Studies of Interfacial Flow Behavior Using Langmuir Monolayers

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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.