Atomic force microscopy reveals liquids adjust viscosity when confined, shaken

A liquid cell containing water allows viscosity measurements of the water to be collected with an atomic force microscope. A study led by Elisa Riedo, an assistant professor in the... Click here for more information.

Getting ketchup out of the bottle isn’t always easy. However, shaking the bottle before trying to pour allows the thick, gooey ketchup to flow more freely because it becomes more fluid when agitated. The opposite is not typically true – a liquid such as water does not become a gel when shaken.

However, new research published in the March 14 issue of the journal Physical Review Letters shows that when fluids like water and silicon oil are confined to a nanometer-sized space, they behave more like ketchup or toothpaste. Then, if these confined liquids are shaken, they become fluidic and exhibit the same structural and mechanical properties as those in thicker layers.

The study – the first to use an atomic force microscope to measure the viscosity of confined fluids – revealed that these liquids can respond and modify their viscosity based on environmental changes.

As graduate student Tai-De Li (left) looks on, Elisa Riedo, an assistant professor in the Georgia Tech School of Physics, holds a liquid cell containing water that allows viscosity measurements... Click here for more information.

“Knowing this could be very important,” said Elisa Riedo, an assistant professor in the Georgia Tech School of Physics. “If a lubricant used in a piece of machinery becomes thick and gelatinous when squeezed between two solid surfaces, serious problems could occur. However, if the machine vibrated, the liquid could become fluidized.”

With funding from the National Science Foundation and the U.S. Department of Energy, Riedo and graduate student Tai-De Li used atomic force microscopy (AFM) to measure the behavior of thin and thick layers of liquids while they were vibrated. A nanometer-size spherical silicon tip was used to approach a mica surface immersed in water or silicon oil, while small lateral oscillations were applied to the cantilever support.

“Some researchers have measured the force it takes to squeeze out a fluid, but we took a different approach,” explained Riedo. “We are the first group to use AFM to study the viscosity of confined fluids from direct high-resolution lateral force measurements.”