Bistable wetting on a smooth surface overturns a 200 year old law

May 21, 2026

Researchers show water droplets can adopt sticky or repellent states on the same smooth surface, challenging a 200 year old law of interface chemistry.

(Nanowerk News) Researchers at Japan’s National Institute for Materials Science have observed bistable wetting on a single smooth surface, with water droplets settling into either a sticky or a repellent state depending on the order of the experiment (Advanced Materials Interfaces, “Bistable Wetting States on a Smooth Surface”). The finding contradicts a principle of interface chemistry that Thomas Young formulated in 1805, and it points toward water and oil repellent materials made without PFAS chemicals.

Key Findings

Background

Wetting describes how a liquid behaves when it meets a solid. A droplet may spread out and adhere to the surface, or it may roll away as a bead. The phenomenon underpins industrial technologies such as inkjet printing and coatings, and it shapes everyday observations of liquids on solid materials. In 1805, Thomas Young formulated a law stating that for any given solid and liquid pair, only one wetting state is possible. The rule has been treated as a fixed principle of interface science ever since. The NIMS team showed that the rule does not hold under certain conditions. Working with a smooth, non-textured substrate immersed in oil, the researchers cast water droplets onto the surface and found that the resulting state depended entirely on the order of operations. Casting water first and then adding the oil produced a sticky droplet. Immersing the substrate in oil first and then adding the water produced a repellent droplet. The combination of materials was identical in both cases. (a) Droplets in repellent and sticky states are simultaneously observed on the same non-textured substrate. The repellent state is formed by first immersing the substrate in oil, and then casting a water droplet. The sticky state is formed by first casting a water droplet, and then immersing the substrate in oil. (b) Switching from a sticky state to a repellent state by applying an external stimulus. A water droplet in a sticky state can be switched to a repellent state by applying stress in the direction parallel to the substrate with a Teflon needle. (Image: NIMS) The team also identified the design rule that produces this behavior. The substrate must offer specific hydrogen bond sites, which the researchers describe as “hands,” that interact with the oil phase, and the density of those sites must be precisely controlled. When the condition is met, the system supports two coexisting wetting outcomes. A droplet sitting in the sticky state can also be flipped into the repellent state by applying lateral mechanical stress with a Teflon needle.

Future outlook

The finding points toward water and oil repellent surfaces that do not depend on per- and polyfluoroalkyl substances, the PFAS family of chemicals that are highly effective at repelling water but raise environmental and health concerns. Because the effect is produced on a smooth substrate rather than a microtextured one, the resulting surfaces should also be more mechanically robust than conventional textured coatings, retaining their properties in environments where scratching would otherwise degrade performance. The researchers suggest that the same design principle could yield smooth surfaces that become liquid repellent simply by applying oil, with no specialized surface structuring required. The switchable behavior also points toward use in microfluidic devices, where dynamic control over wetting could underpin a new class of programmable smart surfaces that move beyond a fixed liquid repellent or sticky character. The study was carried out by Mizuki Tenjimbayashi of the Research Center for Materials Nanoarchitectonics (MANA) and Shunto Arai of the Research Center for Macromolecules and Biomaterials, both Independent Researchers at NIMS. A single solid and liquid combination can now be made to settle into two stable wetting outcomes, contradicting a rule that has governed interface science since Young first wrote it down. The mechanism is concrete enough — a controlled density of hydrogen bond sites on a smooth substrate — to give engineers a starting point for designing durable, switchable, fluorine free surfaces.