Molecular dynamics simulation of droplet nucleation and growth on a rough surface: revealing the microscopic mechanism of the flooding mode

Enhancement in Heat Transfer Performance of Water Vapor Condensation on Graphene-Coated Copper Surfaces: A Molecular Dynamics Study

The condensation of water vapor plays a crucial role in various applications, including combating water scarcity. In this study, by employing molecular dynamics simulations, we delved into the impact of graphene coatings on water vapor condensation on copper surfaces. Unique to this work was the exploration of various levels of graphene coverage and distribution, a facet largely unexplored in prior investigations. The findings demonstrated a notable increase in the rate of water vapor condensation and heat transfer performance as the graphene coverage was reduced. Using graphene coverages of 84%, 68%, and 52%, the numbers of condensed water molecules were 664, 735, and 880 molecules/ns, respectively. One of the most important findings was that when using the same graphene coverage of 68%, the rate of water vapor condensation and heat transfer performance increased as the graphene coating became more distributed. The overall performance of the water condensation correlated well with the energy and vibrational interaction between the graphene and the copper. This phenomenon suggests how a hybrid surface can enhance the nucleation and growth of a droplet, which might be beneficial for tailoring graphene-coated copper surfaces for applications demanding efficient water vapor condensation.

Uniform and Persistent Jumping Detachment of Condensed Nanodroplets

Realizing jumping detachment of condensed droplets from solid surfaces at the smallest sizes possible is vital for applications such as antifogging/frosting and heat transfer. For instance, if droplets uniformly jump at sizes smaller than visible light wavelengths of 400-720 nm, antifogging issues could be resolved. In comparison, the smallest droplets experimentally observed so far to jump uniformly were around 16 μm in radius. Here, we show molecular dynamics (MD) simulations of persistent droplet jumping with a uniform radius down to only 3.6 nm on superhydrophobic thin-walled lattice (TWL) nanostructures integrated with superhydrophilic nanospots. The size cutoff is attributed to the preferential cross-lattice coalescence of island droplets. As an application, the MD results exhibit a 10× boost in the heat transfer coefficient (HTC), showing a -1 scaling law with the maximum droplet radius. We provide phase diagrams for jumping and wetting behaviors to guide the design of lattice structures with advanced antidew performance.

Nanoarray-Embedded Hierarchical Surfaces for Highly Durable Dropwise Condensation

Durable dropwise condensation of saturated vapor is of significance for heat transfer and energy saving in extensive industrial applications. While numerous superhydrophobic surfaces can promote steam condensation, maintaining discrete microdroplets on surfaces without the formation of a flooded filmwise condensation at high subcooling remains challenging. Here, we report the development of carbon nanotube array-embedded hierarchical composite surfaces that enable ultra-durable dropwise condensation under a wide range of subcooling (ΔT_sub = 8 K–38 K), which outperforms existing nanowire surfaces. This performance stems from the combined strategies of the hydrophobic nanostructures that allow efficient surface renewal and the patterned hydrophilic micro frames that protect the nanostructures and also accelerate droplet nucleation. The synergistic effects of the composite design ensure sustained Cassie wetting mode and capillarity-governed droplet mobility (Bond number < 0.055) as well as the large specific volume of condensed droplets, which contributes to the enhanced condensation heat transfer. Our design provides a feasible alternative for efficiently transferring heat in a vapor environment with relatively high temperatures through the tunable multiscale morphology.

Role of the microridges on cactus spines

Cactus spines have inspired a wide range of micro- and nano-structures that cause droplets to move spontaneously and directionally. The conical shape and the surface wettability gradient are two typical characteristics in such systems. The cross section of the existing conical fibers is usually assumed to be an ideal circle. In fact, microridges are observed on the spine surface of the cactus, and the function is not yet fully understood. The present work thus focuses on how microridges affect droplet self-transport. Structures mimicking microridges are first investigated by constructing pyramidal cross sections with concave or convex lateral faces. The dissipative particle dynamics method is then employed to numerically investigate and theoretically analyze the dynamic behaviors of droplets on these conical fibers with different cross sections. The results show that the microridges reduce the base radius and the contact area of the droplet, thereby increasing the driving force and reducing the friction force. Moreover, by mimicking the microridges structure, we propose a conical fiber with a triple concave cross section, which increases the droplet velocity and the distance traveled over the traditional circular fiber. This work reveals the role of the microridges in the droplet self-transport, which opens up new prospects for the manufacture of fiber systems for microfluidics and liquid manipulation.

