  In studies of the dynamic failure of brittle hydrogels, a bound has been
placed on the process zone scale - the scale where material separation and
ultimate failure occur. For the polyacrylamide hydrogel system under study,
this bound is set at 20 microns. Thus, any subtle alterations to the material
at a \emph{smaller} scale should not in principle alter the dynamic fracture
response of the hydrogel. Here we test this directly by embedding
sub-micron-scale latex polystyrene microspheres within the brittle
polyacrylamide hydrogel at a solids fraction of 0.1 \%. We verify that the
spheres are well-distributed throughout the hydrogel material at this
concentration with optical microscopy, and reconstruct the 3D distribution of
these spheres using laser scanning confocal microscopy in backscatter mode.
Finally, we test the fracture behavior of this gel with the dilute, embedded
sub-micron spheres, and find that the brittle material failure modality common
to this material \emph{without} the sub-micron spheres is indeed retained. By
comparing the crack tip opening displacement, fracture energy and the crack's
speed with established data from prior experimental work, we demonstrate that
this material's failure is brittle, as it is in good agreement with the pure
hydrogel system.
