Electrowetting makes droplets spread. Apply voltage between a droplet and a surface, and the contact angle decreases — the drop flattens, wets more area, adheres more strongly. This is the operating principle behind electronic paper displays, lab-on-a-chip devices, and adaptive lenses. Electrowetting equals spreading. The sign of the effect is not in question.
Deepak and colleagues applied voltage to droplets on densely textured PDMS surfaces with 5–10 μm post spacing and on lubricant-infused surfaces. The droplets didn't spread. They ejected — shooting laterally off the surface at high speed.
The mechanism requires two conditions: minimal contact line pinning and asymmetric electric stress. On a dense texture or lubricant-infused surface, the droplet's contact line is nearly free — it can't lock onto the posts because they're too closely spaced to provide mechanical resistance. When voltage creates an electrocapillary pressure gradient that would normally drive symmetric spreading, the free contact line allows any small asymmetry to become a net lateral force. Instead of spreading in all directions, the droplet accelerates in one direction and detaches.
On the same surfaces with wider post spacing, or in the Wenzel state where the liquid fills the texture, the conventional behavior returns: voltage drives spreading and stronger adhesion. The pinning that spreading requires is also the pinning that keeps the droplet in place. Remove the pinning, and the force that was supposed to anchor the drop launches it instead.
The same voltage, the same liquid, the same chemistry — spreading or ejection, depending on the texture spacing. The surface decides whether the electric field is glue or a catapult.