Atomic force microscopy reveals liquids adjust viscosity when confin...(re measured directly and simultaneously ...)

Riedo and Li measured the relaxation times of two wetting liquids: water and silicone oil (octamethylcylotetrasiloxane), which is primarily used as a lubricant or hydraulic fluid, and is the main ingredient in Silly Putty.

The relaxation time describes how active the molecules are. A longer relaxation time means it takes longer for the molecules to rearrange themselves back into their original shape after shaking them, said Li. Liquids have very short relaxation times as soon as one stops shaking a bottle of water, it reverts to its original configuration.

Experimental results showed that the relaxation time became orders of magnitude longer in water and silicone oil when they were confined, meaning they behaved more like gels or glass. The researchers also showed that the relaxation times depended on the shaking speed when the liquids were confined. However, in thick layers that were not confined, the molecules showed no dependence on the shaking speed and always relaxed very quickly, meaning they behaved like a normal liquid.

Longer relaxation times were observed when the water film was less than one nanometer thick, composed of about three molecules of water stacked on top of each other. Otherwise, its properties were the same as in a bottle of water. For silicone oil, a thickness of four nanometers was required before the properties were like those of a glassy material.

We observed a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids, such as gels or supercooled liquids, noted Riedo. Because we observed these phenomena in both water and silicone oil, we believe they are very general phenomena and may apply to all wetting liquids.