Electrochemical CO2 reduction (CO2R) is a promising technology for carbon recycling and energy storage. While gas-fed CO2R is currently the best practice because it facilitates fast mass transport, CO2R in water offers potential advantages such as avoiding salt formation, facile water control, and easier integration with CO2 capture. In this work, we enhance mass transport in an aqueous CO2 electrolyzer using fast pressure pulses (50 Hz, 1.2 bar) with a vibratory pump typically found in coffee machines. We demonstrate a limiting current density of 87 mA cm−2 toward CO2R products—nearly three times higher than without pulses. The current density can be further increased by leveraging the peak-to-peak pressure amplitude or pump frequency, as shown through particle image velocimetry (PIV) and an order-of-magnitude scaling analysis. Although challenges remain, such as pump energy consumption, contamination, heating, and pressure-wave damping, the pressure-pulsed concept is a promising direction for aqueous CO2R. ... Read more

We investigate and model the initiation of motion of a single particle on a structured substrate within an oscillatory boundary layer flow, following a mechanistic approach. By deterministically relating forces and torques acting on the particle to the instantaneous ambient flow, the effects of flow unsteadiness are captured, revealing rich particle dynamics. Laboratory experiments in an oscillatory flow tunnel characterise the initiation and early stages of motion, with particle imaging velocimetry measurements yielding the flow conditions at the motion threshold. The experiments validate and complement results from particle-resolved direct numerical simulations, combining an immersed boundary method with a discrete element method that incorporates a static friction contact model. Within the parameter range just above the motion threshold, the mobile particle rolls without sliding over the substrate, indicating that motion initiation is governed by an unbalanced torque rather than a force. Both experimental and numerical results show excellent agreement with an analytical torque balance including hydrodynamic torque derived from the theoretical Stokes velocity profile, and contributions of lift, added mass and externally imposed pressure gradient. In addition to static and rolling particle states, we identify a wiggling regime where the particle moves but does not leave its original pocket. Our deterministic approach enables prediction of the phase within the oscillation cycle at which the particle starts moving, without relying on empirical threshold estimates, and can be extended to a wide range of flow and substrate conditions, as long as turbulence is absent and interactions with other mobile particles are negligible. ... Read more

Colloidal particles at fluid interfaces can enhance the stability of drops and bubbles. Yet, their effect on mass transfer in these multiphase systems remains ambiguous, with some experiments reporting strongly hindered diffusion, while others show nearly no effect, even at near-complete surface coverage. To resolve this ambiguity, we solve the Fick-Jacobs equation for unsteady diffusion, allowing us to treat the particle-laden interface as a locally reduced cross-sectional area for mass transfer. Our numerical solutions reveal two limiting regimes, with the particle layer hindering diffusion only at short times. Guided by analytical solutions for a homogeneous layer with reduced diffusivity, we derive quantitative expressions for the transport regimes and associated transition times for diffusion across the particle layer. This analysis yields a simple criterion for long-term hindrance that accurately distinguishes between conflicting experimental results, providing a unifying framework for mass transfer in particle-laden multiphase systems. ... Read more