<p>Flowable slurry electrodes play important roles in a few promising technologies for energy and water systems including electrochemical flow capacitors (EFCs) and capacitive deionization. The electrochemical performance of slurry electrodes depends on the hydrodynamic behavior of the porous particles in the slurry that serve as individual capacitors, whose dynamic interactions with the stationary electrodes and between each other are essential for spreading charges to the bulk of the slurry. So far, it has been difficult to directly observe the slurry flow and particle interactions because of the opacity of the activated carbon, a typical particle material in slurries. We previously reported a microfluidic electrochemical flow capacitor (µ-EFC), which is made of transparent materials and allows for simultaneous electrochemical characterization and optical observation of slurry electrodes under continuous flow conditions. However, the placement of the stationary Indium Tin Oxide (ITO) electrodes at the top and bottom of the µ-EFC renders it challenging to directly observe the particle dynamics in the boundary layer of the ITO electrode. Here we report on inclusion of three-dimensional (3D) gold electrodes in the µ-EFC channels, which allows for better visualization of the particle behavior in the boundary layer of stationary electrodes. The results show unique particle behaviors as they flow along and interact with the 3D electrodes of different lengths, which have important implications to the electrochemical performance of slurry electrodes.</p>