  Fickian yet non-Gaussian diffusion is observed in several biological and soft
matter systems, yet the underlying mechanisms behind the emergence of
non-Gaussianity while retaining a linear mean square displacement remain
speculative. Here, we characterize quantitatively the effect of spatial
heterogeneities on the appearance of non-Gaussianity in Fickian diffusion. We
study the diffusion of fluorescent colloidal particles in a matrix of
micropillars having a range of structural configurations: from completely
ordered to completely random. We show that non-Gaussianity emerges as a direct
consequence of two coupled factors; individual particle diffusivities become
spatially dependent in a heterogeneous randomly structured environment, and the
spatial distribution of the particles varies significantly in such
environments, further influencing the diffusivity of a single particle. The
coupled mechanisms lead to a considerable non-Gaussian nature even due to weak
disorder in the arrangement of the micropillars. A simple mathematical model
validates our hypothesis that non-Gaussian yet Fickian diffusion in our system
arises from the superstatistical behavior of the ensemble in a structurally
heterogeneous environment. The two mechanisms identified here are relevant for
many systems of crowded heterogeneous environments where non-Gaussian diffusion
is frequently observed, for example in biological systems, polymers, gels and
porous materials.
