结构简单、成本低廉等优点,可用于生化分析、生物传感器等领域的芯片制备.     

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.     

仿生各向异性聚(N-异丙基丙烯酰胺)水凝胶智能响应驱动器的研究进展

各向异性水凝胶在外界的响应刺激下可以具有不同的反应机制与驱动过程. 本文综述了近期基于PNIPAM水凝胶智能响应驱动器的设计方法, 总结了多种各向异性结构对驱动性能的影响, 并对该领域所面临的挑战进行了讨论.     

具有特殊浸润性的各向异性微纳米分级结构表面构筑的研究进展

自然界存在许多具有各向异性表面结构的生物,其表面表现出典型的对液体操控的方向性的差异。近年来,这种表面微结构的构筑引起了广泛的研究兴趣,已成为一个热点研究方向。天然的各向异性浸润表面是由复杂的异质微纳米结构组成,基于基础研究和应用推广的目的,可以将其简化为一些有序的方向性结构表面。本文介绍了现在应用广泛的几种各向异性微纳米分级结构的构筑方法,并对比分析其可行性。同时,文中还深入讨论了各向异性微纳米分级结构表面对于液体行为的调控。这种各向异性微纳米分级结构表面在微流体运输、微流控芯片等领域将有重要应用,也会对生命科学(比如生物芯片和重大疾病的早期诊断)、能源(比如电极材料的可控制备)和环境(比如污染物的分离及定向转化)等研究做出巨大的贡献。     

具有各向异性微纳米分级结构表面的构筑及其特殊浸润性的研究进展

自然界存在许多具有各向异性表面结构的生物,其表面表现出典型的对液体操控的方向性的差异。近年来,这种表面微结构的构筑引起了广泛的研究兴趣,已成为一个热点研究方向。天然的各向异性浸润表面是由复杂的异质微纳米结构组成,基于基础研究和应用推广的目的,可以将其简化为一些有序的方向性结构表面。本文介绍了现在应用广泛的几种各向异性微纳米分级结构的构筑方法,并对比分析其可行性。同时,文中还深入讨论了各向异性微纳米分级结构表面对于液体行为的调控。这种各向异性微纳米分级结构表面在微流体运输、微流控芯片等领域将有重要应用,也会对生命科学(比如生物芯片和重大疾病的早期诊断)、能源(比如电极材料的可控制备)和环境(比如污染物的分离及定向转化)等研究做出巨大的贡献。     

模拟乳状液中微小液滴间的相互作用力

对商品化的DCAT21表面/界面张力仪进行改造, 用于直接测量液滴间相互作用力, 同时用数码摄像头Digital 3.0观察记录两液滴接近, 挤压, 排液, 聚并等过程. 研究发现, 溶液中微小液滴间的相互作用力随距离的变化曲线能够提供分散液滴的行为特征信息: 曲线上不同阶段的斜率反映力的大小; 从液滴接触后到聚并前的挤压距离反映液滴的稳定性. 表面活性剂种类不同, 对两液滴聚并所起的稳定作用不同, 非离子表面活性剂具有较好的稳定作用. 溶液中聚合物分子在薄液膜中形成具有一定强度的层状结构, 阻碍液滴聚并, 受力曲线呈阶梯状.     

Recent progress in experiments for sessile droplet wetting on structured surfaces

Wetting of a sessile droplet on structured or patterned surface can be found in a broad range of applications. The researchers have been promoted to keep working on the topic. The review is on the basis of the recent experimental advances on the sessile droplet wetting on the hydrophobic, hydrophilic, or combined hydrophobic and hydrophilic surfaces under isothermal conditions, and on heating or cooling substrates having nonisothermal conditions. More attention has been paid on the wetting configuration between the sessile droplet and the structured substrate; the research gap has been discussed on identifying the three-phase line shape. Further, the three-dimensional measurement for the sessile droplets on the patterned surfaces with focusing more on the contact line of sessile droplets might reveal new physical insights. This review targets at building a holistic overview on the sessile droplet wetting behaviors on the structured substrate in the past 2 years.     

