UTFacultiesTNWEventsPhD Defence Shantanu Maheshwari

PhD Defence Shantanu Maheshwari

Molecular dynamics simulations of nanobubbles and nanodrops 

Shantanu is a PhD-student in the Physics of Fluids research group. His supervisor is Detlef Lohse from the faculty of Science and Technology.  

Understanding of bubbles and drops at the nanoscale is of primary importance to many technological applications. Although lot of theoretical understanding has been developed in the last few decades for larger size bubbles and drops, fundamental understanding of nanobubbles and nanodrops in some aspects is still inadequate. In this thesis we revealed and explained a few phenomena related to the stability and growth/dissolution of nanobubbles and nanodrops with the help from molecular dynamics (MD) simulations.

We first addressed the stability of a single surface nanobubble on a heterogeneous surface. We tested the conditions for a stable surface nanobubble developed in our group by systematically performing MD simulations. We showed that the contact angle of surface nanobubbles is not given by Young’s law and instead follows the theoretical expression as a function of gas oversatu- ration in the bulk liquid. We also showed that dissolution of surface nanobub- bles exhibit "stick-jump" motion of the contact line similar to the dissolution of surface nanodrops.

Continuing our work on stability of single surface nanobubble, we studied the more realistic scenario, i.e. stability of multiple surface nanobubbles. We gave a potential explanation to many experimental observations of Ostwald ripening of surface nanobubbles despite theoretical calculations suggesting the stability of multiple surface nanobubbles. For higher gas-solid interaction energies, gas particles form a layer of gas particles at the solid surface which connects the liquid-vapour interface of two nanobubbles to overcome the con- tact line pinning barrier. We showed that the gas-solid interaction energy plays a crucial role in the stability of surface nanobubbles which hitherto has not been considered in any macroscopic theories.

In the next chapter, we studied the dynamics of formation of a vapour nanobubble around a heated nanoparticle. We investigated the conditions required to nucleate a vapour nanobubble in terms of the nanoparticle temperature and the temperature "far away" from the nanoparticle surface. We investigated the role of dissolved gas on nucleating conditions and found that it enhances the formation of nanobubble around a heated nanoparticle. A prediction for the nucleation conditions was made by using a simple heat balance argument with additional assumptions. Theoretical predictions suggest that enhancement in nanobubble formation is primarily due to the change in the critical point of the gas-liquid mixture. However, we showed that beyond a certain gas concentration, enhancement in the nanobubble nucleation cannot be explained by the change in the critical point of the mixture alone. Higher gas concentration increases the oversaturation of gas particles in the bulk liquid which form the weak gaseous spots that help in the formation of a vapour nanobubble.

Next we moved on from nanobubbles to nanodrops. We performed simulations of a nanodrop on curved surfaces to estimate the magnitude of line tension to study the effect of surface curvature on the line tension of nanodrops. We showed that Young contact angle is independent of the surface curvature and the radius of the nanodrop can be predicted from simple geometric arguments for a fixed volume. The magnitude of the line tension is found to be dependent on the sign of the surface curvature but not on its magnitude. The line tension was maximum for nanodrops on planar surface followed by concave surfaces and minimum for convex ones.

In the last chapter we studied the dissolution of a single- and multi-component nanodrop in the bulk liquid. We studied the dissolution behaviour for various interaction strengths between the drop components. We showed that the dis- solution behaviour of a single component nanodrop can be well described by a macroscopic diffusion model. However, this diffusion model deviates from MD simulation for multicomponent drop at lower interaction energies between the two components of the drop. Upon analysing the concentration field of the two drop components, we revealed that the deviation from the macroscopic model is due to the non-uniform concentration of the more soluble component within the drop. We observed an increase in the concentration near the liquid-liquid interface which is due to the very high curvature of the nanodrop.