  We employ a modified cascade hydrodynamics code to simulate the phase
transition of an expanding quark-gluon plasma and the passage of a charmed
particle through it. When inside the plasma droplets, the charmed quark
experiences drag and diffusion forces. When outside the plasma, the quark
travels as a $D$ meson and experiences collisions with pions. Additional energy
transfer takes place when the quark enters or leaves a droplet. We find that
the transverse momentum of $D$ mesons provides a rough thermometer of the phase
transition.
