Nanosize water droplets wet surfaces differently than their macroscopic counterparts. A new study by Hajime Tanaka’s group at the University of Tokyo uncovers the microscopic reason behind the difference—a finding with implications for industrial coatings and catalysis (Nat. Phys. 2026, DOI: 10.1038/s41567-026-03299-z).
At the edge of a droplet of water sitting on a surface, three phases of matter come into contact: the solid surface, liquid water, and gaseous air. The interaction along the line of contact between the three phases creates a force known as line tension. For relatively large droplets, this force is tiny and negligible. But for microscopic droplets, the contact line is comparable to the droplet size, and line tension plays a greater role in the droplet’s dynamics.
Nanodroplets naturally minimize line tension and surface tension—that is, the tension on the droplet skin—by altering their shape. On hydrophobic surfaces, the tension at the liquid-solid interface is high, which causes the droplet to pull itself into a round bead, shrinking contact with the surface and keeping the line of contact small.
In contrast, on hydrophilic surfaces, the tension is low, so the droplet spreads out and makes more contact with the surface. After a certain amount of spreading, researchers expect the droplet to pull itself inward to keep the contact line from increasing further.
But sometimes, as nanodroplets spread thin on hydrophilic surfaces, the spread unexpectedly accelerates instead of stopping. This occurs because, as a droplet thins, line tension reverses its action, which drives the droplet to increase its contact line, thereby spreading the droplet further. The reason for this reversal has remained unexplained until now.
“Using molecular simulations of water nanodroplets, we found that this reversal is closely linked to a structural change of water molecules at the contact line,” Tanaka says. The simulations suggest that as the droplet thins, the local tetrahedral structure of the hydrogen-bond network formed by water molecules collapses at the edges. This enables those water molecules at the edges to interact strongly with the hydrophilic surface, thereby promoting further spread.
“This is a great paper that addresses important questions in the nanoscience of water wetting on surfaces,” says Roland Netz, a physicist at the Free University of Berlin who was not involved in the research.
“Our work is mainly fundamental, but it may have implications for any technology where nanoscale wetting is important,” Tanaka says. “Some examples are coating, printing, antifogging and anti-icing surfaces, water harvesting, and the design of surfaces for controlling nucleation or crystallization.”
The mechanism is also relevant to catalysis. “Catalytic reactions often occur at solid–liquid interfaces where local water structure can influence ion transport, adsorption, proton transfer, and reaction pathways,” Tanaka explains.
Tanaka thinks that, in principle, this line tension reversal can be engineered in surfaces by tuning properties such as surface hydrophilicity and lattice geometry. “However, this is still a conceptual design principle at this stage,” he says. “Experimental validation will be important.”
