A Study on Boiling inside Nanopores through Localized Joule Heating*

We employ the nanopore bubble generation platform to study post-nucleation explosive boiling at the single cavity limit, having dimensions below the Abbe diffraction limit. When a voltage bias is applied across a thin nanopore filled with electrolyte solution, localized Joule heating accumulates superheat in the liquid within the pore volume. Nucleate, transition, and film boiling structures within nanopores are examined at nanosecond resolutions using acoustic and resistive pulse sensing. During nucleate boiling, a homogeneous bubble nucleates at the pore center followed by its departure. During film boiling, a heterogeneous nano-torus vapor bubble blankets the cylindrical pore surface and undergoes pinned volumetric oscillations in thermal resonance with the Joule heating. As the voltage is increased, nucleate boiling within nanopore transitions towards stable film boiling, wherein intermittent torus bubble oscillations are observed during the transition regime. However, beyond a critical voltage, the nanopore undergoes a reverse transition from film boiling to nucleate boiling. This contrasting boiling structure is theoretically explained through the pinning effect, wherein excess Joule heat production leads to destabilization and collapse of the nano-torus bubble.