  The mechanical properties of crystalline materials can be substantially
modified under confinement. Such modified macroscopic properties are usually
governed by the altered microstructures and internal stress fields. Here, we
use a parallel plate geometry to apply a quasi-static squeeze flow crushing a
colloidal polycrystal while simultaneously imaging it with confocal microscopy.
The confocal images are used to quantify the local structure order and, in
conjunction with Stress Assessment from Local Structural Anisotropy (SALSA),
determine the stress at the single-particle scale. We find that during
compression, the crystalline regions break into small domains with different
geometric packing. These domains are characterized by a pressure and deviatoric
stress that are highly localized with correlation lengths that are half those
found in bulk. Furthermore, the mean deviatoric stress almost doubles,
suggesting a higher brittleness in the highly-confined samples.