Predictive model for ice formation on superhydrophobic surfaces

The prevention and control of ice accumulation has important applications in aviation, building construction, and energy conversion devices. One area of active research concerns the use of superhydrophobic surfaces for preventing ice formation. The present work develops a physics-based modeling framework to predict ice formation on cooled superhydrophobic surfaces resulting from the impact of supercooled water droplets. This modeling approach analyzes the multiple phenomena influencing ice formation on superhydrophobic surfaces through the development of submodels describing droplet impact dynamics, heat transfer, and heterogeneous ice nucleation. These models are then integrated together to achieve a comprehensive understanding of ice formation upon impact of liquid droplets at freezing conditions. The accuracy of this model is validated by its successful prediction of the experimental findings that demonstrate that superhydrophobic surfaces can fully prevent the freezing of impacting water droplets down to surface temperatures of as low as -20 to -25 °C. The model can be used to study the influence of surface morphology, surface chemistry, and fluid and thermal properties on dynamic ice formation and identify parameters critical to achieving icephobic surfaces. The framework of the present work is the first detailed modeling tool developed for the design and analysis of surfaces for various ice prevention/reduction strategies.     

Self-propelled swimming droplets

Self-propelled droplets are a class of active matter systems composed of one fluid dispersed in another immiscible fluid. Despite the inherent spherical symmetry in the initial droplet shape and composition, self-propulsion in these systems is achieved by a spontaneous symmetry-breaking bifurcation. Either a chemical reaction, micelle-induced solubilization, or a phase transition may induce gradients in the interfacial tension, generating a Marangoni convection and thereby resulting in self-propulsion. The simplicity associated with these self-propelled droplet systems makes them excellent candidates for investigating the solitary and collective behaviour of several biological swimmers, ranging from single-celled bacteria to school of fishes. Additionally, due to their tunable mobility characteristics, these swimmers have immense potential as smart materials designed to execute intricate tasks in microscopic domains. In this review, we present state-of-the-art experimental and theoretical research relevant to self-propelled swimming droplets.     

Dielectrophoresis-based programmable fluidic processors

Droplet-based programmable processors promise to offer solutions to a wide range of applications in which chemical and biological analysis and/or small-scale synthesis are required, suggesting they will become the microfluidic equivalents of microprocessors by offering off-the-shelf solutions for almost any fluid based analysis or small scale synthesis problem. A general purpose droplet processor should be able to manipulate droplets of different compositions (including those that are electrically conductive or insulating and those of polar or non-polar nature), to control reagent titrations accurately, and to remain free of contamination and carry over on its reaction surfaces. In this article we discuss the application of dielectrophoresis to droplet based processors and demonstrate that it can provide the means for accurately titrating, moving and mixing polar or non-polar droplets whether they are electrically conductive or not. DEP does not require contact with control surfaces and several strategies for minimizing surface contact are presented. As an example of a DEP actuated general purpose droplet processor, we show an embodiment based on a scaleable CMOS architecture that uses DEP manipulation on a 32 x 32 electrode array having built-in control and switching circuitry. Lastly, we demonstrate the concept of a general-purpose programming environment that facilitates droplet software development for any type of droplet processor.     

Droplet on a liquid substrate: Wetting,dewetting, dynamics,instabilities

Droplets on a liquid substrate (‘liquid lenses’) play an important role in various branches of engineering, including microfluidics, chemical engineering, environment protection, etc. In the present paper, we discuss basic phenomena characteristic for liquid lenses. We recall classical results on the shape of an equilibrium droplet and the kinds of droplet wetting. We overview briefly the main theoretical approaches used for the analysis of droplet dynamics, discuss the phenomena accompanying a droplet impact, physical effects used for droplet manipulations, and the factors that determine the interaction between droplets. We describe the main types of droplet instabilities leading to oscillations, self-propulsion, and disintegration of droplets. Some promising directions of further research are listed.     

