| Dynamic wetting, the displacement of one fluid by another immiscible fluid, is important in many natural phenomena and technological applications. Complexity and modeling difficulties arise due to the unique hydrodynamics which operate in the microscopic region near the contact line and control the macroscopic spreading of a liquid. Most previous experimental work measuring the detailed hydrodynamics near contact lines has focused on the spreading of fluids over bare solid surfaces. However, in many situations, fluids spread over surfaces with pre-coated thin films. In this thesis, we have moved to regimes not treated before where thick fluid films and thin molecular scale films exist near the contact line at higher velocities. We study the hydrodynamics using video microscopy and digital image analysis to measure the liquid/vapor interface shape in a small region near a contact line advancing over a pre-existing film. In the process of our investigations, we have also developed techniques for dip-coating and characterizing uniform molecular scale and micron scale films. Several models, which describe the interface shape close to the moving contact line in the presence of a film, are experimentally examined. Our studies reveal the hydrodynamic behavior of the fluid in the film and near the moving contact line and give the dependence of dynamic contact angle on contact line speed and film thickness in the advancing case. We have also studied effects of possible complex material properties of molecular scale films (101 A∼102 A) on the macroscopic spreading of the liquid. We have begun to develop a unified picture of the wetting behavior of liquids on dry surfaces, molecular scale films, and thick fluid films. |
