  We study a strongly interacting dense hard-sphere system confined between two
parallel plates by event-driven molecular dynamics simulations to address the
fundamental question of the nature of the 3D to 2D crossover. As the fluid
becomes more and more confined the dynamics of the transverse and lateral
degrees of freedom decouple, which is accompanied by a diverging time scale
separating 2D from 3D behavior. Relying on the time-correlation function of the
transversal kinetic energy the scaling behavior and its density-dependence is
explored. Surprisingly, our simulations reveal that its time-dependence becomes
purely exponential such that memory effects can be ignored. We rationalize our
findings quantitatively in terms of an analytic theory which becomes exact in
the limit of strong confinement.
