Energy stored in nanoscale water capillary bridges formed between chemically heterogeneous surfaces with circular patches*

Abstract

The formation of nanoscale water capillary bridges (WCBs) between chemically heterogeneous (patchy) surfaces plays an important role in different scientific and engineering applications, including nanolithography, colloidal aggregation, and bioinspired adhesion. However, the properties of WCB of nanoscale dimensions remain unclear. Using molecular dynamics simulations, we investigate the geometrical and thermodynamic properties of WCB confined between chemically heterogeneous surfaces composed of circular hydrophilic patches on a hydrophobic background. We find that macroscopic capillary theory provides a good description of the WCB geometry and forces induced by the WCB on the confining surfaces even in the case of surface patches with diameters of only 4 nm. Upon stretching, the WCB contact angle changes from hydrophobic-like values (θ > 90°) to hydrophilic-like values (θ < 90°) until it finally breaks down into two droplets at wall separations of ∼ 9–10 nm. We also show that the studied nanoscale WCB can be used to store relevant amounts of energy E P and explore how the walls patch geometry can be improved in order to maximize E P. Our findings show that nanoscale WCB can, in principle, be exploited for the design of clean energy storage devices as well as actuators that respond to changes in relative humidity. The present results can also be of crucial importance for the understanding of water transport in nanoporous media and nanoscale engineering systems.

Harvesting Energy from Changes in Relative Humidity Using Nanoscale Water Capillary Bridges

“…From a practical point of view, our MD simulations show that macroscopic thermodynamics (capillarity theory) holds for nanoscale WCB formed between patchy walls separated by only h ≥ 3 nm. 23,24 In particular, we find that a simple modified version of the macroscopic Laplace−Kelvin equation is consistent with the MD simulations. This implies, for example, that one could use macroscopic thermodynamics to explore capillarity phenomena at (1) nm-scales, and at different RH.…”

“…This is motivated by a previous work where we show that, surprisingly, nanoscale WCB can be used to store energy at a given temperature, with non-negligible energy densities. 23,24 If the energy exchanges between the vapor and the nanoscale WCB due to changes in RH are indeed relevant, then it would be possible, in principle, to create a water responsive (WR) material that would host a large number of nanoscale WCB per unit volume, which could harvest energy from variations in the RH of the surroundings. For example, such a material could be composed of stacked monolayers decorated with hydrophobic and hydrophilic nanoscale domains.…”

How Small Is Too Small for the Capillarity Theory?

“…Similar results were obtained in the case of more complex heterogeneous surfaces, composed of a hydrophilic patch on a hydrophobic background. 12,13 Interestingly, we found that the CT can also predict the thermodynamic limit of stability of such capillary bridges 11 as a function of h, at large wall separations (h > 50 Å). In this work, we focus on homogenous hydrophobic and hydrophilic surfaces and test whether the CT can also predict the properties of WCBs at very 'small' wall separations, h = 15−50 Å.…”

Experimental investigations of liquid bridge rupture between a sphere and a spherical concave

“…A simulation model was constructed in the FLUENT environment to explore the gravitational effect of liquid rupture. In a nanoscale dimension, the geometry of the liquid bridge formed by water was investigated by Tang et al [21]. Even when the diameter of the plate surface was only 4 nm, the formed liquid bridge complies with the capillary theory on a macroscopic scale.…”