Increasing electrification of mechanically controlled or driven systems has created a demand for the development of compact, lightweight electronics. Removing waste heat from these high volumetric and gravimetric power dense assemblies, especially in mobile applications, requires non-traditional thermal management strategies with high heat flux potential and low integration penalty. Here, we develop and study confined subcooled pool boiling on nanoengineered surfaces which enables self-assembly of liquid bridges capable of high heat flux dissipation without external pumping. Using high-speed optical imaging coupled with high-fidelity heat transfer experiments in pure vapor environments, we study the physics of liquid bridge formation, bridge lifetime, and heat transfer. We demonstrate heat flux dissipations >100 W/cm(2) from a gallium nitride (GaN) power transistor residing above a horizontally parallel superhydrophobic nanostructured aluminum cold plate. To understand the confined bridge dynamics, we develop a hydrodynamic droplet bridging model and design rules capable of predicting the effects of gravity, intrinsic contact angle, contact angle hysteresis, and device heat flux. Our work not only demonstrates an ultra-efficient mechanism of heat dissipation and spreading using nanoengineered surfaces coupled to fluid confinement, but also enables the development of fully three-dimensional integrated electronics. (C) 2018 Elsevier Ltd. All rights reserved.