Coat a surface with a copper-magnesium alloy (amalgam?).<p>Then remove the magnesium. The microscopic voids in the solid material, where the magnesium used to be, dramatically increase the effective area of the copper.<p>This caught my attention as I was thinking about making my own vapor chambers for heat sinks. A vapor chamber is basically a sealed tube of copper with a rough interior surface, charged with a few drops of distilled water.<p>I still haven't figured out how to make such things, but it might lead to a fun project.