For over 50 years, plastic deformation in crystals has been
understood to be governed by dislocations. Despite several decades of
work, a coherent picture of plasticity in amorphous materials has only
recently begun to emerge. This picture is based on local zones which
are particularly susceptible to shear; so-called shear transformation
zones (STZs). The visco-plastic response of a host of amorphous
solids ranging from metallic glasses, to granular materials, to soft
matter such as disordered emulsions or colloidal glasses are believed
to be governed by this same underlying STZ mechanism. At the same
time, purely repulsive systems, like granular materials, can lose
rigidity below a certain packing density: the so-called jamming
transition. It is not at all clear how the nature of the plastic
response in repulsive systems changes on approach to the jamming
transition as the system loses its shear modulus and yield stress.
I will discuss computer simulations of various model amorphous solids
under shear which probe the complex spatio-temporal organization
exhibited by these STZs. In particular, I will show how correlated
avalanches of shear zones can give rise to a system-size dependent
diffusion constant in sheared 2D Lennard-Jones glasses and how this
picture is modified for model bubble rafts near their jamming
transition. |