Beyond the Mean Squared Displacement: Revealing Fickian Non-Gaussian Diffusion with Differential Dynamic Microscopy

by Andrea de Marco (DFA UniPD)

Monday 21 Sept 2026, 14:30 → 15:30 Europe/Rome

P1A (Dipartimento di Fisica e Astronomia - Edificio Ricci-Curbastro (ex-Paolotti))

Fickian non-Gaussian diffusion (FnGD) combines a mean-squared displacement growing linearly with time with a non-Gaussian displacement distribution. It is conventionally identified through single-particle tracking (SPT), a well-established technique that requires reliable tracer detection and trajectory reconstruction. These requirements become limiting when particles cannot be individually resolved or consistently linked across successive frames.

Differential dynamic microscopy (DDM) offers a complementary approach. Without resolving or tracking individual particles, DDM extracts ensemble dynamics from intensity fluctuations in microscopy videos in a form analogous to a scattering experiment, while retaining the advantages of real-space imaging.

Here, we study colloidal tracers diffusing in a static optical speckle landscape and use SPT as a benchmark for the DDM analysis. From a single experiment, DDM probes dynamics over multiple length scales, or scattering wave vectors q. This wave-vector-resolved description reveals reciprocal-space signatures of FnGD and provides a route for identifying non-Gaussian transport when conventional particle tracking is impractical.