  We analyze the properties of naturally formed nano-bubbles in Lennard-Jones
molecular dynamics simulations of liquid-to-vapor nucleation in the boiling and
the cavitation regimes. The large computational volumes provide a realistic
environment at unchanging average temperature and liquid pressure, which allows
us to accurately measure properties of bubbles from their inception as stable,
critically sized bubbles, to their continued growth into the constant speed
regime. Bubble gas densities are up to 50$%$ lower than the equilibrium vapor
densities at the liquid temperature, yet quite close to the gas equilibrium
density at the lower gas temperatures measured in the simulations: The latent
heat of transformation results in bubble gas temperatures up to 25$%$ below
those of the surrounding bulk liquid. In the case of rapid bubble growth -
typical for the cavitation regime - compression of the liquid outside the
bubble leads to local temperature increases of up to 5$%$, likely significant
enough to alter the surface tension as well as the local viscosity. The
liquid-vapor bubble interface is thinner than expected from planar coexistence
simulations by up to $50%$. Bubbles near the critical size are extremely
non-spherical, yet they quickly become spherical as they grow.
