when I say "heat pipe" I mean a tube that is filled with a working fluid that is under a sufficient vacuum to be very near its boiling point. like this. when the working fluid at the bottom boils, the vapor fills the tube and begins to condense wherever the tube is cooler, which creates a very fast transfer of heat.

it seems like they rely on gravity to pull the condensed working fluid back to the hot end at the bottom.

I'm curious if there is a way to allow it to work in reverse, where the top side is hotter and the bottom side is cooler, while still being totally passive.

obviously pumping a working fluid in a circuit would work, but I'm wondering if there is any clever method to do it passively.