Manipulating small liquid droplets is central to microfluidics, with applications ranging from medical diagnostics to chemical analysis. Established techniques such as electrowetting and dielectrophoresis offer precise control but often require external voltage supplies and electrode-control hardware.

Researchers at Kyung Hee University, South Korea, have developed a self-sustained electric-field-based electrophoretic platform (SELF-EP) that uses precharged polymer films to charge and manipulate droplets. Each film, called a SELF-emitter, retains surface charge following an initial corona-charging step, allowing it to generate a persistent electric field without a continuous electrical supply.

The key physics is electrostatic induction. As a grounded liquid is dispensed near the emitter, the electric field drives charge redistribution and exchange with ground. The droplet then detaches, carrying a net charge. The film supplies the electric field rather than directly transferring its stored charge to the liquid. By controlling the droplet charge and emitter polarity, the researchers can select electrostatic attraction or repulsion.

Using these emitters, the team demonstrated simultaneous transport of multiple droplets and charge-selective sorting. They also suspended and moved droplets within silicone oil by manually repositioning the emitters, demonstrating battery-free manipulation without direct mechanical contact with the droplets.

A further advance was to pattern the surface charge on the polymer films. These patterns create electrostatic potential-energy landscapes that cause droplets to organize into ordered arrangements and provide passive guides for predefined routes. In the reported long-distance routing experiment, an externally applied electric field supplied the driving force, while the patterned film defined the path. This avoided the need to sequentially switch an array of control electrodes.

The study shows how charge-retaining materials can provide both persistent electric fields and spatial guidance for droplet handling. By encoding part of the control function into the material itself, the approach offers a route towards simpler, portable microfluidic systems with reduced reliance on powered control hardware.