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Nanosize water droplets wet surfaces differently than their macroscopic counterparts. A new study by Hajime Tanaka<\/a>\u2019s group at the University of Tokyo uncovers the microscopic reason<\/a> behind the difference\u2014a finding with implications for industrial coatings and catalysis (Nat. Phys. 2026, DOI: 10.1038\/s41567-026-03299-z).<\/p>\n 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\u2019s dynamics.<\/p>\n Nanodroplets naturally minimize line tension and surface tension\u2014that is, the tension on the droplet skin\u2014by 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n \u201cUsing molecular simulations of water nanodroplets, we found that this reversal is closely linked to a structural change of water molecules at the contact line,\u201d 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.<\/p>\n