On the mobility of dislocations intersecting {112} free surfaces in Cu

We perform atomistic simulations to study the motion of edge dislocations intersecting {112} free surfaces in Cu. For dislocations of length > 11 nm, intersections with the free surface exert a drag force that reduces mobility. This force increases with velocity and appears to diverge below the Rayleigh wave speed ($c_R)$. New dislocations emitted from the surface effectively prevent the intersections from exceeding $c_R$. For line lengths equal to or below 11 nm, dislocations additionally excite Lamb (bending) waves in the medium. Resonances with these waves generate new subsonic branches in the stress-velocity relation, giving rise to intermittent fluctuations in dislocation velocity at constant imposed strain rate. These findings exhibit qualitative differences to the behavior of dislocations under periodic boundaries, suggesting that experiments measuring mobilities from the displacements of dislocation intersections with free surfaces may not fully represent the behavior of dislocations within crystal interiors, especially at near-sonic velocities and above.