Universality Classes for Interface Growth with Quenched Disorder

We present numerical evidence that there are two distinct universality classes characterizing driven interface roughening in the presence of quenched disorder. The evidence is based on the behavior of $\lambda$, the coefficient of the nonlinear term in the growth equation. Specifically, for three of the models studied, $\lambda \rightarrow \infty$ at the depinning transition, while for the two other models, $\lambda \rightarrow 0$.