Boiling Bifurcations and Hysteresis in Solid-state Nanopores

Boiling is a complex nonequilibrium dissipative process, which undergoes thermodynamic bifurcations and hysteresis. Although pool boiling has been widely studied, much less is known about thermodynamics at nanoscale, where imaging techniques suffer due to the diffraction limit. To fill this knowledge gap, we utilize the nanopore Joule heating system that enables the generation of nanobubbles and simultaneous diagnosis of their nanosecond resolution dynamics using resistive pulse sensing. When a bias voltage is applied across a silicon nitride nanopore immersed in aqueous salt solutions, Joule heat is generated owing to the flow of ionic current. With increasing voltage, the superheat within nanopore increases, leading to the first bifurcation point with the onset of periodic nucleate boiling. Upon further increasing the voltage, transition boiling commences with torus-shaped vapor films intermittently blanketing the pore surface. Finally, a stable vapor film is formed beyond a threshold voltage, marking the second bifurcation point. These boiling structures are summarized in the form of an 'M'-shaped boiling curve, experimentally obtained from the Joule heat variation with applied voltage. We also studied the hysteresis of bifurcation points for increasing and decreasing bias voltages at different voltage rise rates and pore sizes. These results offer insights into the nonlinear characteristics of nanoscale boiling, paving the way for further research using stochastic thermodynamics.