Emergence of branched wetting states on a smooth surface

When the droplet is cast on the solid surfaces, part of the droplet/solid surface is replaced by their interface, and a wide variety of shapes and adhesion modes, from spreading1, sticky2, to super-repellent3, appears. In principle, droplets on a smooth surface exhibit a single wetting state. According to Young's law in 1805 4, the shape of a droplet, quantified with contact angle, is determined by the balance of substance-specific interfacial energies between three phases of a droplet, contacting solid surface and surrounding media4. Contact angles fluctuate due to external disturbance and the mobility of contact line 5, but the applicable range is generally fixed, which features the adhesion mode of the droplet. Here we show the emergence of branched wetting states with different contact angle ranges using the same droplet and surrounding media on a smooth, homogeneous surface. The applicable wetting states are high-contact angle droplet sticks to the surface or repellent, akin to the Wenzel 6 or Cassie 7 states observed on a nano/micro-textured surface, respectively. The phenomenon is commonly observed when site-specific molecular interactions trap the droplet metastable —the wettability branches when the interface formation order changes. This is because the molecular interactions at interfaces cause hysteresis in phase replacement under certain wettability conditions. Our experiment varied the combination and number of interaction molecules in phases, unravelling the design principles of the branched wettability. This work suggests that the site-specific molecular interaction can bifurcate macroscopic wetting modes, which advances the fundamental understanding of the molecular effect on macro-wettability.