The above photo shows a few different heatpipe wick structures used with high temperature water-based heat pipes made by the company Advanced Cooling Technologies. These include, clockwise, axial grooves, mesh screen, sintered metal powders, sintered metal powder grooves (fine grooves), sintered slab and sintered metal powder grooves (large grooves).
Additionally, ACT have an interesting write up on heatpipe life tests, here. Extra info on water heatpipes, intermediate temp. & high temp heatpipes, vapor chambers, pressure controlled heatpiped, loop heat pipes here.This 30minute webinar on Heat Pipe basics is a good refresher if you've been confused on how a sealed metal tube conducts such a high amount of energy.At 8:20 the presenter talks about heat pipe limits and the effect Gravity has on the performance of these devices.
"The Capillary Limit is affected by gravity; it is the wicks' ability to overcome pressure drop in the system and return the working fliud to the heat source.
The largest pressure drop is typically caused by gravity. This will affect the maximum power the heatpipe can transfer, however it will not affect the thermal resistance of the heatpipe...."