On the early and developed stages of surface condensation: competition mechanism between interfacial and condensate bulk thermal resistances

Near-Wall Cavitation Effect: A Molecular Dynamics Study

Cavitation has been the subject of abundant studies, but the internal mechanism of cavitation is less well known. In this article, a microlevel near-wall model was established by using LAMMPS to present the process of cavitation effect. The results of molecular dynamics simulation revealed the fluctuation process of the liquid near the wall with the change in pressure. Molecular dynamics was also used to evaluate the void volume fraction and density distribution of the system. The results exhibited that the cavitation process can be divided into two stages: the initial cavitation stage and the rapid growth stage. Based on these results, the effects of wettability and initial system temperature on the near-wall cavitation effect were demonstrated. The results indicated that the hydrophobic near-wall forms a gas layer to weaken the density fluctuation, while the hydrophilic wall is opposite. Increasing the temperature could positively affect molecular motion and cavitation. This work provides a theoretical basis for further exploration of the cavitation effect.

Molecular Dynamics Simulations of Water Condensation on Surfaces with Tunable Wettability

Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.

Generation and Evolution of Nanobubbles on Heated Nanoparticles: A Molecular Dynamics Study

Molecular dynamics simulations were conducted to investigate the generation and evolution of nanobubbles on heated gold-like nanoparticles (GNPs). The effects of surface wettability (β) and heating intensity (Q) of the GNPs are studied. We found that nanobubbles are generated faster on the superhydrophobic GNP than on the superhydrophilic GNP where nanobubble formation appears after a delay. In the case of the superhydrophilic GNP, the nanobubble is observed to grow explosively because it is initially generated at a distance from the GNP surface instead of on its surface. In the case of the superhydrophobic GNP, the faster generation of the nanobubble is promoted by the larger temperature difference between the GNP and the surrounding fluid and an ultrathin low-density layer that exists before the GNP is heated. For a given β, faster generation and growth of nanobubbles are observed with increasing Q. Furthermore, the maximum radius of the nanobubble is found to be dependent on β and not Q. The mechanism is elaborated based on the thermal resistance analysis at the melting point of GNPs. Additionally, it was found that there exists a threshold Q for nanobubble generation and the threshold value for the case of the superhydrophobic GNP is lower than that for the case of the superhydrophilic GNP. The present results have demonstrated that the superhydrophobic GNP is favorable for fast and energy-saving nanobubble generation. Our work provides further understanding in the generation and evolution of nanobubbles and potentially offers a new insight for nanobubble manipulation.

Dependences of Formation and Transition of the Surface Condensation Mode on Wettability and Temperature Difference

In this work, we use molecular dynamics (MD) simulations to investigate the dependences of formation and transition of surface condensation mode on wettability (β) and vapor-to-surface temperature difference (ΔT). We build a map of different surface condensation modes against β and ΔT based on plenty of MD simulation results and reveal five formation mechanisms and two transition mechanisms. At low β and ΔT, the high free energy barrier (ΔG*) prevents any surface clusters from surviving, therefore no-condensation (NC) is observed. The formation of dropwise condensation (DWC) could evolve from either nucleation or film rupture. Similarly, the formation of filmwise condensation (FWC) could evolve from either nucleation or the adsorption-induced film. The transition between NC and DWC is determined by ΔG* according to classical nucleation theory. The transition between DWC and FWC depends on the stability of condensate film; there emerges the competition between the trend that the uneven condensate film contracts and ruptures to droplets favored by lower β and the trend that the uneven condensate film continues growing promoted by higher ΔT. We finally present a schematic overview of all of the mechanisms revealed for a better understanding of the physical phenomenon of the surface condensation mode.

Hybrid Wettability-Induced Heat Transfer Enhancement for Condensation with NonCondensable Gas

Heat transfer enhancement in dropwise condensation is widely investigated on a superhydrophobic surface with the advances in surface engineering, but the influence of a large amount of noncondensable gas (NCG) has not been clarified. In this work, the condensation heat transfer with a large amount of NCG is investigated by developing a multiphase lattice Boltzmann model for a multicomponent system. First, the condensation of a single droplet on a hydrophobic surface with NCG is simulated, demonstrating the capacity of the present model to capture the behaviors of different components during phase change and predict the significant influence of even a small fraction of the NCG on heat transfer. Then, solid surfaces with mixed wettability are built by introducing a fraction of hydrophilic parts to enhance heat transfer. It is found that there exists an optimized proportion which could maximize the condensation heat transfer efficiency corresponding to a specific mass fraction of NCG. Furthermore, the mechanism of this optimized proportion is revealed by examining the dynamic behaviors of condensation in a typical case, as a balance between a promotion of the nucleation rate and a put off of transition to filmwise condensation.

Self-shedding and sweeping of condensate on composite nano-surface under external force field: enhancement mechanism for dropwise and filmwise condensation modes

In this work, we propose the concept to use the hydrophilic or neutral surface for condensation heat transfer and to use the superhydrophobic surface for enhancement by self-shedding and sweeping of condensate. Molecular dynamics simulation results show that no matter the vapor condenses on the solid surface in dropwise or filmwise mode, the grown-up condensate self-sheds and falls off the superhydrophobic surface, sweeping the growing condensate on the condensing surface downstream. We characterize the dynamics of condensate that the continuous self-shedding and sweeping effectively remove the droplets from the solid surface in dropwise mode or thin the condensate film on the solid surface in filmwise mode, which significantly enhances the condensation heat transfer. We reveal that the mechanism for self-shedding is two-fold: (1) that the external force on condensate bulk defeats the adhesive force between the condensate and the solid surface triggers the self-shedding; (2) the release of the surface free energy of condensate promotes the self-shedding. We also reveal that the mechanism of heat transfer enhancement is essentially due to the timely suppression over the growing condensate bulk on the condensing surface through the self-shedding and sweeping. Finally, we discuss the possible applications.