Current fluctuations reveal transport mechanisms in a colloidal micropore

The stochastic processes that govern transport through nanoscale pores generate noisy, fluctuating currents. Interpreting such fluctuations in experimental data offers a powerful route to uncover transport mechanisms, but unambiguously linking fluctuation phenomena to physical mechanism is very challenging in molecular systems. Here, we investigate current fluctuations in an experimental colloidal model 'micropore', which allows for direct visualisation of transport at the single particle level in a highly controlled setting. Fluctuations are quantified via the power spectral density (PSD) – the central correlation function used in nanopore measurements. In dilute systems, characteristic spectral features arise from the random arrival times of particles and distribution of particle velocities within the pore. We show this behaviour can be described using expressions for shot noise with a finite transit time – a model borrowed from electronic systems – and that this model can also be extended to describe transport of DNA through a solid-state nanopore. For densely packed systems, spectra display clear changes at low frequencies arising from interparticle interactions, demonstrating the sensitivity of the PSD to collective particle effects. Overall, we show that the colloidal micropore can be used to establish concrete and translatable links between power spectral scaling and underlying mechanisms, making it an excellent platform with which to explore fluctuation behaviour in porous media.