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• Boiling on flat mixed-wettability surface with an increasing temperature is studied. • The condition is consistent with the situation of radiation from photons, plasma, or gas molecules with high temperature. • Compared to hydrophobic surface, it enhances solid/liquid interaction and promotes bubble nucleation. • Compared to hydrophilic surface, it delays vapor film formation. • An appropriate proportion of area favorable for bubble nucleation and heat transfer enhancement was found. This paper introduces a molecular dynamics investigation of the influence of mixed-wettability on the boiling of a water layer over a flat plate surface with nonlinearly increasing wall temperature. The first-type Dirichlet temperature condition, which is considered for the first time in the analysis of the wettability influence on nucleation and boiling at the atomic scale, is consistent with the case of irradiation from photons, plasma, or high-temperature gas molecules. The simulation of the density evolution of water molecules in the nucleation zone shows that the surface with an optimal proportion (60 %∼70 %) of hydrophilic walls is most efficient for nucleation on the studied mixed-wettability substrates, attributing to the increased surface potential energy on the walls. Compared to the hydrophilic or hydrophobic surfaces, the mixed-wettability surfaces transfer more energy from the wall to the liquid. This is because the hydrophobic portion retards the forming of vapor layer and the hydrophilic portion induces an efficient liquid/solid interaction, with a lowered interfacial heat resistance and promoted nucleation. The results also indicate that there exists an appropriate area ratio that is most advantageous for nucleation and boiling intensification under such temperature-increasing boundary conditions. Based on this work, it may be possible to successfully manage boiling at the nanoscale by exploiting the coupled effects of hybrid wettability and irradiation. [ABSTRACT FROM AUTHOR]

• The heat transfer characteristics of flow boiling with inlet subcooling have been investigated using a "heat pump model". • The effect of the depth of the groove structure on the heat transfer characteristics at the nanoscale is investigated. • The bubble growth process at different wall temperatures is studied, and the mechanism of bubble nucleation promoted by the nanostructures is analyzed. As a prominent limiting factor in the performance of microelectronic devices, the improvement of heat dissipation is of utmost importance. The enhancement of heat and mass transfer during boiling by nanostructured surfaces has been demonstrated in many studies. In this study, a nanoscale flow-boiling model is developed by molecular dynamics method in order to deeply investigate the microscopic mechanism of nanostructure-enhanced heat transfer. Simple liquid argon is heated by grooved substrate to explore the effect of the depth of the cavities on the boiling heat transfer. The results indicate that the grooved surface improves heat transfer performance through two key mechanisms. Firstly, it induces bubble nucleation, and secondly, it delays the onset of film boiling. The presence of cavities allows the argon atoms within to absorb additional energy from the surrounding walls and be in closer proximity to the heating layer. This reduction in solid thermal resistance results in increased heat transfer efficiency, leading to earlier bubble nucleation and a greater propensity for nucleation within the cavities. The low horizontal velocity of argon atoms within the cavities suggests their ability to retain a significant number of atoms, thereby effectively mitigating the deterioration of heat transfer caused by the formation of a vapor film on the solid wall. The comprehensive performance is observed to improve with increasing depth of the cavities within the range investigated in this study. [ABSTRACT FROM AUTHOR]

• A revised L-J potential for the bubble nucleation study of liquid argon is proposed. • The onset temperature of bubble nucleation of liquid argon is reduced to 90 K. • A pressure control method based on force balance is firstly introduced. • The availability of the revised L-J potential is verified by properties of liquid argon. Bubble nucleation in boiling heat transfer is a microscale phenomenon, which is usually investigated by the molecular dynamics simulation method with the study object of argon. The simple potential model of argon favors revealing the bubble nucleation mechanism. However, the published works indicate that the required heating temperature for achieving bubble nucleation inside liquid argon is unreasonable and usually over the critical temperature. Therefore, in this study, a revised Lennard-Jones (L-J) potential for bubble nucleation study of argon is proposed based on the "PK" norm that bubble nucleation happens when the average kinetic energy of a group of liquid atoms exceeds their potential barrier (absolute value of atomic potential energy). The parameters α and β are added to the original L-J potential to weaken the phase transition barrier and reduce atomic balance distance, corresponding to the functions of lowering onset nucleation temperature and avoiding the unreasonable change of liquid argon properties, respectively. Besides, a moving top wall based on the force balance is conducted to maintain the simulation system pressure at the expected value. Simulation results indicate that based on the revised L-J potential and pressure controlled at one atmosphere, only the evaporation phenomenon happens when the heat source temperature is fixed at 85 K within the simulation time of 10 ns, while it converts into the bubble nucleation phenomenon as the thermostat temperature is raised to 90 K, which is very close to the onset boiling temperature of liquid argon (87.2 K). In addition, the relative differences between the original and revised potentials in the density, thermal conductivity, and thermal diffusion coefficient are 1.31%, 33.50%, and 233.33%, respectively. The difference in the third property is significant, but compared with the benchmark value obtained by the popular property database RefProp, the computational accuracy of the revised potential is higher than the original potential. Hence, the revised L-J potential is more suitable for the bubble nucleation study of argon by using the molecular dynamics simulation method. [ABSTRACT FROM AUTHOR]

• The nucleation of water on a substrate with different sizes of grooved substrate is investigated. • An idea based on molecular energy competition is extended to explain water nucleation mechanism. • The idea is extended to obtain some important parameters for bubble nucleation further. Generation of a bubble nucleus is the initial stage of nucleate boiling and an essential part of the nucleate boiling theory. In the present study, the molecular dynamics simulation method is conducted to study the bubble nucleation of water on a grooved copper substrate with hydrophilicity. An idea based on the competition between molecular kinetic energy and molecular potential restriction, corresponding to the motivity and resistance for bubble nucleation at the nanoscale, is extended to identify the intrinsic regime of bubble nucleation of water. If a group of water molecules obtain enough kinetic energy such that their potential restriction is broken and become activated molecules at some time, the bubble nucleus is taken shape. Simulation results of bubble nucleation behaviors on the substrate with different sizes of grooves indicate that this idea works well in clarifying the intrinsic regimes of bubble nucleation and the nucleation difference between different cases. In addition, this idea can be used to obtain the incipient nucleation position, nucleation time, and the volume of the bubble nucleus without any subjective threshold. [ABSTRACT FROM AUTHOR]

To promote the engineering application of additive in enhancing nucleate boiling, the effect of foreign particles on bubble nucleation is investigated by the molecular dynamics simulation method. Some foreign atoms are added into the pure argon system with a proportion of 5%. The liquid is heated by a hydrophilic smooth platinum substrate to achieve bubble nucleation. The results show that foreign atoms have significant impacts on bubble nucleation. Compared with the pure liquid system, the bubble nucleation efficiencies in the aspects of the incipient nucleation time and temperature are promoted by the foreign atom with a smaller energy parameter than argon atom but decreased with the increasing energy parameter. Then, the intrinsic regime of the difference between pure liquid and mixed liquid in bubble nucleation is fully illustrated based on the competition between atomic potential energy and atomic kinetic energy, which are the restriction and impetus for bubble nucleation, respectively. Furthermore, based on the influencing regime of foreign atoms in bubble nucleation, a further application of foreign atoms in controlling the nucleation position is developed. • The comparisons between pure liquid and mixed liquid are conducted. • Foreign atom with a smaller energy parameter improves nucleation efficiency. • Intrinsic regime of the differences between pure liquid and mixed liquid is explained. • An application of foreign atoms in controlling nucleation position is developed. [ABSTRACT FROM AUTHOR]