  Mechanical properties of disordered materials are governed by their
underlying free energy landscape. In contrast to external fields, embedding a
small fraction of active particles within a disordered material generates
non-equilibrium internal fields, which can help to circumvent kinetic barriers
and modulate the free energy landscape. In this work, we investigate through
computer simulations how the activity of active particles alters the mechanical
response of deeply annealed polydisperse colloidal gels. We show that the 'swim
force' generated by the embedded active particles is responsible for
determining the mode of mechanical failure, i.e., brittle vs. ductile. We find,
and theoretically justify, that at a critical swim force the mechanical
properties of the gel decrease abruptly, signaling a change in the mode of
mechanical failure. The weakening of the elastic modulus above the critical
swim force results from the change in gel porosity and distribution of
attractive forces among gel particles. Below the critical swim force, the
ductility enhancement is caused by an increase of gel structural disorder.
Above the critical swim force, the gel develops a pronounced heterogeneous
structure characterized by multiple pore spaces, and the mechanical response is
controlled by dynamical heterogeneities. We contrast these results with those
of a simulated monodisperse gel that exhibits a non-monotonic trend of
ductility modulation with increasing swim force, revealing a complex interplay
between the gel energy landscape and embedded activity.
