Micro-centrifuge: Controlling coffee ring effect with surface acoustic waves on a patterned substrate

The coffee ring effect, where particles in an evaporating droplet migrate to the edges, forming a ring-like deposit, has been of great interest in colloid and interface science. It presents significant challenges in applications that require uniform particle deposition, such as inkjet printing, biosensing, and nanomaterial assembly. Existing methods to mitigate this effect often suffer from limitations, including the risks of chemical contamination and complex environmental requirements. This study proposes an acoustic centrifuge device with microfabricated multiple concentric circular grooves designed to maintain high contact angles when a droplet is actuated by surface acoustic waves (SAWs).

Acoustic force is employed to rotate the droplet, resulting in nanoparticle accumulation at the center of the droplet. Thin PDMS substrates having high-aspect-ratio microstructures are fabricated to maintain a high contact angle when the droplet dries. We visualized that the proposed device concentrates nanoparticles during the droplet's evaporation under a microscope.

The proposed method effectively mitigates the coffee ring effect, reducing the residue diameter to 20% of that on a flat surface. Furthermore, the device accumulates nanoparticles at the center of the droplet with a uniform distribution. Unlike previous acoustic approaches limited to 5 μl droplets, our design accommodates volumes exceeding 30 μl due to the novel patterned surface. Additionally, the device is adaptable due to the absence of permanent bonding between the droplet holder, enabling repeated use of the acoustic device, significantly reducing production time and costs.