Young’s modulus of nanoconfined liquids?

Abstract

In material science, bioengineering, and biology, thin liquid films and soft matter membranes play an important role in micro-lubrication, ion transport, and fundamental biological processes. Various attempts have been made to characterize the elastic properties, such as Young's modulus, of such films using Hertz theory by incorporating convoluted mathematical corrections. We propose a simple way to extract tip-size independent elastic properties based on stiffness and force measurement through a spherical tip on a flat surface. Using our model, the Young's modulus of nanoconfined, molecularly-thin, layers of a model liquid TEHOS (tetrakis 2-ethylhexoxy silane) and water were determined using a small-amplitude AFM. This AFM can simultaneously measure the stiffness and forces of nanoscale films. While the stiffness scales linearly with the tip radius, the measured Young's modulus essentially remains constant over an order of magnitude variation in the tip radius. The values obtained for the elastic modulus of TEHOS and water films on the basis of our method are significantly lower than the confining surfaces' elastic moduli, in contrast with the uncorrected Hertz model, suggesting that our method can serve as a simple way to compare elastic properties of nanoscale thin films as well as to characterize a variety of soft films. In addition, our results show that the elastic properties (elastic modulus) of nanoconfined liquid films remain fairly independent of increasing confinement.

The Influence of Mechanical Deformations on Surface Force Measurements

“…Irrespective of the particular system or technique used, it is very important to know the degree of convolution, i.e., whether the solid compression is negligible or dominant compared to the liquid compression, in order to interpret properly the structural force profile regarding the compressibility of the liquid layers [33,70,[75][76][77][78][79]. Indeed, this question of the "elasticity" of a thin liquid film is connected to a strong debate in the community, to understand how a liquid can exhibit a solid-like behaviour in nanoconfinement [8,[80][81][82][83]. Our study suggests that the finite slope of the structural force profile in each layer is not necessarily due to a change in load of packing fraction or structure of the molecules in the structured film, but can be attributed, in some cases, and at least in part, to the indentation of the confining solids.…”

The influence of mechanical deformations on surface force measurements

“…Indeed, our study shows that the finite slope of the structural force profile in each layer is not necessarily due to a change with load of packing fraction or structure of the molecules in the structured film. In particular, this question of the "elasticity" of a thin liquid film is connected to a strong debate in the community, to understand how a liquid can exhibit a solid-like behaviour in nanoconfinement 6,[72][73][74][75] . We propose a general criterion to distinguish the two opposite regimes of convolution, for a generic system exhibiting a structural force profile.…”

Confinement of a polymer chain: An entropic study by Monte Carlo method

“…In the case of macromolecule‐containing systems, the high molecular weight and the possibility for the long chain to experience a number of conformations are additional features that should be taken into account when studying thermodynamics or properties of confined systems. In particular, the confining constraints (such as the imposition of volume constraints) applied to a free unperturbed polymer chain reduce the number of available conformations for the macromolecules and could introduce additional interactions between the chain segments and the confining interface . Among other properties, this reduction influences the conformational entropy of the polymer chain, determining a change in many characteristics of a polymer, such as its mobility and its miscibility .…”