Programmable Generation of Droplet Array with Volumetric Gradient by Modulating the Smearing Velocity

The development of droplet-array-based microreactors has emerged as a key technology in high-throughput and low-consumption microfluidic platforms, which is at the forefront of scientific innovation. With hydrophilic sites engraved on hydrophobic substrates with a femtosecond laser, this study leverages a smearing-driven droplet array dispensing platform for the simultaneous and programmable generation of droplet arrays with tunable volume gradients. Proof-of-concept experiments with hydrophilic sites of 1-4 mm diameters demonstrate a 100-fold increase in the droplet volume, enabling real-time control of the volume distribution by programming the smearing velocity. To reduce the reliance on complex programmable translation stages, we further implement a rotational platform that exploits radial velocity at constant angular velocity and generates unidirectional volume gradients along the disc radius. Droplet arrays generated on this platform exhibit a predictable scaling of volume with smearing velocity and radial position. By decoupling the droplet volume from fixed dimensions of hydrophilic patterns and enabling gradient programming via simple mechanics, the system eliminates dependencies on complex external facilities. This work provides a versatile and resource-efficient solution compatible with droplet-based microfluidic systems with potential applications in point-of-care diagnostics, combinatorial assays, and high-throughput screening.