Tokyo, Japan – Why does water roll off a duck's back but spread on clean glass? For macroscopic (mm-scale) drops, this behavior can be explained using continuum theory. However, when nanoscale (10–9 mm) droplets spread on surfaces, a force called the line tension becomes relevant and mysteriously changes sign. Questions about the nature of this force and its relevance to water's interaction with surfaces have remained unanswered.
Now, researchers from the Institute of Industrial Science, The University of Tokyo, have carried out computational studies that explain the origin of line tension in water nanodroplets at a molecular level. This discovery will be reported in Nature Physics.
On hydrophobic surfaces like Teflon, water forms spherical beads and slides off, a process known as nonwetting. On a waxy leaf, water forms stable, round drops, corresponding to partial wetting. On clean glass, water spreads into a thin film, referred to as complete wetting. Substrate wettability is the ability of liquids to maintain contact with a solid surface. Changing the substrate wettability can drive wetting from partial to complete.
At the edge of a water droplet on a surface, three phases come into contact. The shape of a macroscopic droplet is determined by the balance of forces at three interfaces (solid–air, liquid–solid, and liquid–air) along the contact line.
An additional force acts along the contact line – line tension. For a macroscopic drop, the line tension is much smaller than the interfacial tension. However, for nanoscale drops, the contact line is comparable in size to the drop. At complete wetting, line tension can become important and reverse sign, strongly affecting nanoscale wetting behavior. This phenomenon cannot be explained within a purely continuum description.
The team performed computer experiments (molecular dynamics simulations) to examine how water molecules organize at a surface during wetting. The team then quantified the line tension for different substrate wettabilities.
"In liquid water, hydrogen bonds tend to organize water molecules into a local, short-range tetrahedral structure—a transient four-neighbor pattern," explains lead author, Mohd Moid. "It is difficult to perform physical experiments to probe how this tetrahedral order evolves when water completely wets a surface."
However, the computer experiments showed that the tetrahedral order collapses at the contact line at complete wetting. This structural change is linked to a change in the sign of the line tension.
"We also performed computer experiments on an ice bilayer on a hydrophilic surface," reports Hajime Tanaka, senior author. "Ice bilayers consist of two layers of water molecules. We found that the bilayer did not wet the surface, showing that local order can outweigh surface chemistry."
This work shows that the interfacial liquid structure is a key determinant of wettability, providing a new design principle for controlling interfacial mechanics and wetting in inorganic and biological systems.
