  Moir\'e superlattices in van der Waals (vdW) heterostructures have given rise
to a number of emergent electronic phenomena due to the interplay between
atomic structure and electron correlations. A lack of a simple way to
characterize moir\'e superlattices has impeded progress in the field. In this
work we outline a simple, room-temperature, ambient method to visualize
real-space moir\'e superlattices with sub-5 nm spatial resolution in a variety
of twisted vdW heterostructures including but not limited to conducting
graphene, insulating boron nitride and semiconducting transition metal
dichalcogenides. Our method utilizes piezoresponse force microscopy, an atomic
force microscope modality which locally measures electromechanical surface
deformation. We find that all moir\'e superlattices, regardless of whether the
constituent layers have inversion symmetry, exhibit a mechanical response to
out-of-plane electric fields. This response is closely tied to flexoelectricity
wherein electric polarization and electromechanical response is induced through
strain gradients present within moir\'e superlattices. Moir\'e superlattices of
2D materials thus represent an interlinked network of polarized domain walls in
a non-polar background matrix.
