  When glassy materials are rapidly quenched from the liquid to the solid state
upon flow cessation or cooling, they solidify in an out-of-equilibrium
configuration, retaining the memory of the processing conditions for very long
times. This is the origin of various phenomena, such as residual stresses and
directional memory, which greatly affect their properties. At the same time,
annealing the mechanical history encoded in disordered materials constitute a
great challenge. Here, we address this problem for the case of colloidal
glasses made of soft particles densely packed at a high volume fraction, using
experiments and particle dynamic simulations. We demonstrate that periodically
training soft particle glasses with a sequence of stress-controlled
oscillations successfully anneals residual stress and directional memory when
the stress amplitude corresponds to the yield point. At the microscopic level,
annealing provides a fine tuning of the local distribution of the stress
carried by the particles. Through the simulations, we show that the first
moments of this distribution have precise physical meaning: the mean value of
the distribution corresponds to the macroscopic stress; the skewness carries
information about directional memory; and the standard deviation is related to
mechanical aging. The same methodology is successfully applied to silica gels
with thixotropic properties, suggesting that it is general and may be extended
to other classes of disordered materials.
