Posted in | Nanomaterials | Nanoanalysis

New Method Could Help Estimate the Amount of Negative Pressure Produced by Liquid Crystals in Nanopores

In addition to governing the quantum vacuum or the Universe, negative pressure, despite being of a different nature, also appears in liquid crystals restrained in nanopores.

The negative pressure produced in nanopores by liquid crystals can significantly exceed 100 atmospheres. Above: The glass of the nematic phase of liquid crystal studied by scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow. (Image credit: IFJ PAN)

Now, a technique has been presented at the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow which makes it viable to predict the amount of negative pressure present in spatially restricted liquid crystal systems, for the first time.

Initially, negative pressure seems to be an unusual phenomenon. As a matter of fact, it is typical in nature, and moreover, happens on numerous scales. The cosmological constant—on the scale of the Universe—is accountable for speeding up the expansion of spacetime. In the plant world, attracting intermolecular forces—not increasing thermal motions—ensure water flow to the treetops of all trees higher than 10 m. On the quantum realm, an attractive force is created by the pressure of virtual particles of a false vacuum. This force appears, for instance, between two parallel metal plates (the well-known Casimir effect).

The fact that a negative pressure appears in liquid crystals confined in nanopores was already known. However, it was not known how to measure this pressure. Although we also cannot do this directly, we have proposed a method that allows this pressure to be reliably estimated.

Dr Tomasz Rozwadowski, Study First Author, Institute of Nuclear Physics of the Polish Academy of Sciences

The study has been published in the Journal of Molecular Liquids.

A liquid crystal called 4CFPB, composed of molecules measuring 1.67 nm in length and having a molecular diameter of 0.46 nm, were examined by the Polish physicists. At the University of Silesia in Katowice, experiments without nanopores, under elevated and normal pressure conditions (up to about 3000 atmospheres), were performed. Consecutively, at the University of Leipzig (Germany), systems in silicon membranes with non-intersecting nanopores measuring 6 and 8 nm in diameter were investigated. The nanopores’ geometry meant that there was space for just a small number of liquid crystal molecules next to one another, with the long axes placed along the channel’s walls.

Furthermore, the experiments observed the variations in numerous factors of liquid crystal (including dielectric absorption and dispersion). Based on the measurements, it was concluded that increased pressure was accompanied by a deceleration of molecular mobility. Yet, the narrower the channels in which the liquid crystal molecules in the nanopores were present, the quicker they moved. In addition, the data demonstrated that as the pressure increased, the density of the liquid crystal molecules also increased while it decreased in the nanopores. A change was also observed in the temperatures at which point the liquid crystal changed from the liquid isotropic phase (with molecules haphazardly arranged in space) to the most basic liquid crystalline phase (nematic; the molecules are still haphazardly arranged; however, they position their long axes in the same path), and subsequently to the glassy solid phase. The temperatures of the phase transitions increased as the pressure increased; however, the temperatures decreased in the nanopores.

With increasing pressure, all the parameters of the liquid crystal we examined changed conversely to how they changed in nanopores with decreasing diameters. This suggests that the conditions in the nanopores correspond to a reduced pressure. Since the liquid crystal molecules in the channels try to stretch their walls, as if they were expanding, we can talk about negative pressure, relative to atmospheric pressure which constricts the walls.

Dr Tomasz Rozwadowski, Study First Author, Institute of Nuclear Physics of the Polish Academy of Sciences

For the first time, the changes observed in physical parameters made it viable to predict the value of the negative pressure appearing in the liquid crystal that fills the nanopores. Assuming the changes are linear, it was seen that the negative pressure present in nanopores is capable of reaching nearly –200 atmospheres. This is an order of magnitude higher than the negative pressure accountable for the flow of water in trees.

Our research is fundamental in nature, it provides information about the physics of phenomena occurring in liquid crystals constrained in nanopores of varying diameters. However, liquid crystals have many applications, for example in displays, optoelectronics, and medicine, so each new description of how these substances behave on the nanoscale in such specific spatial conditions may carry practical information.

Dr Tomasz Rozwadowski, Study First Author, Institute of Nuclear Physics of the Polish Academy of Sciences

The SONATA grant from the National Science Centre funded the study on liquid crystals under spatial limitations.


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