A new hypothesis for cavitation nucleation in gas saturated solutions:Clustering of gas molecules lowers significantly the surface tension

Cavitation in water generally takes place at much lower negative pressure than predicted from theories.In this work,we try to stress the discrepancy from the influence of the dissolved gas on cavitation nucleation.By combining molecular dynamics simulation and thermodynamic analysis,we evaluated the lowering of surface tension as a function of density of gas molecules in gas clusters formed in aqueous solution.We found that the obtained surface tension of small gas clusters is much more substantially reduced than expected.The surface tension lowering and the non-ideality of gas molecules in the clusters are then taken into account in determining the nucleation of cavitation,and as a consequence,the required negative pressure for cavitation becomes comparable to experimental values.Thus,we give an alternative explanation for the discrepancy of cavitation pressure between experiment and theory,i.e.,it is the substantially reduced surface tension for small gas nuclei,which have not been taken into account in theory,along with the ideal gas approxiamtion that induce its deviation from the experimental values.

Surface active agents stabilize nanodroplets and enhance haze formation

Although many organic molecules found commonly in the atmosphere are known to be surface-active in aqueous solutions, their effects on the mechanisms underlying haze formation remain unclear. In this paper, based on a simple thermodynamic analysis, we report that the adsorption of amphiphilic organics alone not only lowers the surface tension,but also unexpectedly stabilizes nanodroplets of specific size under water vapor supersaturation. Then we determine how various factors, including relative humidity, water activity effect due to dissolution of inorganic components as well as surface tension effect due to surface adsorption of organic components, cooperatively induce the stability of nanodroplets.The nanodroplet stability behaviors not captured in the current theory would change the formation mechanism of haze droplets, from the hygroscopic growth pathway to a nonclassical two-step nucleation pathway.

Multiplicity of thermodynamic states of van der Waals gas in nanobubbles

The gas-containing nanobubbles have attracted extensive attention due to their remarkable properties and extensive application potential.However,a number of fundamental aspects of nanobubbles,including thermodynamic states for the confined gas,remain still unclear.Here we theoretically demonstrate that the van der Waals(vd W)gases confined in nanobubbles exhibit a unique thermodynamic state of remarkably deviating from the bulk gas phase,and the state transition behavior due to the sizedependent Laplace pressure.In general,the vd W gas inside nanobubbles present multiple stable or transient states,where 0–2 states are for supercritical gas and 0–4 for subcritical gas.Our further analysis based on Rayleigh–Plesset equation and free energy determination indicates that the gas states in nanobubbles exhibits different levels of stability,from which the coexistence of multiple bubble states and microphase equilibrium between droplets and bubbles are predicted.This work provides insight to understand the thermodynamic states appeared for gas in nanobubbles.

Role of substrate softness in stabilizing surface nanobubbles

The contact line pinning and supersaturation theory for the nanobubble stability has attracted extensive concerns from experimental investigators,and some experimenters argue that the contact line pinning is unnecessary.To interpret the experimental observations,we have proposed previously through molecular dynamics simulations that the deformation of soft substrates caused by surface nanobubbles may play an important role in stabilizing surface nanobubbles,while yet no quantitative theory is available for explanation of this mechanism.Here,the detailed mechanism of self-pinning-induced stability of surface nanobubbles is investigated through theoretical analysis.By manipulating substrate softness,we find that the formation of surface nanobubbles may create a deformation ridge nearby their contact lines which leads to the self-pinning effect.Theoretical analysis shows that the formation of nanobubbles on sufficiently rigid substrates or on liquid-liquid interfaces corresponds to a local free energy maximum,while that on the substrates with intermediate softness corresponds to a local minimum.Thus,the substrate softness could regulate the surface nanobubble stability.The critical condition for the self-pinning effect is determined based on contact line depinning,and the effect of gas supersaturation is explored.Finally,the approximate stability range for the surface nanobubbles is also predicted.

The existence and stability of bulk nanobubbles:a long-standing dispute on the experimentally observed mesoscopic inhomogeneities in aqueous solutions

In theory,nanobubbles can stably exist with a lifetime of microseconds at most,but numerous experimental observations demonstrate that nanobubbles in bulk solution can be stable from hours to weeks.Although various conjectures on the stability mechanism of bulk nanobubbles,such as the contaminant mechanism,skin mechanism,surface zeta potential mechanism,are proposed,there has not yet been a unified conclusion.Since bulk nanobubbles show great potential in a wide spectrum of applications and are relevant to a number of unsolved questions on cavitation and nucleation,the debate over their stability mechanisms has been active.In the past,extensive studies have been carried out to understand the mechanism of nanobubble stability,and important insights have already been provided.This paper will provide a brief overview of our current understanding of the unexpected stability of bulk nanobubbles.

Stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions

In our previous work[2022 Phys.Chem.Chem.Phys.249685],we used molecular dynamics simulations to show that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces.This observation closely matches the origin of stability of microemulsions and inspired us to propose here that,in certain cases,stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions:the nanobubble phase and the bubble-containing solution phase coexist with the external gas phase.This three-phase coexistence is then validated by molecular dynamics simulations.The stability mechanism for bulk nanobubbles is thus given:the formation of a compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension,and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles.With the monolayer model,we further interpret several strange behaviors of bulk nanobubbles:gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension,and the typical nanobubble size of 100 nm can be explained through the small bending constant of the monolayer.Finally,through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can exist ubiquitously in aqueous solutions.

Many-body dissipative particle dynamics simulation of wetting phenomena

With the development of the simulation of particle dynamics,the traditional dissipative particle dynamics(DPD)method can not satisfy the needs of research in static or dynamic wetting phenomena.However,the Many-body DPD approach extends the ability of the traditional method to simulate the interface between solid and liquid or some other situation.In this paper,we propose a Many-body DPD program to simulate the solidliquid interface and get satisfactory results.