Capillary Surface Instability
Project: Walking Drop Instability and the Schrodinger Equation
Solution of a Schrodinger-like equation for the behavior of drops on solid surfaces reveals the 'walking instability' among other motions. According to this instability, the energy stored in the liquid shape can be converted into the energy of liquid motion representing a heretofore unknown pathway of energy conversion of potentially vast significance. To the extent that droplets on solid surfaces are found throughout our world, understanding and organizing their behaviors can serve many purposes including appreciating how nature works, engineering superior manufacturing processes and solving our pressing energy challenges. The mode shapes and frequencies of the motion of a free drop held by surface tension when viscosity is not important were determined by Rayleigh (1879). Still today Rayleigh's result serves as the central tool in interpreting meniscus motions , invoked for constrained and unconstrained cases, viscous ans inviscid situations. Sometimes it appears adequate, often it does not. We have solved for the behavior of the supported Rayleigh drop-inviscid motions of a drop constrained by a planar support-and have tested the predictions against experiment. The predictions are quantitatively accurate.
Figure 1. Walking Instability Schematic-sessile drop on planar solid substrate: (left) Renditions of 3D shapes (color) and contact line (gray), exaggerated to visualize. (right) Vertical sections through shapes.
Figure 2. Top view snapshots of observed mode shapes (first and third rows) and rendered mode shapes (second and fourth rows) for ten 'elements' of a periodic table of droplet motions. The rendered shapes are solutions of a Schrodinger-like equation. Shapes are classified by their number of layers n and number of sectors l as (n,l). Sectors are the number of points in the star-like pattern and layers are the number of superposed patterns.
Project: Manipulating Droplets during Condensation for Heat-Transfer Enhancement
Ashley Macner, a NASA Space Technology Research Grants Program researcher, is developing condenser surface design conditions for use in a two-phase heat transfer cycle on board a spacecraft to efficiently manage large waste heat loads in a low gravity environment. The key to condensing a large amount of liquid (i.e. removing the most heat) is to clear large drops from the surface as quickly as possible to make room for renucleation of small drops. Because heat transfer at any point in time is influenced by the size and the number of drops on a surface, understanding the effects of surface wettability is critical. The strategy then is to chemically functionalize a surface to yield a radial surface energy gradient (i.e. center is hydrophobic, perimeter is hydrophilic) that passively sweeps drops to a gutter without the aid of gravity. Populations of condensed drops on a uniformly treated surface (left) and a surface energy gradient surface (right) are shown in snapshots. Each photo is a 1 cm x 1 cm view field.