Flow Condensation Heat Transfer Characteristics of Nanochannels with Nanopillars: A Molecular Dynamics Study

Flow condensation in nanochannels is a high-efficiency method to deal with increasingly higher heat flux from micro/nanoelectronic devices. Here, we study the flow condensation heat transfer characteristics of nanochannels with different nanopillar cross-sectional areas and heights using molecular dynamics simulation. Results show that two phases containing vapor in the middle of the channel and liquid near walls can be distinguished by obvious interfaces when the fluid is at a stable state. The condensation performance can be promoted by adding nanopillars. With the increase in nanopillar cross-sectional areas or heights, the time that the fluid spends to reach stability will be put off, while the condensation performance enhances. Different from the small enhancement of nanopillar cross-sectional areas, the condensation heat transfer performance improves significantly at a higher nanopillar height, which increases the heat transfer rates by 11.6 and 35.8% when heights are 6a and 8a, respectively. The preeminent condensation heat transfer performance is ascribed to the fact that nanopillars with a higher height disturb the vapor-liquid interface and vapor region, which not only allows vapor atoms with strong Brownian motion to collide with nanopillar atoms directly but also increases deviations of vapor-liquid potential energy to facilitate condensation heat transfer in nanochannels.

Effects of frost formation on the ice adhesion of micro-nano structure metal surface by femtosecond laser

In this work, we study the effect and evolution of frost condensation on the ice adhered micro-nano structured Ni metal surface. By measuring the ice adhesion strength to the porous-structured surface under different frosting coverage, it is found that the ice adhesion on the surface changes not only with the morphology but also with the different evolution stages of the frosting coverage. Combining the changes in surface morphology and low-temperature wettability, the reasons for the changes in the ice adhesion strength are attributed to the change of wettability of the porous-structured surface under different frosting coverage, which affects the infiltration state of the surface and results in different mechanical interlocking, air-pockets and cracks states at the ice-solid interface. Moreover, it is found that the ice adhesion strength of the hydrophobic and hydrophilic porous-structured surface tends to be the same when the surface is heavily frosted. In addition, we demonstrate that both the hydrophobic and hydrophilic columnar-structured surfaces have smaller ice adhesion strength than the original surface without any micro-nano structures regardless of whether the surface is frosted or not, and the measured lowest ice adhesion strength of the hydrophobic columnar-structured surface can be down to 30 kPa in all frosted evolution stages, which is less than one tenth of the original surface. The findings in this work suggest that the influence of frosting should be fully considered when designing the icephobic micro-nano structured metal structure, and it can further serve as an important guidance for the design of passive icephobic surfaces expected to be free from the frosting effect.

Failure and Recovery of Droplet Nucleation and Growth on Damaged Nanostructures: A Molecular Dynamics Study

The condensate flooding during dropwise condensation causes serious deterioration in heat transfer performance. In this study, the three-dimensional large-scale molecular dynamics simulation is carried out to investigate the droplet state transition from local flooding mode to Wenzel or from Wenzel to Cassie due to the droplet coalescence under the effect of nanostructure size. In particular, the effect of nanostructure breakage on droplet nucleation and growth is discussed to reveal the mechanism of dropwise condensation heat transfer deterioration. As a potential solution, the lubricant-impregnated surface is proposed to recover the preferred Cassie state by regulating the dynamic wetting characteristics of droplets, and thus the detrimental effect of nanostructure breakage could be effectively avoided.

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities

The model of vapor condensation heat transfer on downward-facing surfaces with different wettabilities is built by a two-dimensional (2D) lattice Boltzmann method. Dynamic evolution of condensate microdroplets on different wettability surfaces is simulated and the influence on heat transfer performance is analyzed. Moreover, the mechanism of a heterogeneous wettability surface enhancing condensation heat transfer is explored by investigating the condensate behaviors in the process of condensation. The numerical results indicate that as the contact angle of the homogeneous wettability surface increases, the initial nucleation time of the condensate is prolonged, while the departure time of the condensate is reduced significantly. The temperature adjacent to the gas-liquid interface, especially in the three-phase contact line region, is much higher than elsewhere due to the release of latent heat during condensation. Coalescence and detachment behaviors of condensate droplets cause the average heat flux to fluctuate locally with time. For the hybrid wettability surface, if the proportion of hydrophobic regions is small, the condensation heat transfer performance will be deteriorated. However, increasing the hydrophobic-hydrophilic ratio has a positive effect on enhancing heat transfer. It is found that a critical hydrophobic-hydrophilic ratio exists to optimize the heat transfer performance. For the gradient wettability surface, directional migration induced by capillary force facilitates the removal of condensate droplets, thereby enhancing the condensation heat transfer. Furthermore, a larger wetting gradient benefits to further improve the heat transfer performance. The results are valuable for optimally designing the heat transfer enhancement of vapor condensation on functionalized surfaces with heterogeneous wettability.

