  Cessation of flow in simple yield stress fluids results in a complex stress
relaxation process that depends on the preceding flow conditions and leads to
finite residual stresses. To assess the microscopic origin of this phenomenon,
we combine experiments with largescale computer simulations, exploring the
behavior of jammed suspensions of soft repulsive particles. A spatio-temporal
analysis of microscopic particle motion and local particle configurations
reveals two contributions to stress relaxation. One is due to flow induced
accumulation of elastic stresses in domains of a given size, which effectively
sets the unbalanced stress configurations that trigger correlated dynamics upon
flow cessation. This scenario is supported by the observation that the range of
spatial correlations of quasi-ballistic displacements obtained upon flow
cessation almost exactly mirrors those obtained during flow. The second
contribution results from the particle packing that reorganize to minimize the
resistance to flow by decreasing the number of locally stiffer configurations.
Regaining rigidity upon flow cessation then effectively sets the magnitude of
the residual stress. Our findings highlight that flow in yield stress fluids
can be seen as a training process during which the material stores information
of the flowing state through the development of domains of correlated particle
displacements and the reorganization of particle packings optimized to sustain
the flow. This encoded memory can then be retrieved in flow cessation
experiments.
