Dynamics and Stability of Capillary Surfaces

Capillary surfaces are gas/liquid interfaces whose shape is primarily determined by surface tension. Change of shape can drive an underlying flow. Alternatively, flow can induce shape change. Capillary surface dynamics are important to a broad range of forming and coating operations, including immersion lithography in semiconductor manufacturing, microfluidic drop manipulation in lab-on-chip platforms and low-gravity two-phase flow configurations.

*Courtesy of A. Davidhazy.

Rapid Solidification Processing

The planar flow casting (PFC) process is a single-stage metal casting process, able to rapidly solidify metal from the molten state into thin, continuous metal ribbons. These ribbons have a thickness often less than 100 µm and can be produced at high speeds, often in excess of 20 m/s. Liquid metal is forced through a nozzle held in proximity to a rotating copper wheel. This proximity increases the capillary forces in the liquid metal, holding it in place and differentiating PFC from other continuous casting processes, such as melt-drag casting. It’s within this puddle region that solidification of the molten metal takes place; as molten metal is fed to this region, solid metal is removed at an equal rate. The balance of feed rate and solidification rate is of critical importance. Solidification rate is, in part, determined by the heat transfer at the wheel; the copper wheel is used as a rotating heat sink, pulling the latent and sensible heat from the liquid metal and encouraging solidification, often cooling at rates in excess of 10^5 K/s. Depending on the alloy composition, solidification will result in either an amorphous (”glassy”) or crystalline metal product. Our goal is to understand the processing of these materials by examining the heat flow in each of the systems, revealing their inherent differences and similarities. Ultimately, the goal is to use the knowledge of the heat and fluid transport to predict (and ultimately control) nanocrystalline features in the ribbon.

(a) Puddle images of crystalline (top) and noncrystalline (bottom) alloys                                                  (b) Schematic of the PFC process with inset of the puddle              with length scales noted                                                                                                                                                           region highlighted

Last edited 5/03/2019