Molecular Dynamics of Nanobubbles’ Collapse in Ionic Solutions

Abstract

Bubble collapse is a complex phenomenon that results in a variety of effects, such as, for instance, luminescence with different features, [1][2][3][4] and has practical implications, such as the possible use of bubbles as carriers or in micropumps, [5] and their daily use in inkjet printers to eject ink. Herein, we investigate the effect of salts during bubbles' collapse in water. To this end, we performed molecular dynamics (MD) simulations of the collapse of empty cavities of initial radius 10 created in aqueous ionic solutions of LiCl and CsCl ( % 6 m) at 280, 300, and 360 K. The large salt concentration is a computational expedient to facilitate the analysis of the MD simulations. The SPC model [6] implemented in TINKER [7][8][9] was chosen for water because the diffusion coefficients are very close to the experimental values.[6] The OPLS-AA force field [10] is used for the ions. A similar model successfully reproduced the variation of surface tension of aqueous electrolytic solutions.[11] Ewald summation was applied to calculate long-range electrostatic interactions. Constant temperature and constant pressure molecular dynamics algorithms were used for the equilibration of the systems. The SHAKE algorithm was active. All systems were made by cubic boxes of % 45 of side filled with 232 cations, 232 anions and 2199 water molecules. Bubbles were created and then equilibrated by the introduction of a single repulsive Lennard-Jones potential located at the centre of the empty cavity. Collapse starts after the removal of the repulsive potential and the temperature of the system varied freely. At least five statistically independent collapses for each system were run and the data presented are the average over the runs.Ions have long been classified in terms of the Hofmeister scale [12] as either kosmotropes (from "kosmos" meaning order makers) or chaotropes (from "chaos" meaning order breakers) according to their relative abilities to create ordered structures in water. Small ions are kosmotropic, have high charge densities, form strong electrostatic ordering of nearby water molecules, and break hydrogen bonds of water molecules. Large ions are chaotropic, have low charge densities, are not able to order the nearby water molecules, and the surrounding water molecules retain their pattern of hydrogen bonds.[13] Li + is strongly kosmotropic; Cs + is strongly chaotropic, Cl -is weakly chaotropic. Experimentally, the degree of water structuring is reflected in its viscosity, which is higher for the solvation of kosmotropic ions and lower for chaotropic ones. Viscosity, in turn, is one of the parameters that influence the dynamics of bubbles. Salts have a tendency to slow down bubble coalescence, [14][15][16][17][18][19] although as stated recently the general behaviour of "bubble coalescence process in aqueous electrolyte solutions remains unresolved". [18] Collapse events were initially analyzed by monitoring the pouring of the water molecules into the empty cavity and fitting the results with the logistic function ...

Interface dynamics of microscopic cavities in water

“…We note here that recently, Lugli and Zerbetto studied nanobubble collapse in ionic solutions by MD simulations on similar length scales. 23 While their MD data compares favorably with our results their interpretation and conclusions in terms of the RP equation are different. We will resume this discussion in the conclusion section.…”

Nonequilibrium all-atom molecular dynamics simulation of the bubble cavitation and application to dissociate amyloid fibrils

“…When the pressure is negative, bubbles are formed in the liquid and crashed when the pressure becomes positive. 12 All these techniques have been developed and employed to study the bubble nucleation, 11,[13][14][15] to investigate the nanobubble collapse in water, 16 and to validate the well-known Rayleigh-Plesset equation 17,18 which was theoretically developed to describe the bubble dynamics driven by a low amplitude sound wave in an infinity fluid. [8][9][10]19 As mentioned above, methods for simulating the bubble inertial cavitation have been developed, but to our best knowledge, there are currently no methods for simulating the bubble stable cavitation, where bubbles undergo harmonic vibration in size, and the first aim of this work is to develop such a method.…”

Ab initio based polarizable force field parametrization

“…Understanding the molecular level detail of anion-water interaction is of fundamental importance to explain ͑i͒ the behavior of water interfaces ͑that affect biological, atmospheric, pharmaceutical, and industrial processes͒, 1-4 ͑ii͒ the dynamics of solvation shell in the bulk ͑which can now be probed with ultrafast spectroscopic techniques͒, [5][6][7][8][9][10][11] and ͑iii͒ the mechanism for nanobubbles' coalescence ͑salts show the tendency to slow it down͒. 12,13 It is now accepted that all the above mentioned phenomena are strongly affected by the molecular polarization that plays a fundamental role in determining the structural, orientational, dynamical, and thermodynamical properties of the system. Accounting for nonaddititive forces in liquid water allows one to accurately predict the relative contribution of hydrogen-bond interactions and gives a reasonable description of water-liquid interface.…”