Can Electroactive Tracer Molecules Reveal Viscoelastic Structure by Measuring Non‐Fickian Diffusion?

Abstract We find that viscous and viscoelastic fluids are distinguishable by gauging Non‐Fickian diffusion of dissolved electroactive molecules. Typically, such fluids are differentiated by measuring the mean‐squared‐displacement <Δ r 2 > of embedded tracer particles (~1 μm) diffusing over time ( t ). From the relationship <Δ r 2 >=6D t α ( D =particle diffusivity), log plots of <Δ r 2 >vs. t α reveal regimes encoded in the slope α . For Fickian diffusion α =1, whereas α <1 and α >1, indicate Non‐Fickian sub‐ and super‐diffusion, respectively. Here, we electrolyzed redox reporters as molecular tracers in selected fluids. The current ( I ) relationship I v 1/2 ( v =scan‐rate) was recast as I 2 vs.1/ t α to introduce α as Non‐Fickian quantifier in log plots. When viscosity increased at high concentration of small‐molecules, D for the redox reporter declined but α remained constant at ~1 (Fickian). In contrast, both D and α (<1) decreased in viscoelastic hydrogels confirming a molecular sub‐diffusive regime. These results agree with particle microrheology on the same fluid types using optical methods that are inapplicable to molecules. By quantifying Non‐Fickian diffusion of electroactive molecular tracers, our method can uncover diffusion‐structure relationships to identify regulators in neurodegenerative liquid‐solid transitions of protein aggregates. Unlike tracer particles, the diffusivity of tracer molecules is controlled by the applied potential and electrode size.