Measurements of monolayer hydrodynamics at an air/water interface

Growing interest in monomolecular films is driven in part by their numerous applications, which include coating technologies, chemical and bio-sensors, and optoelectronic devices. In the present research, a study involving several different experiments has focused on an improved understanding and quantification of the physics of monolayer-influenced flows. Measurements were made with laser-based nonintrusive techniques, including boundary-fitted digital particle image velocimetry (BFDPIV) to obtain interfacial velocity and shear data, and reflected second-harmonic generation (SHG) to directly measure surfactant concentration at the interface.; A simple geometry consisting of uniform bulk flow and a planar surface-piercing barrier which resulted in the phenomenon commonly referred to as a Reynolds ridge was used to study the elasticity of a monolayer. A novel technique was developed in which velocity and surfactant concentration measurements are made simultaneously with a single laser beam which is scanned along the interface. Additionally, a theoretical model balancing surface elasticity and bulk shear at the interface was developed to predict the concentration profile for any insoluble monolayer. The predicted concentration profiles were found to be in agreement with experimental results. Additionally, global predictions from the model for four different insoluble surfactant systems also showed agreement with experimental measurements.; In order to study the interfacial dilatational viscosity (κ s) of a monolayer, for which there are no consistently measured values in the literature, a cavity flow was utilized in which the floor oscillates in the direction parallel to itself. Initially, a baseline study was performed to establish the range of parameters for which the flow is essentially two-dimensional (2D). Three flow regimes were found in the parameter space considered: an essentially 2D time-periodic flow, a time-periodic three-dimensional (3D) flow with a cellular structure in the spanwise direction, and a 3D irregular flow (in both space and time). The surface motion was then measured and compared to (2D) computational results in order to determine values of κ s for different concentrations of surfactant systems. Departures from a Newtonian interfacial behavior (Boussinesq-Scriven surface model) were found in several instances.