Experiments were performed using the laser extinction method to directly measure the thickness of the liquid film between the bubbles and heating plates in a mini-gap formed by two parallel vertical quartz glass plates for the dielectric fluid HFE-7200 at gap sizes of 0.15, 0.3 and 0.5mm. High-speed movies were also taken to measure the velocity of bubble forefront simultaneously. It was confirmed that the microlayer thickness was determined by the gap size and velocity of bubble forefront. At the region of small Weber number, microlayer thickness of HFE-7200 was obviously thicker than that of water, toluene and ethanol at the same velocity and gap size for its small surface tension. Furthermore, by nondimensional analyzing of experimental data, the empirical correlation proposed in previous study which was based on water, toluene and ethanol is still reliable for HFE-7200.

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