The boundary between two liquids is like an invisible skin: it's there, but you can't touch it with your finger. Scientists have found a way to measure its elasticity without touching it. They made a tiny probe oscillate above the surface, and between the probe and the interface lay a thin film of liquid. This film works as both a spring and a damper: it resists compression and dampens oscillations.
First, the method was tested on a liquid-solid interface. There, everything matched theory: the measured film thickness and its resistance to oscillations exactly matched calculations for elastic materials. Then they moved to a liquid-liquid interface. It turned out to behave like a very viscous syrup: it barely springs but strongly resists motion. The thickness of this layer is a few micrometers, about the thickness of a human hair.
Such a contactless method could be useful for studying living cells: their membranes are also interfaces between liquids. You could measure how they deform without damaging the cell. Or test the strength of nanoparticle films used in new materials.
The method is reminiscent of tapping a wall to find a hollow spot, but instead of sound, it uses probe oscillations. Now invisible liquid interfaces can not only be seen but also felt.
The thickness of the liquid layer between the probe and the interface is just a few micrometers, which is tens of times thinner than a human hair.