Fig. 15.  Interface instabilities under nonhydrostatic stress.  (a) Energetically-driven interface instability.  It has long been known that a corrugation will relieve elastic strain energy in the "tips" of the ripples (the so-called Asaro-Tiller-Grinfeld instability).  (b) We have proposed a new, kinetically-driven interface instability.  Under compressive in-plane stress, stress concentrations in the grooves slow down the motion of the grooves relative to the peaks, causing the corrugations to grow.  Note that in tension, (a) still leads to an instability whereas (b) leads to stability.