Understanding Stability, Rheology and Interfacial Properties of Bulk Nanobubbles by Molecular Simulations and Experiments

Abstract

Since the first image of a nanobubble was published in 2000, researchers have increasingly explored them, further proving their existence. It is reported that nanobubbles have unique properties, including high concentration and super-stability, which can be used in various applications. However, the mechanism of nanobubbles is still unclear.

Precise measurement of bulk nanobubble (BNB) properties is key to understanding their stability. To address these questions, we employed molecular dynamics simulations to quantify the density, internal pressure, charge distribution, and structural characteristics of various nanobubble types and sizes. A key objective is to disentangle the contributions of surface tension and interfacial electrostatic forces to internal pressure. Additionally, we calculated surface tension both directly and via Young-Laplace's Theorem, and investigated how nanobubble conditions and gas type influence it. After this, we used the results to discuss factors that help stabilize BNBs. Furthermore, we conducted both molecular simulations and experiments to analyse the rheological behaviours of BNBs-water systems under different volume fractions. Our simulations reveal that the zero-shear viscosity of BNB-water systems exceeds that of pure water and increases significantly with volume fraction. We derived a relative viscosity model grounded in classical theories to describe this behaviour. Our experimentally validated model predicts zero-shear viscosity within 8.93% of measured values. Under Couette flow, bulk nanobubbles become unstable at high shear rates, coalescing into larger bubbles. Prior to coalescence, shear viscosity also rises with increasing volume fraction. Lastly, we predicted the zeta potential of BNBs and examined the effect of electric fields.

In conclusion, this study systematically characterises the internal, interfacial, and rheological properties of BNBs, thereby guiding potential applications of these materials.