  Shear transformations, as fundamental rearrangement events operating in local
regions, hold the key of plastic flow of amorphous solids. Despite their
importance, the dynamic features of shear transformations are far from clear.
Here, we use a colloidal glass under shear as the prototype to directly observe
the shear transformation events in real space. By tracing the colloidal
particle rearrangements, we quantitatively determine two basic properties of
shear transformations: local shear strain and dilatation (or free volume). It
is revealed that the local free volume undergoes a significantly temporary
increase prior to shear transformations, eventually leading to a jump of local
shear strain. We clearly demonstrate that shear transformations have no memory
of the initial free volume of local regions. Instead, their emergence strongly
depends on the dilatancy ability of these regions, i.e., the dynamic creation
of free volume. More specifically, the particles processing the high dilatancy
ability directly participate in subsequent shear transformations. These results
experimentally support the Argon's statement about the dilatancy nature of
shear transformations, and also shed insight into the structural origin of
amorphous plasticity.