Wetting phenomena on micro-grooved aluminum surfaces and modeling of the critical droplet size

The behavior of water droplets on aluminum surfaces with parallel grooves tens of microns in width and depth is considered, and a mechanistic model is developed for predicting the critical droplet size-droplets at incipient sliding due to gravity. The critical droplet size is nearly 50% smaller on micro-grooved surfaces than on the same surface without micro-grooves. The application of existing models fails to predict this behavior, and a new model based on empiricism is developed. The new model provides reasonable predictions of the critical droplet size for a given inclination angle, advancing contact angle, and maximum contact angle. When the grooves are aligned parallel to gravity, the maximum apparent contact angle does not occur at the advancing front but rather along the side of the droplet because of contact-line pinning. Droplets on these surfaces are elongated and possess a parallel-sided base contour shape. Novel data are provided for droplets in a Wenzel state, a Cassie-Baxter state, and combined state on micro-grooved surfaces, and the ability of the empirical model to handle these variations is explored. These findings may be important to a broad range of engineering applications.     

Shear-induced coalescence of emulsified oil drops

Crude oil droplets, when suspended in water, possess negative surface charges which give rise to double-layer repulsive forces between the drops. According to conventional DLVO theory, the magnitude of this repulsion (based on the measured zeta potential) is more than sufficient to prevent coalescence of the droplets. Indeed, when two such droplets were brought together on direct (i.e., "head-on") approach, coalescence was rarely observed. Upon oblique approach, however, the same droplets were seen to coalesce readily. An oblique encounter must necessarily give rise to lateral relative motion-or shearing-between the droplet surfaces. It is speculated that, if the charge distributions at the droplet surfaces were heterogeneous, lateral shearing would facilitate many encounters between surface patches of different zeta potentials across the intervening water film. If the repulsion across any local region were sufficiently weak to allow formation of an oil bridge across the water film, coalescence of the drops would follow inevitably. With the hypothesis of surface heterogeneity, it is not necessary to invoke any additional colloidal interactions (such as "hydrophobic forces") to account for the observed droplet-droplet coalescence. This finding may have important implications for the underlying mechanisms of emulsion stability in general and the commercial extraction of bitumen from oil sands in particular.     

Dynamic effects of bouncing water droplets on superhydrophobic surfaces

Superhydrophobic surfaces have considerable technological potential for various applications due to their extreme water repellent properties. Superhydrophobic surfaces may be generated by the use of hydrophobic coating, roughness, and air pockets between solid and liquid. Dynamic effects, such as the bouncing of a droplet, can destroy the composite solid-air-liquid interface. The relationship between the impact velocity of a droplet and the geometric parameters affects the transition from the solid-air-liquid interface to the solid-liquid interface. Therefore, it is necessary to study the dynamic effect of droplets under various impact velocities. We studied the dynamic impact behavior of water droplets on micropatterned silicon surfaces with pillars of two different diameters and heights and with varying pitch values. A criterion for the transition from the Cassie and Baxter regime to the Wenzel regime based on the relationship between the impact velocity and the parameter of patterned surfaces is proposed. The trends are explained based on the experimental data and the proposed transition criterion. For comparison, the dynamic impact behavior of water droplets on nanopatterned surfaces was investigated. The wetting behavior under various impact velocities on multiwalled nanotube arrays also was investigated. The physics of wetting phenomena for bouncing water droplet studies here is of fundamental importance in the geometrical design of superhydrophobic surfaces.     

Induced detachment of coalescing droplets on superhydrophobic surfaces

Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on the superhydrophobic surface is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid (VOF) method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spread length of two coalesced droplets along their original center is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, and droplet size on contact time and restitution coefficient are studied and compared to the experimental results. Finally, the wetting and the self-cleaning properties of superhydrophobic surfaces have been investigated. It has been found that impinging water drops with very small amount of kinetic impact energy were able to thoroughly clean these surfaces.