The hydrodynamic formation of a microlayer at the base of a vapor bubble growing on a heated wall was experimentally and theoretically studied. Single bubble nucleate boiling experiments were conducted in a pool of saturated water under atmospheric pressure. A complete picture of the bubble geometry was obtained, including the three-phase contact line, microlayer and macroscopic bubble. This was performed using integrated laser interferometry, infrared thermometry and shadowgraph techniques. Existing models of the initial microlayer thickness that use an idealized hemispherical bubble shape and neglect surface tension force significantly overestimate microlayer thickness measured in experiment. The visualization results revealed that the non-hemispherical shape and surface tension of the bubble play a critical role in determining initial microlayer thickness. Theoretical analysis also indicated that the short-lived residual flow could play a key role in microlayer formation. Finally, a sophisticated model of initial microlayer thickness, developed to take into account the three identified mechanisms, provides predictions in agreement with experiments for different fluids. (C) 2017 Elsevier Ltd. All rights reserved.