We demonstrate the use of fluorescence microscopy as a tool for mapping the spatial distribution of fluid flow and electrochemical reactions in an operating aqueous quinone flow cell. 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is a reversibly redox active molecule with a reduced form (H2AQDS) that fluoresces when excited by UV light. Visualization of AQDS/H2AQDS within commercial porous carbon electrode papers enables a direct comparison of their performance. In particular, this technique illuminates surprisingly large-scale heterogeneous fluid flow profiles present in several carbon papers, leaving substantial areas of the electrode mass-transport limited. In others, more homogeneous flow distribution is observed, but limitations such as low electronic conductivity and limited accessible electrode surface area limit the performance. This work provides insights into improving structural properties of porous electrodes for high-power density electrochemical flow cells.

We observe nonmonotonic aging and memory effects, two hallmarks of glassy dynamics, in two disordered mechanical systems: crumpled thin sheets and elastic foams. Under fixed compression, both systems exhibit monotonic nonexponential relaxation. However, when after a certain waiting time the compression is partially reduced, both systems exhibit a nonmonotonic response: the normal force first increases over many minutes or even hours until reaching a peak value, and only then is relaxation resumed. The peak time scales linearly with the waiting time, indicating that these systems retain long-lasting memory of previous conditions. Our results and the measured scaling relations are in good agreement with a theoretical model recently used to describe observations of monotonic aging in several glassy systems, suggesting that the nonmonotonic behavior may be generic and that athermal systems can show genuine glassy behavior.

We measure the response of cylindrical shells to poking and identify a stability landscape, which fully characterizes the stability of perfect shells and imperfect ones in the case where a single defect dominates. We show that the landscape of stability is independent of the loading protocol and the poker geometry. Our results suggest that the complex stability of shells reduces to a low dimensional description. Tracking ridges and valleys of this landscape defines a natural phase-space coordinates for describing the stability of shells.