Gating of Droplets in Open Surface Microfluidics

We present an open-surface droplet microfluidic platform that enables precise and reconfigurable manipulation of aqueous droplets through electromagnetically induced substrate tilting. Building on a Laplace-pressure-driven design, the system employs controlled gravitational components to guide droplet motion on a PDMS-grafted surface without enclosed channels, external pumps, or mechanical motion.

The substrate consists of a glass plate chemically grafted with a thin polydimethylsiloxane (PDMS) layer. Prior to grafting, an adhesion primer promotes covalent bonding with surface hydroxyl groups, producing a hydrophobic surface that exhibits a 180° contact angle for water droplets when the substrate is covered with a layer of silicone oil. The minimal contact angle hysteresis and adhesion enable smooth and sustained motion of water drops on the PDMS surface.

Droplet transport arises from gravity and gradients in Laplace pressure, which develop when the substrate is slightly tilted. This tilt alters the Laplace pressure distribution, producing a net capillary force that drives the droplet toward regions of lower potential energy. Controlling the substrate orientation allows dynamic control of the direction and speed of droplet movement.

Tilt is generated using four electromagnets positioned beneath the substrate, facing four permanent magnets attached underneath it. This configuration allows modulation of tilts in two orthogonal directions. By varying the voltages applied to the electromagnets, the tilt angle can be controlled in the horizontal plane. This setup enables continuous, reversible droplet motion that can be controlled manually or through programmed voltage inputs.

To enhance spatial precision, the PDMS surface incorporates shallow microchannels and traps that control droplet trajectories or isolate droplets for specific tasks such as fusion, splitting, or localized reactions.

This integrated system combines surface-energy gradients, electromagnetic control, geometric confinement, and localized thermal zones to achieve multifunctional droplet manipulation on an open platform. Its modular and contamination-resistant design provides a promising foundation for lab-on-a-chip applications, point-of-care diagnostics, and miniaturized chemical or biological reactions.