Molecular Dynamics Simulations Of The Effect Of Wettability On Explosive Boiling Of A Nanoscale Thin Liquid Film

The rapid development of microelectromechanical technology leads to quite miniaturized and compacted microelectronic devices,which can produce a huge amount of heat and burn the devices.Boiling heat transfer has been widely used in the thermal management of micro electronic devices owing to its high heat transfer coefficient with small temperature variation.The boiling phase change can only be tuned precisely from the micro/nano scale bubble nucleation or dynamics.In addition,the boiling phase change in the microelectronic devices usually occurs in micro/nano scale.Therefore,it is of great significance to study the nucleation and bubble behavior of boiling from micro/nano scale.In this thesis,the effects of the wettability and the liquid film thickness on the nucleate boiling of the nanoscale thin liquid film are studied by molecular dynamics simulation.The main contents of the study include three parts:(1)The effect of wettability on the onset of nucleate boiling(ONB)was investigated.The boiling behavior of nano thin film only exhibits evaporation and explosive boiling as the superheat increasing,which is quite different from the traditional pool boiling.The required superheat that triggers the onset of nucleate boiling is smaller for the liquid film on hydrophobic surface than that on hydrophilic surface.This findings in nanoscale are consistent with classical nucleation theory.However,if the superheat exceeds the ONB for both hydrophobic and hydrophilic surfaces,the ONB will occurs faster on the hydrophilic surface than the hydrophobic surface.The reason can be attributed to the unique heat transfer mode in nanoscale.The thermal conduction dominates the heat transfer mode in nanoscale due to the molecular collision in nanoscale,thus,the heat transfer can be enhanced if the solid-liquid interaction force is lager.Therefore,the ONB of thin liquid film on hydrophilic surface is triggered faster because of the larger solid-liquid interaction force.(2)The thickness also affects the boiling behavior of the liquid film.For very thin liquid films(less than 1.5 nm)on both the hydrophilic and the hydrophobic surface,free surface evaporation only occurs.For a medium thickness range(2 nm-3 nm),the explosive boiling occurs on hydrophilic surface,while the evaporation occurs hydrophobic surface.Explosive boiling occurs on both hydrophilic and hydrophobic surfaces if the liquid film is thick enough(larger than 6 nm in this study).The results can be explained as follows:the explosive boiling results from the energy accumulation beneath the liquid film,as well as the temperature gradient along the thickness of the thin liquid film.Both the energy accumulation and temperature gradient are more remarkable for the thick liquid films on hydrophilic surfaces than thin liquid films on hydrophilic surfaces.(3)A hydrophilic-hydrophobic hybrid surface is proposed to precisely tune the boiling behaviors.It is found that a lager average temperature of the liquid film can be obtained if the hydrophilic proportion is large,so does the accumulating energy.In this case,the liquid film favors to explosive boiling.When the thickness of the liquid film reaches a certain value,the "liquid film brake" will be observed at the junction of the hydrophilic and hydrophobic wall.To sum up,a new phenomenon that the nanoscale boiling exhibits different behaviors from traditional pool boiling was reported.Wettability plays different roles on the required superheat and trigged time of ONB.In a certain liquid film thickness range,the classical nucleation theory is suitable for the ONB of liquid film in nanoscale,if the thickness of the liquid film below a certain value it will no longer be applied.The accumulation of energy and the temperature gradient inside the liquid film response to the explosive boiling.The hydrophilic-hydrophobic hybrid surface was proposed to to precisely tune the boiling behaviors of thin liquid films.Therefore,it is important to reveal the new mechanism of nucleation of liquid film boiling in nanoscale for the development of microelectronics industry.