Towards using nanoelectrospray for evaporative heat transfer enhancement

For a number of demanding applications, the performance of electronic devices is hampered by the inability to remove generated heat at a sufficient rate to increase transistor density or operating frequency without exceeding thermal limits. Two-phase cooling, in particular thin film evaporation, exploits the latent heat of phase change to provide an effective means for high heat flux dissipation while keeping device junction temperatures nearly constant over a range of heating loads. Delivering a coolant at a precise location on the heated surface and forming a thin film needed for efficient evaporation from a free liquid surface are key requirements for using evaporative cooling for thermal management of spatially non-uniform heat fluxes. The electrospray process enables production and delivery of micro to nanoscale electrically charged droplets towards the heated surface to produce the liquid films, and therefore it has a potential to be a promising method for evaporative cooling. A relatively low power consumption needed to generate the electrospray provides further benefits in terms of energy efficiency as compared to conventional mechanically pumped liquid spray approaches. We report on experimental observations of electrosprayed droplet impingement, coalescence, and film formation on a heated ITO (Indium Tin Oxide) surface acting as an optically transparent heating element, focusing primarily on visualization and mapping of the impacting spray-jet behavior and its impact on the resulting film thickness and shape, which directly affect an expected performance of evaporative cooling.