Studies of Interfacial Flow Behavior Using Langmuir Monolayers

ABSTRACTCTS-9874701Michael Dennin/U。卡尔。 @伯克利提出了使用光学（布鲁斯特角显微镜）和流变技术相结合的朗缪尔单分子层的流动行为。 流变测量将使用新开发的基于 Couette 粘度计的 Langmuir 槽进行。 该提案由两组相关实验组成：朗缪尔单层膜固有流动行为的研究；以及作为三维泡沫和乳液流动模型的单层泡沫流动研究。 这两组实验的共同主题是拓扑对流变学的贡献。朗缪尔单分子层本质上是二维的，由局限于空气-水界面的两亲分子组成。 我们开发的 Couette 槽由两个垂直定向的同心圆柱体组成。 内筒固定，外筒可自由旋转。 水的上表面是自由的，单分子层放置在该表面上。 外筒由弹性带组成，用于单层的压缩和膨胀。 内筒由两部分组成。 水亚相中的静止圆柱体和刚好与水面接触的扭摆。 通过旋转外筒并用扭摆测量内筒上的应力来测量直流粘度。 通过固定外筒并使扭摆振荡来测量交流粘度。 通过使用布鲁斯特角显微镜直接观察速度分布和域动力学可以获得有关单层流动特性的附加信息。 此外，通过连续旋转外圆筒，单层的剪切排列使得获得高度有序的 Langmuir 单层样品成为可能。最近，人们对 Langmuir 单层的流变学重新产生了兴趣，部分原因是其液态凝聚 (LC) 相的阐明。 LC 相是三维近晶液晶的二维类似物。 它们具有六方有序，并且在分子相对于表面倾斜的阶段中，倾斜方位角表现出取向顺序。 由于 LC 相在 Langmuir 单分子层中普遍存在，因此了解其流变学与涉及表面活性剂单分子层在界面处流动的一系列过程相关，包括泡沫排水和乳液稳定性。 此外，泡沫、乳液和胶体悬浮液在受到外部剪切力作用下的宏观粘弹性行为通常强烈依赖于它们的界面性质。有关液晶相粘弹性行为的两个基本问题仍未得到解答：介观结构对粘度的贡献是什么，以及对粘度的主要微观贡献是什么？ 这里提出的工作重点是拓扑对液晶相粘度的贡献。 LC 相通常由 100 mm 数量级的随机取向区域组成。 每个域对应于一个统一顺序的区域。 我们建议研究域动力学的贡献，并将研究域之间的耗散对测量粘度的影响。 此外，外部剪切对域结构和拓扑的影响。 由于朗缪尔单分子层是二维的，因此可以直接观察域动力学。 相比之下，由域组成的三维系统是不透明的，必须间接探测域动态。 这是使用朗缪尔单层膜研究流动行为的一个显着优势。 除了界面流变学的重要性之外，泡沫和乳液的粘弹性特性通常由构成系统的域或气泡的拓扑结构决定。 除了我们对液晶相域动力学的研究之外，我们还将使用朗缪尔单分子层研究二维气液泡沫的流动。 建议解决有关气泡动力学的许多问题。 应力和应变或剪切率之间有什么关系？ 人们能否定义流动泡沫的有效“温度”？ 人们是否观察到泡沫的剪切熔化？ 该计划获得的见解预计将推广到存在类似问题的三维系统。

ABSTRACTCTS-9874701Michael Dennin/U. Cal. @ BerkeleyThe flow behavior of Langmuir monolayers using a combination of optical (Brewster angle microscopy) and rheological techniques is proposed. The rheological mesurements will be made using a newly developed Langmuir trough that is based on a Couette viscometer. This proposal consists of two sets of related experiments: a study of the intrinsic flow behavior of Langmuir monolayers; and a study of flow in monolayer foams as a model for flow in three-dimensional foams and emulsions. The common theme in both of these sets of experiments is the contribution of topology to rheology.Langmuir monolayers are intrinsically two-dimensional and consist of amphiphilic molecules that are confined to the air-water interface. The Couette trough we have developed consists of two concentric cylinders that are oriented vertically. The inner cylinder is fixed and the outer cylinder is free to rotate. The top surface of the water is free, and the monolayer is placed on this surface. The outer cylinder is composed of an elastic band that is used for compression and expansion of the monolayer. The inner cylinder consists of two parts. A stationary cylinder in the water subphase, and a torsion pendulum that just makes contact with the water surface. The dc viscosity is measured by rotating the outer cylinder and measuring the stress on the inner cylinder with the torsion pendulum. The ac viscosity is measured by holding the outer cylinder fixed and oscillating the torsion pendulum. Additional information about the flow properties of the monolayer is obtained by direct observation of velocity profiles and domain dynamics with a Brewster angle microscope. Also, by continuously rotating the outer cylinder, shear alignment of the monolayer makes it possible to obtain highly ordered samples of Langmuir monolayers.Recently, there has been a renewed interest in the rheology of Langmuir monolayers, in part, due to the elucidation of their liquid condensed (LC) phases. The LC phases are two-dimensional analogs of three-dimensional smectic liquid crystals. They posses hexatic order, and in phases where the molecules are tilted with respect to the surface, the tilt azimuth exhibits orientational order. Because LC phase are ubiquitous in Langmuir monolayers, understanding their rheology has relevance to a range of processes that involve surfactant monolayer flow at interfaces, including foam drainage and emulsion stability. Furthermore, the macroscopic viscoelastic behavior of foams, emulsions, and colloidal suspensions subjected to external shear forces is often strongly dependent on their interfacial properties.Two fundamental questions regarding the viscoelastic behavior of LC phases remain unanswered: what is the contribution of the mesescopic structure to the viscosity, and what is the dominant microscopic contribution to the viscosity? The work proposed here forcuses on the contribution of topology to the viscosity of the LC phases. The LC phases are generally composed of randomly oriented domains on the order of 100 mm. Each domain corresponds to a region of uniform order. We propose to study the contribution of the domain dynamics and the dissipation between the domains to the measured viscosity will be investigated. Also, the effect of external shear on the structure and topology of domain. Because Langmuir monolayers are two-dimensional, the domain dynamics are directly observable. In contrast, systems in three-dimensional that are composed of domains are opaque, and the domain dynamics must be probed indirectly. This is a significant advantage of studying advantage of studying flow behavior using Langmuir monolayers. In addition to the importance of interfacial rheology, the viscoelastic properties of foams and emulsions are often dominated by the topology of the domains, or bubbles, that comprise the system. In addition to our studies of the domain dynamics in the LC phase we will also study the flow of two-dimensional gas-liquid foams using Langmuir monolayers. It is proposed to address a number of questions concerning the dynamics for the bubbles. What is the relation between stress and strain or shear rate? Can one define an effective "temperature" for a flowing foam? Does one observe shear melting of the foam? Insights gained by this program are expected to generalize to three-dimensional systems where similar questions exist.