Micro-Centrifuge: Suppressing Coffee Ring Effect via Surface Acoustic Waves on Patterned Substrates

The coffee-ring effect—a persistent challenge in droplet evaporation—leads to non-uniform particle deposition, limiting its use in biosensing and nanofabrication. We present an acoustofluidic "micro-centrifuge" that integrates surface acoustic waves (SAWs) with high-aspect-ratio concentric microstructures on a PDMS substrate to manipulate droplet drying dynamics. Acoustic radiation and streaming forces rotate the droplet, concentrating nanoparticles at the center. Simultaneously, the microgrooved substrate preserves high contact angles, enabling stick-jump motion of the contact line during evaporation and minimizing residue between grooves. Unlike prior approaches limited to sub-5 µL volumes, our design accommodates >30 µL droplets and reduces coffee-ring diameter to 20% of flat surfaces. The droplet holder is detachable and reusable, improving practicality and cost-efficiency. This hybrid strategy enables precise, uniform deposition and expands the utility of acoustic manipulation to larger droplet volumes, offering applications in SERS, biosensors, and printed electronics.