Explain the working principle of a heat pipe and its applications.

A heat pipe is a passive two-phase heat transfer device consisting of a sealed container with working fluid and wick structure. At the evaporator end, heat vaporizes the fluid; vapor travels to the cooler condenser end where it liquefies, releasing latent heat. The wick (sintered, grooved, or mesh) returns liquid to the evaporator via capillary action. Heat pipes offer extremely high effective thermal conductivity (10,000-100,000 W/m-K), are passive with no moving parts, and work against gravity with proper wick design. Applications include laptop/smartphone cooling, satellite thermal control, permafrost stabilization, and CPU heat sinks.