Bistable Wetting States on a Smooth Surface

ABSTRACT Droplets on a non‐textured surface typically exhibit a monostable wetting state, as represented by Young state. Here, we show that droplets can exhibit bistability without surface texture by tuning molecular interactions. We investigated the behavior of water droplets on a smooth substrate when immersed in oil. The oil contains hydrophilic interaction components, and their hydrophobic–hydrophilic balance was systematically varied. For oils with a specific hydrophobic–hydrophilic balance, droplet states bifurcate between repellent or sticky depending on the order of droplet casting and oil immersion. These states are neither transient nor one‐off, indicating molecular interactions can create an energetic barrier separating the two states rather than surface textures. Under other oil conditions, the droplet remains monostable, either repellent or sticky, regardless of the order. This work advances the fundamental understanding of molecular effects on droplet behavior and expands surface design strategies in functional materials without compromising mechanical durability.