Practical engines such as liquid rockets and diesel engines operate at high pressures. As a result, the injected fuel may be at transcritical or supercritical state during the injection, mixing and vaporization processes. Previous studies have provided important insights on supercritical behaviors, as well as limitations of current theoretical modeling and numerical simulations of such behaviors.  The introduction of thermodynamic nonidealities and transport anomalies near the critical point is one of the main challenges to model these phenomena. Very limited experimental data are available for either quantitative understanding of the physics or for assessment of various models.  Although we have good knowledge of the classical two-phase atomization regime and the supercritical one-phase mixing regime, the transition from the former to the latter is less understood.  This research intends to understand the physics of fuel injection, mixing and vaporization at high pressures by applying novel diagnostic methods and molecular dynamics simulations.