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An understanding of the fundamentals of the boiling mechanism is essential if phase-change liquid immersion cooling is to emerge as a cooling option for the next generation of data servers and other high performance electronics. The present work is an experimental study on the effect of cavity mouth size on the nucleation characteristics of a single isolated micro-pyramidal reentrant cavity. Cavity mouth size has long been known to be a primary variable determining the ebullition characteristics of microscopic structures present on a pool boiling heat sink surface. Isolated pyramidal shaped cavities with square mouth sizes of 7 μm, 19 μm and 25 μm etched in polished silicon using an anisotropic etch were evaluated in this study. Serpentine thin film heaters (6.9 mm×6.9 mm) deposited on a Borofloat® glass substrate and anodically bonded to the silicon cavity section served as heat sources. All experiments were conducted at atmospheric pressure in dielectric fluids, HFE-7100 and FC-72, popular in thermal management applications. High speed photography (up to 600 frames/sec) was used to record and quantify the effect of heat flux on bubble departure frequency and departure diameters under subcooled and saturated conditions. The bubble departure diameters increased with an increase in the cavity mouth size. Frequency and bubble departure size, both decreased with increased subcooling.