Studies of Interfacial Flow Behavior Using Langmuir Monolayers

使用 Langmuir 单层膜研究界面流动行为

• 批准号: 9874701
• 负责人: Michael Dennin
• 金额: $ 10万
• 依托单位: University of California-Irvine
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-07-01 至 2000-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9874701&HistoricalAwards=false
• 关键词: Studies; Interfacial; Flow; Behavior; Using

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.

摘要-9874701 Michael Dennin/U. @ BerkeleyThe流动行为的朗缪尔单分子膜，使用光学（布鲁斯特角显微镜）和流变学技术的组合提出。 将使用基于库埃特粘度计的新开发的朗缪尔槽来制备流变测量仪。 该建议包括两组相关的实验：朗缪尔单分子层的固有流动行为的研究;和单层泡沫中的流动作为三维泡沫和乳液中的流动模型的研究。 这两组实验的共同主题是拓扑学对流变学的贡献。Langmuir单分子膜本质上是二维的，由局限于空气-水界面的两亲分子组成。 我们开发的库埃特槽由两个垂直取向的同心圆柱体组成。 内筒是固定的，外筒可以自由旋转。 水的上表面是自由的，并且单分子层被放置在该表面上。 外筒由用于压缩和膨胀单层的弹性带组成。 内筒由两部分组成。 一个静止的圆柱体在水的亚相中，一个扭摆刚好与水面接触。 直流粘度通过旋转外筒并用扭摆测量内筒上的应力来测量。 交流粘度是通过保持外圆筒固定并摆动扭摆来测量的。 通过直接观察速度分布和域动态与布鲁斯特角显微镜的单层的流动特性的其他信息。 此外，通过连续旋转外圆柱体，单层的剪切排列使得有可能获得高度有序的样品的Langmuir monolayers.Recently，有一个新的兴趣在流变学的Langmuir monolayers，在一定程度上，由于其液体凝聚（LC）相的阐明。LC相是三维近晶液晶的二维类似物。 它们是六角晶系有序的，并且在分子相对于表面倾斜的相位中，倾斜方位角表现出取向有序。 由于LC相在朗缪尔单分子膜中普遍存在，因此了解其流变学与涉及界面处表面活性剂单分子膜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外部剪切力作用下的宏观粘弹行为往往强烈依赖于它们的界面性质，关于液晶相粘弹行为的两个基本问题仍然没有答案：介观结构对粘度的贡献是什么，以及占主导地位的微观结构对粘度的贡献是什么？ 本文的工作是研究拓扑结构对液晶相粘度的影响。 LC相通常由约100 mm的随机取向的畴组成。每个畴对应于均匀有序的区域。 我们建议研究的贡献域动力学和域之间的耗散测量粘度将进行调查。 研究了剪切力对畴结构和拓扑的影响. 因为朗缪尔单分子膜是二维的，域动力学是直接观察到的。 相比之下，由域组成的三维系统是不透明的，域动力学必须间接探测。 这是使用朗缪尔单分子膜研究流动行为的一个显著优势。 除了界面流变学的重要性之外，泡沫和乳液的粘弹性通常由构成系统的域或气泡的拓扑结构支配。 除了我们在LC相的域动力学的研究，我们也将研究二维气液泡沫使用朗缪尔单分子膜的流动。 建议解决一些关于气泡动力学的问题。 应力和应变或剪切速率之间的关系是什么？ 可以定义流动泡沫的有效“温度”吗？ 是否观察到泡沫的剪切熔化？ 通过这个程序获得的见解预计将推广到三维系统中存在类似的问题。