Studies of Interfacial Flow Behavior Using Langmuir Monolayers

摘要-9874701迈克尔·丹宁/加州大学。@Berkeley结合光学(布鲁斯特角显微镜)和流变学技术研究朗缪尔单分子膜的流动行为。流变学测量将使用基于Couette粘度计的新开发的朗缪尔槽进行。该方案由两组相关实验组成：朗缪尔单分子层本征流动行为的研究，以及作为三维泡沫和乳状液流动模型的单层泡沫流动的研究。这两组实验的共同主题是拓扑对流变学的贡献。朗缪尔单分子膜本质上是二维的，由限制在空气-水界面的两亲性分子组成。我们开发的Couette槽由两个垂直定向的同心圆柱体组成。内筒是固定的，外筒是自由转动的。水的顶面是自由的，单层放置在这个表面上。外筒由用于压缩和展开单层的弹性带组成。内筒由两部分组成。水相中有一个静止的圆柱体，还有一个正好与水面接触的扭转摆。直流粘度的测量是通过旋转外筒并用扭摆测量内筒上的应力来实现的。交流粘度是通过固定外筒并摆动扭转摆来测量的。通过用布鲁斯特角显微镜直接观察速度分布和磁区动力学，获得了关于单分子膜流动特性的其他信息。此外，通过不断旋转外筒，单分子膜的剪切对准使获得高度有序的朗缪尔单分子膜样品成为可能。最近，人们对朗缪尔单分子膜的流变学重新产生了兴趣，部分原因是对其液体凝聚(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，在分子相对于表面倾斜的相中，倾斜方位显示出取向顺序。由于LC相在朗缪尔单分子膜中普遍存在，因此了解其流变性与涉及表面活性剂单分子层界面流动的一系列过程相关，包括泡沫排水和乳状液稳定性。此外，泡沫、乳液和胶体悬浮液在外部剪切力作用下的宏观粘弹性行为往往强烈依赖于它们的界面性质。关于LC相的粘弹性行为的两个基本问题仍然没有得到回答：介观结构对粘度的贡献是什么，以及主要的微观贡献是什么？本文的工作强调了拓扑结构对LC相粘度的贡献。LC相通常由100 mm量级的随机取向的磁区组成。每个域对应于一个具有统一顺序的区域。我们建议研究磁区动力学的贡献，并将研究磁区之间的耗散对测量粘度的影响。此外，还讨论了外剪切力对电畴结构和拓扑的影响。因为朗缪尔单分子膜是二维的，所以磁区动力学是直接可见的。相反，由领域组成的三维系统是不透明的，必须间接地探查领域的动力学。这是利用朗缪尔单分子膜研究流动行为的显著优势。除了界面流变学的重要性外，泡沫和乳液的粘弹性特性通常由组成体系的区域或气泡的拓扑结构决定。除了研究液晶相区的磁区动力学外，我们还将利用朗缪尔单分子膜来研究二维气-液泡沫的流动。建议解决与气泡动力学有关的一些问题。应力和应变或剪切率之间的关系是什么？人们能为流动的泡沫定义一个有效的“温度”吗？有没有观察到泡沫的剪切融化？通过这个项目获得的见解有望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.