Simulation And Experiment Investigation Of Bubble Nucleation Dynamics

Thermal bubble nucleation initiates the liquid-vapor phase transition,which exists widely in scientific research and is used extensively in engineering applications.It involves the most basic state change in nature.Due to the very short nucleation time(microseconds to nanoseconds),it is difficult to observe the complete process of thermal bubble nucleation in the experiment.In this article,molecular dynamics simulations and theoretical analysis are carried out to investigate the thermal bubble nucleation processes inside a graphene nanochannel,from which the mechanism behind thermal bubble nucleation is revealed.In addition,from the perspective of medical practical applications,the bubble dynamics were studied by experimental method to obtain the flow field around the bubble and its dynamics information.We also studied the influence of the rigid wall in a confined channel on the dynamics of bubble collapse,and the following main findings were obtained.(1)Molecular dynamics simulations were carried out to investigate the thermal bubble nucleation process and mechanism in the water system in nanoscale homogeneous system and heterogeneous confined channel.The simulation results show that when liquid water is confined in a nanoscale channel,the thermal bubble nucleation temperature increases sharply with the decrease of channel height.More importantly,the heterogeneous bubble nucleation temperature can be significantly higher than the corresponding homogeneous bubble nucleation temperature when the channel height is below 3.1 nm,which is in contrast to the common understanding in the classical nucleation theory that homogeneous thermal bubble nucleation sets an upper limit for nucleation temperature under a given pressure.By investigating the number density and potential energy distribution of water molecules along with the channel height in heterogeneous systems,we attributed this unexpected observation to the near-wall layering structure of the water molecules in nanoconfined systems.It is believed that this more ordered arrangement will hinder thermal bubble nucleation due to the lower potential energy.In this paper,the thermal bubble nucleation processes of homogeneous and heterogeneous systems are compared from the perspective of energy.The underlying mechanism is attributed to the fact that a thermal bubble is more easily nucleated at the position where the local potential energy is relatively high in the whole system.(2)Molecular dynamics simulations were carried out to study the thermal bubble nucleation process in the confined nanochannel with different channel height and different surface properties in heterogeneous systems.The simulation results show that the temperature of thermal bubble nucleation is not only highly correlated with the channel height,but also strongly dependent on the surface properties of the channel.The results show that the nucleation site is also strongly dependent on the surface property of the channel.In the weak interaction case,the weaker solid-liquid interaction strength reduces the nucleation temperature because it is easier for water molecules to escape from the weak bonding at the solid-liquid interface.Therefore,voids can be generated at lower temperatures,which promote phase-transition possibility and reduce the nucleation temperature.When the solidliquid interaction strength increases,more ordered arrangement of the near-wall molecules tends to suppress void generation near the solid plate and prevents phase transition from occurring.The thermal bubble nucleates above the layered structure and far away from the solid wall.In fact,once the interaction strength increases to a certain value,the temperature of thermal bubble nucleation of the heterogeneous system saturates.It is found that the nucleation temperature of the system can be reduced and the bubble nucleation site can be accurately regulated by adjusting the surface property of the channel.Moreover,the bubble can be driven to make directional motion along the channel wall by the wettability gradient.These findings provide us with a method for controlling and manipulating the thermal bubble nucleation site in a micro-/nanoscale confined space.(3)Using the experimental platform of Particle Image Velocimetry(PIV),the flow field information of bubble dynamics in the rectangular confined channel is obtained.The evolution process of the bubble flow field is visualized.At the same time,the bubble morphology in the free field and the rectangular confined channel is compared.It is found that the existence of the rigid wall will affect the equivalent radius of bubble,bubble shape,and bubble existence period.(4)The effect of rigid wall on bubble dynamics is studied by using PIV.The effect of different boundary conditions on the dynamics behavior during bubble collapsing is further studied.The results show that the rigid wall in the confined channel changes the direction of the high-speed jet generated when the bubble collapse.It can change the object motion in the flow field.It is found that the boundary conditions not only determine the bubble nucleation temperature and site but also affect its morphology and dynamics.In the background of medical application,the rectangular channel is used to simulate the confined space of the ureter.We measure the displacement of the stone model produced by holmium laser lithotripsy.The influence of rigid wall in the confined channel on bubble dynamics is verified.The stone model displacement is compared with the instantaneous flow field synchronically.We found one of the reasons for stone retropulsion during laser lithotripsy,which has a guiding significance for the surgery and the design of medical equipment.