Orbital Electrowetting: From Continuous Droplet Transport to Programmable Microfluidics.

Droplet transport on solid surfaces underpins applications in microfluidics, thermal management, and water harvesting. Conventional electrowetting (CEW) has enabled digital droplet control, but reliance on numerous individually addressed electrodes, complex driving circuitry, and step-wise motion has limited scalability and practical deployment. Orbital electrowetting (OEW) has recently been introduced as a complementary paradigm, where asymmetric electrowetting forces and electrostatic energy gradients drive droplets into continuous, high-speed motion along predefined orbital pathways using only a minimal number of electrodes and simple global excitation. In this perspective, the physical mechanisms of OEW are summarized, with emphasis on how surface wettability, electric field distribution, and orbital geometry govern droplet speed, stability, and confinement. Key challenges are identified, including the current lack of robust bidirectional and position-resolved control in complex networks. Future directions and application opportunities for OEW-enabled platforms are outlined, such as programmable microfluidics with low overhead wiring, defogging and self-cleaning of photovoltaic modules, enhancement of condensation heat transfer, and atmospheric water harvesting when combined with passive radiative cooling surfaces. It is argued that, as these challenges are addressed, OEW will substantially expand the electrowetting toolbox for next-generation droplet manipulation and water-energy technologies.