(a) Water boiling on a clean copper heater. Before CHF, a large mushroom of vapor is formed on an entire heater. At CHF, the large dry patch (spot) is observed in the large mushroom. At the edge of heater, the vapor film goes into the large dry path and meets with the large dry patch. Finally, the vapor film covers the entire heater. A film boiling is developed with the rapid wall temperature increase. (b) Water boiling on RGO flakes pre-coated copper heater. At the CHF, the wall temperature starts to increase slowly and the large dry patch can be observed in the large mushroom. Even though the wall temperature reaches to 209°C, the nucleate boiling is still maintained with the large dry patch in the large mushroom, contrary to the film boiling development in water boiling on a clean copper heater.