Heterogeneous nucleation of argon vapor on the nanostructure surface with molecular dynamics simulation

The study of vapor condensation on surface has important engineering significance. The condensation nucleation process of vapor on substrates with different pillar structures is studied through molecular dynamic simulation. The condensation droplets on the pillar with various heights and solid fractions exhibit Wenzel state, Cassie state and the transformation from Wenzel state to Cassie state. The results show that the condensation efficiencies are correlated with the state of droplet and it is explained from the perspective of heat transfer. For the Wenzel state, the droplet fills the gap of pillar and the form of the heat conduction change with the growth of cluster. In the initial of condensation droplets, the heat conduction is similar with various heights of pillar. As the condensation droplets grow, the efficiency of heat conduction enhances with the increasing of height of pillar. For the Cassie state, the form of heat conduction is perpetual during the condensation process with the thermal resistance of the droplet dominated due to the droplets suspended on the pillar. The efficiency of heat transfer is insensitive to the height of pillar. The form of heat conduction for the transformation state transforms from Wenzel state to Cassie state leading to the reduction of condensation rate. The droplet formed in the Wenzel state has the higher transfer efficiency than the Cassie one.

Molecular dynamics study of convective heat transfer mechanism in a nano heat exchanger

With the rapid development of micro/nano electro-mechanical systems, the convective heat transfer at the micro/nanoscale has been widely studied for the thermal management of micro/nano devices. Here we investigate the convective heat transfer mechanism of a nano heat exchanger by the employment of molecular dynamics simulation with a modified thermal pump method. First, the temperature jump and velocity slip are observed at the wall–fluid interfaces of the nano heat exchanger. Moreover, the larger Kapitza resistance in the entrance region weakens the convective heat transfer. Second, the heat transfer performance of the nano heat exchanger can be improved by increasing the surface wettability of the solid walls owing to more fluid atoms being involved in heat transport at the walls when the wall–fluid interaction is enhanced. Meanwhile, the strong surface wettability results in the appearance of the quasi-solid fluid layers, which improves the heat transfer between walls and fluids. Finally, we point out that when the surface wettability of the nano heat exchanger is weak, the heat transfer of the hot fluid side is better than that of the cold fluid side, while the convective heat transfer performances of the cold and hot fluid sides are reversed when the surface wettability is strong. This is because of the feebler temperature jump of the hot fluid side when wall–fluid interaction is small and the greater velocity slip of the cold fluid side for walls with large wall–fluid interaction.

The convective heat transfer mechanism in a nano heat exchanger is investigated using molecular dynamics simulation.

Dynamic Wettability on the Lubricant-Impregnated Surface: From Nucleation to Growth and Coalescence

The surface dynamic wettability during droplet nucleation and growth involved with phase change is different from the static wettability formed from a sessile drop. Revealing this dynamic wettability of the lubricant-impregnated surfaces (LISs) and identification of the consistency between the wettability during condensation and the static wettability are of significant importance. In this study, we investigated condensation of water droplets on LISs using molecular dynamics simulations. All possible morphologies on LISs were investigated considering the effects of interfacial tension and lubricant thickness. The exploration of droplet behaviors from nucleation to growth and coalescence revealed four nucleation mechanisms and six growth modes. The lubricant was observed to be beneficial for the formation of droplets and maintaining dropwise condensation mode. The present investigation also established that the consistency between the wettability during condensation and the static wettability was determined by the solid-water-oil interface and the lubricant thickness. A map was proposed which helps in deciding whether the wettability during condensation is the same as the static wettability on LIS.

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.

Effect of substrate wettability and flexibility on the initial stage of water vapor condensation

Understanding the mechanisms of controlling vapor condensation on surfaces is of significant importance in many fields. Despite many efforts made in the investigation of vapor condensation, few studies concern the condensation on flexible substrates, especially in microscale. In this paper, the condensation of high temperature water vapor on substrate with various flexibilities and wettabilities is investigated using molecular dynamics simulation. The results indicate that when substrates with the same flexibility vary from hydrophobic to hydrophilic, the condensation rate increases and the condensation mode changes from no-condensation to dropwise condensation, incomplete filmwise condensation and filmwise condensation, and meanwhile, the heat exchange between the water vapor and the substrate becomes more efficient; when substrates with the same wettability vary from rigid to flexible, the nucleation density, the condensation rate and the heat exchange efficiency increase significantly. In particular, the condensation modes on rigid and flexible substrates with the same wettability are generally the same except for the substrates with εwater-Cu = 0.4 kcal mol-1; and the critical values of substrate spring constant for the condensation mode transition are about 80-100 kcal mol-1 Å-2 when εwater-Cu = 0.4 kcal mol-1. Therefore, changing the flexibility of the substrate is proposed as a new way to control the condensation mode at the initial stage of water vapor condensation to meet design requirements.

