dislocations appear as distinctive spiral features – thus revealing an important mechanism of crystal growth: where there is a surface step, atoms can more easily add to the crystal, and the surface step associated with a screw dislocation is never destroyed no matter how many atoms are added to it. Chemical etching When a dislocation line intersects the surface of a metallic material, the associated strain field locally increases the relative susceptibility of the material to acid etching and an etch pit of regular geometrical format results. In this way, dislocations in silicon, for example, can be observed indirectly using an interference microscope. Crystal orientation can be determined by the shape of the etch pits associated with the dislocations. If the material is deformed and repeatedly re-etched, a series of etch pits can be produced which effectively trace the movement of the dislocation in question. Dislocation forces Forces on dislocations Dislocation motion as a result of external stress on a crystal lattice can be described using virtual internal forces which act perpendicular to the dislocation line. The Peach-Koehler equation can be used to calculate the force per unit length on a dislocation as a function of the Burgers vector, , stress, , and the sense vector, . The force per unit length of dislocation is a function of the general state of stress, , and the sense vector, . The components of the stress field can be obtained from the Burgers vector, normal stresses, , and shear stresses, . Forces between dislocations The force between dislocations can be derived from the energy of interactions of the dislocations, . The work done by displacing cut faces parallel to a chosen axis that creates one dislocation in the stress field of another displacement. For the and directions: The forces are then found by taking the derivatives. Free surface forces Dislocations will also tend to move towards free surfaces due to the lower strain energy. This fictitious force can be expressed for a screw dislocation with the component equal to zero as: where is the distance from free surface in the direction. The force for an edge dislocation with can be expressed as: References External links Defects in Crystals/ Prof. Dr. Helmut Föll website Chapter 5 contains a wealth of information on dislocations; DoITPoMS Online tutorial on dislocations, including movies of dislocations in bubble rafts; Difference between Edge dislocation and Screw dislocation Difference