Tomographic reconstruction of incompressible flow.

In many experiments facilitating tomography the reconstruction problem is under-determined, meaning there are many possible solutions consistent with the measurements. If the sampling rate is fast relative to the typical evolution time, the known physical dynamics of the system can be used as additional reconstruction constraints. Here we demonstrate that incorporating the requirement of incompressible flow can improve significantly the fidelity of the reconstructed sequence. The incompressibility of the reconstruction is assured by requiring the conservation of the density moments. It is demonstrated that the "incompressible" reconstruction can be significantly more accurate than the reconstruction using standard methods. A consequence of the density moments' conservation is the conservation of the density histogram throughout the reconstructed sequence.

Dislocations deflect and perturb dynamically propagating cracks.

We demonstrate that in single-crystal silicon short-range collisions of a dynamically propagating crack with stationary, intrinsic, "inclined" dislocations generate local crack deflections that grow to a large surface perturbation. Experiments show that when the crack collides with a single dislocation, the perturbation height is about 8 nm, but when it collides with a group of adjacent dislocations, the perturbation may extend to 80 nm in height (approximately 200/b/) and 250 microm in length, visible to the naked eye. A model was developed formulating the maximum velocity at which the crack climbs into the dislocation's core. The model predicts that when a dislocation's line is perpendicular to the crack surface, no interaction takes place.