Molten drop deposition and the dynamics of the molten contact line

Abstract

The development of many advanced manufacturing technologies requires a basic understanding of how molten materials spread over cold, solid surfaces. Of varticular interest are molten drop deposition processes in which the drops arrest by solidification before they reach an equilibrium configuration on the solid. Central to this spreading process is the molten dynamic contact line problem, which involves the relationship between the speed of the contact line and the apparent (visible to the observer) contact angle of the molten liquid. This thesis examines the deposition of millimeter-scale molten drops on cold, solid surfaces. Arrest angle data are obtained for solder drops deposited on oxidized solid solder, octacosane (C28 H58 ) on solid octacosane, and octacosane on glass coated with different types of self-assembled monolayers (SAMs). Direct measurements of the velocity-angle relationship are obtained for microcrystalline wax drops deposited on wax targets, both horizontal and inclined, and for water drops deposited on ice. The observations are consistent with Schiaffino 1:1.nd Sonin's (e.g. 1997, Phys. Fluids, 9 (8), 2217-2226 and 2227-2233, and 9 ( 11 ), 3172-3187) molten contact line model, except for the depositions on inclines, which reveal that the molten contact line may experience "stick­slip" motion and arrest with apparent contact angles that are inconsistent with existing empirical correlations. A theoretical model for quasi-steady molten contact line motion is developed using a simple piecewise approximation of the shape of the solidification front based upon Schiaffino and Sonin's calculations. By considering the flow field very near the contact line, the relationship between the apparent contact angle, the dimensionless advance speed, and other relevant system parameters is established. When the molten contact line i!. very close to arrest this relationship reduces to a simple analytical expression, denoted the near-arrest law. Considering its approximations, the model provides surprisingly good agreement with the experimental data, provided the data meet the requirements of quasi-steady, inertia free flow near the contact line.