to probe diffusion at different length scales . The root mean square displacement 2√𝐷Δ, where Δ is the diffusion time, provides an estimate of how the transportation distance in the gradient direction. We were here able to probe diffusion at the ~1-3 µm length scale (Figure ). The discrepancy between the diffusion coefficient found at short diffusion times (D " ) and long diffusion times (D ! ) represents the diffusion coefficients in the two limiting cases of shortrange (<< 0.1µm) diffusion versus long-range (> 3 µm scale) diffusion lengths (Figure ). This discrepancy can be quantified using tortuosity values , which are here defined as 𝜏 =