Coalescence and wetting mechanism of Al droplets on different types of carbon for developing wettable cathodes: a molecular dynamics simulation

So far, there have been few studies on the microscopic wetting behavior of aluminum liquid on cathode surfaces, which is critical for developing wettable cathode materials. In the present study, an investigation on the coalescence and wetting mechanism of Al droplets on different carbonaceous substrates has been performed via molecular dynamics (MD) simulation for developing wettable cathodes. The growth rate of liquid bridge, the mean squared displacement, the balanced contact angle, and the time of full coalescence were calculated to describe the coalescence and wetting of the Al droplets. The results illustrate the sequence of full coalescence time for the Al droplets: DG < HCNT < VCNT ≈ AC and the corresponding balanced contact angles were 47.98°, 53.32°, 55.02°, and 63.12°, respectively. Furthermore, the presence of defects on DG will increase the time of coalescence and the contact angle but the directions of defects have little influence. The free energy analysis indicates that the defects reduce the solid-liquid interaction and the work done for removing the Al droplet from the substrates so that the wettability is weaker than that for perfect graphene, which also explains the balanced wettability of Al droplets on the other substrates. In addition, the surface roughness increases the contact angle of Al liquid on AC (from 62° to 113°-120°) and hence, the wettability is changed from good to poor. In general, our results can improve the understanding of the wetting of AC and graphene by Al liquid at the atomic level, which can provide direction and theoretical guidance for further research on wettable cathodes.

Influence of surface topography attributes on settlement and adhesion of natural and synthetic species

Surface topographies of various sizes, shapes, and spatial organization abound in nature. They endow properties such as super-hydrophobicity, reversible adhesion, anti-fouling, self-cleaning, anti-glare, and anti-bacterial, just to mention a few. Researchers have long attempted to replicate these structures to create artificial surfaces with the functionalities found in nature. In this review, we decompose the attributes of surface topographies into their constituents, namely feature dimensions, geometry, and stiffness, and examine how they contribute (individually or collectively) to settlement and adhesion of natural organisms and synthetic particles on the surface. The size of features that comprise the topography affects the contact area between the particle and surface as well as its adhesion and contributes to the observed adsorptive properties of the surface. The geometry of surface perturbations can also affect the contact area and gives rise to anisotropic particle settlement. Surface topography also affects the local stiffness of the surface and governs the adhesion strength on the surface. Overall, systematically studying attributes of surface topography and elucidating how each of them affects adhesion and settlement of particles will facilitate the design of topographically-corrugated surfaces with desired adsorption characteristics.

Tuning nanostructured surfaces with hybrid wettability areas to enhance condensation

Vapor condensation is widespread in natural and industrial applications. Rapid and efficient condensation plays an essential role in improving energy efficiency. Despite numerous efforts over the past few decades, the fundamental mechanism of condensation and the microscopic features of condensed droplets are not well understood. Moreover, designing a nanostructured surface with wetting contrast to enhance dropwise condensation remains unclear. Herein, through molecular dynamics simulation, we characterized the condensation processes on various nanopillar surfaces, including the nucleation, growth and coalescence of nanodroplets. During condensation, the droplet size grows linearly with time as V ∝ t, and the coalescence between small droplets can affect the resultant wetting mode of large droplets. The results indicate that the cooperation between spatially ordering nucleation and dropwise growth endows hybrid nanopillar surfaces with better heat and mass transfer performance compared with other homogeneous nanopillar surfaces. Moreover, an interesting dewetting transition occurring on hydrophobic nanopillar surface was observed during droplet growth, the nucleation site and dewetting transition were analyzed based on potential energy field of surface. By varying the geometric parameters of the nanopillar, we found that the condensation rate of the hybrid nanopillar surface increases with the increase of surface solid fraction. The dense nanopillar array can not only restrain the formation of Wenzel mode droplet, but also enhance the condensation rate, which provides a guidance for the design of hybrid nanostructured surfaces.