Alexandros G. Sourais

Superhydrophobic surfaces enable liquid drop mobility with minimal friction but inherently lack control due to their extremely low adhesion. Here, we report a simple, accessible approach for programmable drop manipulation on an air-permeable superhydrophobic substrate using pressure modulation. We use a substrate which is commercially available, in our case, a rough hydrophobic sintered porous PTFE sheet. Its surface is rendered superhydrophobic via manual wet sanding, eliminating the need for micropatterning or chemical treatment. Air injection (positive pressure difference across the sheet) reduces solid–liquid contact, forming an air cushion that induces drop depinning and levitation, similarly with the Leidenfrost effect. Conversely, air suction (negative pressure) enhances adhesion, which increases contact angle hysteresis and roll-off angles and can immobilize drops even on inclined surfaces. Switching between suction and injection enables reversible switching between pinned and mobile states in real time. We demonstrate stepwise motion of drops on inclined surfaces by tuning the duration of positive pressure pulses. On horizontal substrates, directional transport is achieved by synchronizing pressure pulses with horizontal substrate oscillations, enabling motion without gravitational bias. This low-cost, programmable method enables robust and reversible switching of drop mobility, holding promise for applications in open-surface microfluidics, lab-on-chip devices, and adaptive liquid handling systems.

The electrowetting-induced detachment of droplets from solid surfaces is important for numerous applications in the fields of heat transfer and fluid mechanics. The forced oscillations of droplets on solid surfaces and their ability to detach are studied. In this study, the process is efficiently simulated by implementing a powerful methodology developed by our team. Our results agree with experiments showing that optimal detachment, in terms of actuation energy, is achieved when the application of voltage is synchronized with the spreading time of the droplet. Under these conditions, the droplet oscillates with a period close to that of a mirrored Rayleigh droplet. The relationship between the droplet’s oscillation period and its physical properties is examined. During voltage-droplet synchronization, the droplet’s ability to detach depends mostly on its contact angle, its viscosity, and the applied voltage. An energy analysis is also conducted, revealing how energy is supplied to the droplet by electrowetting-induced detachment.

Electrowetting – induced detachment of droplets from solid surfaces is important for numerous applications in the fields of heat transfer and fluid mechanics. The forced oscillations of droplets on solid surfaces and their ability to detach are studied. In this study the process is efficiently simulated by implementing a powerful methodology developed by our team. Our results agree with experiments, showing that optimal detachment, in terms of actuation energy, is achieved when the application of voltage is synchronized with the spreading time of the droplet. Under these conditions, the droplet oscillates with a period close to that of a mirrored Rayleigh droplet. The relation between the droplet’s oscillation period and its physical properties is examined. During voltage-droplet synchronization, the droplet’s ability to detach depends mostly on its contact angle, its viscosity, and the applied voltage. An energy analysis is also conducted, revealing how energy is supplied to the droplet by electrowetting-induced detachment.

Detachment and jumping of liquid droplets over solid surfaces under electrowetting actuation are of fundamental interest in many microfluidic and heat transfer applications. In this study we demonstrate the potential capabilities of our continuum-level, sharp-interface modelling approach, which overcomes some important limitations of convectional hydrodynamic models, when simulating droplet detachment and jumping dynamics over flat and micro-structured surfaces. Preliminary calculations reveal a considerable connection between substrate micro-topography and energy efficiency of the process. The latter results could be extended to the optimal design of micro-structured solid surfaces for electrowetting-induced droplet removal in ambient conditions.