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Is it a Solid or a Liquid? Flow Properties of Water under Nano-Confinement

Karan Kapoor and Shivprasad Patil with the instrument they have developed to measure rheological properties of fluids.

Karan Kapoor and Shivprasad Patil with the instrument they have developed to measure rheological properties of fluids.

Imagine a small glass of water. Imagine the glass to be so small you cannot see it with your naked eyes—a glass the diameter of which is about a million times smaller than a strand of human hair. Do you think the physical properties of water in such a nano-scale glass would be the same as the water you drink everyday?

“The question becomes even more relevant given that nano-confined liquids are common in biological systems,” says Shivprasad Patil, who heads a research group at IISER Pune. “For several years now, there has been no clear consensus on the physical state of a nano-confined liquid. Some reports indicate it remains a liquid whereas others suggest a solid-like behavior. One reason for this lack of agreement is use of experimental approaches that have different sensitivity ranges.”

In a typical experiment to assess the physical state of a material, scientists monitor the resistance that the material offers to an applied force. At a simplistic level, a solid resists an applied force to a larger extent than a liquid. Most measurements aiming at understanding physical state of a material therefore rest on measuring this “resistance” (shear force in technical terms).

When a fluid is confined in a small volume of atto to zeptolitres, the shear forces can be as small as nano Newtons (a measure of force). This is why it is important that an instrument measuring shear force be not just sensitive, but sensitive in a relevant range.

Patil’s research group has now developed a new force sensor to experimentally measure flow properties of nano-confined liquids. The instrument is based on a tuning fork model, a design that is used often in watches and in certain microscopes, but not been previously applied in the making of a force sensor for rheology (study of flow of matter).

In this set-up, one arm of the tuning fork is attached to a long fiber containing a tip. The other arm of the fork is an integral part of the rest of the instrumentation that processes the signal. The tip is immersed in liquid and the tuning fork is used to measure the stress on the fiber tip by measuring the difference in current through the arms. The authors have also developed a mathematical model to understand how the measurements in amplitude and phase relate to shear stress.

Force sensors that are highly sensitive and are designed to measure very small forces also tend to perceive the random forces due thermal vibrations at room temperature in addition to measuring the resistance that the material offers to an applied force. Patil’s team seems to have overcome this classical problem by using sensor which is very stiff but still capable of picking up very small forces.

Describing the results obtained using this device, Patil says, “The new instrument measured the shear response and the phase lag between the strain and stress. We found that water displays complex viscosity under nano-confinement and with large enough shear rates. The physical meaning of this complex viscosity is that part of the liquid gives a solid-like elastic response. By taking the ratio of this viscous and elastic response, one can compute Maxwell’s relaxation time, an important quantity that characterizes rate dependent system behavior.”

Karan Kapoor, an undergraduate student and primary author of the paper, compares this visco-elastic response by nano-confined liquid with experiments performed on Xenon aboard Columbia space shuttle under microgravity “Typically complex viscosity is exhibited by mixtures such as mud slurry, sauce and whipped creams, polymer melts and colloidal suspensions. The complex response of pure water to shearing is very similar to that of Xenon near its critical point. It is interesting that the effects of temperature are brought about by physical confinement by rigid walls,” says Kapoor.

The study titled “A new tuning fork-based instrument for oscillatory shear rheology of nanoconfined liquids” has been published in the journal Review of Scientific Instrumentsand is authored by Karan Kapoor, Vinod Kanawade, Vibham Shukla, and Shivprasad Patil.

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