Molecular Simulation And Theoretical Study On The Dissolution Kinetics And Stability Of Interfacial Nanobubble

In recent years,nanoscale bubbles have been found on solid surfaces and in liquid phase,which are respectively called interfacial nanobubbles and bulk nanobubbles.Studies have shown that the life of these bubbles is hours,days or even weeks.The unexpected stability of nanobubbles is contrary to classical thermodynamics.According to the Laplace equation,due to its small radius of curvature,the pressure inside the nanobubbles will be very high in this case,so bubbles on the nanoscale cannot exist at all.For bubbles of micron or nanometer size,classical theory predicts that they will disappear in an instant.Because the existence of nanobubbles breaks the conventional understanding of gas behavior,and it has potential applications in many fields such as pharmaceuticals,Flotation,and water cleaning,the stability mechanism of nanobubbles is the core issue in this field.This paper mainly studies the stability of nanobubbles through computer simulation methods combined with some theoretical analysis.The research results are as follows:(1)Curved interfaces between coexisting vapor and liquid phases are ubiquitous in nature,and the question of whether the Henry's law is applicable for highly curved vapor/liquid interfaces remains unsolved.Using stable surface nanobubbles that have highly curved interfaces as examples,we investigate the viability of Henry's law with molecular simulations and thermodynamic analysis.We show that the prediction of Henry's law displays an increasingly deviation from the simulation results as nanobubble curvature increases.Such a curvature dependence of Henry's law constant is ascribed to the nonideality of gas in liquid phase because of the required gas supersaturation for stabilizing nanobubbles.Based on the effect,we develop a relationship for determining Henry's law constant from the level of gas supersaturation,as well as a robust relationship between gas supersaturation and nanobubble radius.(2)Although the stability of most surface nanobubbles observed can be well interpreted by contact line pinning and supersaturation theory,there is increasing evidence that at least for certain situations,contact line pinning is not required for nanobubble stability.This raises a significant question of what is the stability mechanism for those sessile nanobubbles.Through molecular dynamics simulations,in this work,we report a mechanism for stabilizing surface nanobubbles on flat and homogeneous substrates.It is attributed to the deformation of a soft substrate induced by the formed nanobubble,which in turn stabilizes the nanobubble via impeding the contact line motion,similar to self-pinning of microdroplets on soft substrates.This mechanism can interpret,depending on the specified conditions,how surface nanobubbles can remain stable in the absence of contact line pinning.(3)To understand the reason behind the amazing stability of nanobubbles,it is very important to understand how nanobubbles will disappear.In previous research,researchers found that interfacial nanobubbles are similar to droplets,and the stick-slip behavior also occur during their dissolution.This raises an important question whether the cause of the stick-slip behavior that occurs during the dissolution of interfacial nanobubbles is consistent with the droplets.If they are not consistent,what is the mechanism.In this work,we have established a theoretical model in which we thought that the friction between the nanobubble contact line and the substrate surface is related to its contact angle.From this model,We explained the stick-slip behavior during the dissolution of interfacial nanobubbles.Then we used molecular dynamics simulation to verify the theoretical model.As we expected,the stick-slip phenomenon of nanobubbles was also observed during the simulation.