Particle Imaging in Electrohydrodynamic Pumps operating at several kV

Electrohydrodynamic (EHD) pumps generate flow of dielectric liquids using electrostatic forces, enabling fluid transport without moving parts or vibration. Pumps based on EHD can be lightweight, compact, and silent. Their ability to generate high flow rates and pressures makes EHD pumps well-suited for driving fluidic actuators in soft robotics and for cooling electronic devices. EHD pumps however require several kV to operate and to date have energy efficiencies of only a few percent. The mechanisms behind energy loss remain poorly understood.

This study aims to visualize and analyze the local flow patterns within mm-diameter EHD pumps to better understand their internal dynamics using particle imaging techniques. Our pump relies on charge-injection EHD, using electrodes spaced by 1 mm and voltages up to 4 kV. A major challenge for imaging lies in the strong electric fields used in EHD pumping, which can influence tracer particles, which then no longer follow the flow lines. To mitigate this, we used sub-micron particles as the dielectrophoretic forces acting on them are lower. We designed a cross-sectional planar EHD pump to enable direct observation of particle trajectories. A high-speed camera was used to capture transmitted light from a backlit setup, providing high-contrast visualization of flow structures.

Using particle image velocimetry (PIV) and particle tracking velocimetry (PTV), we observed complex vortex structures forming between electrode pairs. These vortices exhibited velocities significantly higher than the net flow, suggesting internal recirculation and potential sources of energy loss. We report an effective approach for visualizing flow in EHD systems, highlighting the importance of local flow characterization, and thus aiding in devising new pumps geometries to improve efficiency.