The mechanics and stability of liquid jets and films.

Abstract

The underlying objective of this thesis is to study the process of coating by jet atomization, whose mechanism is divided into the mechanics and stability of liquid jets and the dynamics of thin films. The first step in the analysis of the mechanics of liquid jets is to examine the two-dimensional formation of non-Newtonian liquid jets. Steady-state, free-surface profiles and the subsequent motion of unstable surfaces are analyzed by the Galerkin finite-element method with free-surface parameterization. A constitutive equation is used to approximate Bingham liquids, which incorporates the effects of shear thinning. Increased surface tension and shear thinning destabilize round jets and increase the size of satellite drops, while increased yield stress decreases the size of satellite drops. The increase in satellite drop size is a response to the increased growth rate of disturbances on the free surface. Inertial effects prevent liquid from leaving regions that form satellites under the more rapid advance of disturbances. Nonlinear analysis show that the most unstable wavelength is closely approximated by linear theory. However, the dynamics near jet breakup are controlled by nonlinear effects. Two-dimensional studies are continued with the analysis of electrohydrodynamic jet breakup. An applied electric field will increase the size of the satellite drop formed, despite an increase in breakup time. The contradiction is due to the pressure gradient induced in the jet near the point of disintegration, which causes slowly propagating disturbances to advance rapidly in the final stages of the process. Three-dimensional jets are also analyzed using a unique streamlined finite-element method. The effects of inertia, gravity, and nozzle geometries are considered when producing steady-state jets. Also considered is the problem of die design where the die (or nozzle) shape is determined to produce a jet with a specified final shape, which is important to the extrusion of polymers. The next step of the study of coatings is an analysis of the spreading and leveling of thin films. The constitutive equation used is the modified Oldroyd-B model. Asymptotic analysis, which is exact for long wavelengths of small disturbance amplitudes, is compared with direct nonlinear finite-element analysis. Increased relaxation time decreases the rate of leveling and spreading, enhances surface defects, while increased retardation time reverses the effects of increased relaxation time.