The surface of water drops at 100 nm size changes with temperature. At room temperature, the surface water molecules of these droplets have much stronger interactions than a normal water surface. The structural difference corresponds to a difference in temperature of -50°C.

Nanometric-sized water drops are everywhere, as droplets or aerosols in air, in our bodies as medication, and in rocks and oil fields. How they interact with their hydrophobic environment, at the curved droplet interface, a sub-nanometric region that surrounds the small pocket of water, could boost our understanding of atmospheric, biological and geological processes.

The surface of these droplets that are as thick as one thousandth of a hair, with a volume of an attoliter (that's 18 zeros). Sylvie Roke, director of the Julia Jacobi Chair of Photomedicine - Laboratory for Fundamental BioPhotonics at  Ecole Polytechnique Fédérale de Lausanne, explains. "The method involves overlapping ultrashort laser pulses in a mixture of water droplets in liquid oil and detecting photons that are scattered only from the interface. These photons have the sum frequency of the incoming photons and are thus of a different color. With this newly generated color we can know the structure of the only the interface."

Credit: © EPFL/Julia Jacobi Chair of Photomedicine - Laboratory for Fundamental BioPhotonic

The surface of the water droplets turns out to be much more ordered than that of normal water and is comparable to super cooled (liquid < 0 °C water) water in which the water molecules have very strong hydrogen bond interactions. In ice, these interactions lead to a stable tetrahedral surrounding of each water molecule. Surprisingly, this type of structure was found on the surface of the droplets even at the room temperature - 50 °C above were it would normally appear. 

Further research could target the surface properties of water droplets with adding salt, a more realistic model of marine aerosols that consist of salty water surrounded by a hydrophobic environment. Salt may either enhance the water network or reduce its strength or it may not do anything at all.