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

Effect of Nanopillars on the Wetting State and Adhesion Characteristics of Molten Aluminum Droplets

To solve the adhesion problem between molten aluminum and vacuum ladle liner during the electrolytic aluminum production process, the wetting state and adhesion properties of molten aluminum droplets on substrate surfaces with different nanopillars are investigated based on molecular dynamics. The results show that the adhesion strength of molten aluminum droplets in different wetting states has the pattern Young state > Wenzel state > Cassie state. Effects of increasing nanopillar height or interval are poles apart in the wetting state and adhesion characteristics of aluminum molten droplets. The critical height and critical interval of the nanopillar where the wetting state transition occurs are obtained. The increase of the nanopillar width can induce the wetting state transition from the Cassie state to the Wenzel state. In addition, the phantom wall method is applied to study the variation of the separation force. It is found that a peak in the separation force curve occurs when the molten droplet separates from the bottom of the nanopillar interval or the top of the nanopillar. The separation force curves of the droplets in the Young state and the Cassie state have single peaks, while the droplets in the Wenzel state have double peaks.

Theoretical and Three-Dimensional Molecular Dynamics Study of Droplet Wettability and Mobility on Lubricant-Infused Porous Surfaces

Profiting from their slippery nature, lubricant-infused porous surfaces endow with droplets excellent mobility and consequently promise remarkable heat transfer improvement for dropwise condensation. To be a four-phase wetting system, the droplet wettability configurations and the corresponding dynamic characteristics on lubricant-infused porous surfaces are closely related to many factors, such as multiple interfacial interactions, surface features, and lubricant thickness, which keeps a long-standing challenge to promulgate the underlying physics. In this work, thermodynamically theoretical analysis and three-dimensional molecular dynamics simulations with the coarse-grained water and hexane models are carried out to explore droplet wettability and mobility on lubricant-infused porous surfaces. Combined with accessible theoretical criteria, phase diagrams of droplet configurations are constructed with a comprehensive consideration of interfacial interactions, surface structures, and lubricant thickness. Subsequently, droplet sliding and coalescence dynamics are quantitatively defined under different configurations. Finally, in terms of the promotion of dropwise condensation, a non-cloaking configuration with the encapsulated state underneath the droplet is recommended to achieve high droplet mobility owing to the low viscous drag of the lubricant and the eliminated pinning effect of the contact line. On the basis of the low oil-water and water-solid interactions, a stable lubricant layer with a relatively low thickness is suggested to construct slippery surfaces.

Anti-Icing Mechanism for a Novel Slippery Aluminum Stranded Conductor

The icing of transmission conductor seriously threatens the safe operation of power grids. Slippery lubricant-infused porous surface (SLIPS) has shown great potential for anti-icing applications. However, aluminum stranded conductors have complex surfaces, and the current SLIPSs are almost prepared and studied on small flat plates. Herein, the construction of SLIPS on the conductor was realized through anodic oxidation and the anti-icing mechanism of the slippery conductor was studied. Compared to the untreated conductor, the SLIPS-conductor reduces the icing weight by 77% in the glaze icing test and shows very low ice-adhesion strength (7.0 kPa). The excellent anti-icing performance of the slippery conductor is attributed to the droplet impact dynamics, icing delay, and lubricant stability. The dynamic behavior of water droplets is most affected by the complex shape of the conductor surface. Specifically, the impact of the droplet on the conductor surface is asymmetric and the droplet can slide along the depression in low-temperature and high-humidity environments. The stable lubricant of SLIPS increases both the nucleation energy barriers and the heat transfer resistance, which greatly delays the freezing time of droplets. Besides, the nanoporous substrate, the compatibility of the substrate with the lubricant, and the lubricant characteristics contribute to the lubricant stability. This work provides theoretical and experimental guidance on anti-icing strategies for transmission lines.

Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance

Surfaces with controllable and gradient wettability often require an elaborate design of the microstructure or its response under electrical, thermal, optical, pH, and other stimuli. Generally, the wettability change under these physical or chemical effects relies on a complex mechanism that is difficult to be quantitatively described. In this study, an online controlling strategy for surface wettability and the corresponding theoretical model are put forward based on a bilayer graphene-like atomic structure. Molecular dynamics results indicate that the surface wettability varies toward hydrophilicity after sticking a bottom material regardless of its wettability. But such an influence becomes weak with increasing interlayer distance, and the overall wettability approaches that of the upper layer material gradually. This variation is elucidated by the increase of the work of adhesion, providing new insight into the wetting transparency of graphene. A theoretical model of the governing relationship is established based on the work of adhesion, which correlates the overall surface wettability with the interlayer distance and the wettabilities of individual materials. Moreover, a surface with a uniform wettability gradient is achieved by inclining the bottom material. The spontaneous and steady motion of droplets can be induced by this gradient wettability. The relevant speedup behavior is evaluated through a theoretical model considering the varying interlayer distance, which reveals the critical role of the lower layer. This study proposes a novel strategy for controllable wetting and relevant gradient surfaces using prevailing two-dimensional materials, paving new routes to many applications such as microfluidic chips, virus diagnosis, and intelligent sensors.

Cloaking Transition of Droplets on Lubricated Brushes

We study the equilibrium properties and the wetting behavior of a simple liquid on a polymer brush, with and without the presence of lubricant by multibody Dissipative Particle Dynamics simulations. The lubricant is modeled as a polymeric liquid consisting of short chains that are chemically identical with the brush polymers. We investigate the behavior of the brush in terms of the grafting density and the amount of lubricant present. Regarding the wetting behavior, we study a sessile droplet on top of the brush. The droplet consists of nonbonded particles that form a dense phase. Our model and choice of parameters result in the formation of a wetting ridge and in the cloaking of the droplet by the lubricant; i.e., the lubricant chains creep up onto the droplet and eventually cover its surface completely. Cloaking is a phenomenon that is observed experimentally and is of integral importance to the dynamics of sliding droplets. We quantify the cloaking in terms of its thickness, which increases with the amount of lubricant present. The analysis reveals a well-defined transition point where the cloaking sets in. We propose a thermodynamic theory to explain this behavior. In addition, we investigate the dependence of the contact angles on the size of the droplet and the possible effect of line tension. We quantify the variation of the contact angle with the curvature of the contact line on a lubricant free brush and find a negative value for the line tension. Finally we investigate the effect of cloaking/lubrication on the contact angles and the wetting ridge. We find that lubrication and cloaking reduce the contact angles by a couple of degrees. The effect on the wetting ridge is a reduction in the extension of the brush chains near the three phase contact line, an effect that was also observed in experiments of droplets on cross-linked gels.

Nanoscale Thin-Film Boiling Processes on Heterogeneous Surfaces

Acquiring rapid and efficient boiling processes has been the focus of industry as they have the potential to improve the energy efficiency and reduce the carbon emissions of production processes. Here, we report nanoscale thin-film boiling on different heterogeneous surfaces. Through nonequilibrium molecular dynamics simulation, we captured the triple-phase interface details, visualized the bubble nucleation, and recorded the internal fluid flow and thermal characteristics. It is found that nanoscale thin-film boiling without the occurrence of bubble nucleation shows excellent heat and mass transfer performance, which differs from macroscale boiling. In general, rough structures advance the onset time of stable boiling and improve the efficiency. The heat transfer coefficient and heat flux on a rough hydrophilic surface respectively reach to 7.43 × 10⁴ kW/(m²·K) and 1.3 × 10⁶ kW/m² at a surface temperature of 500 K, which are 100-fold higher than those of micrometer-scale thin-film boiling. However, due to the resultant vapor film trapped between the liquid and the surface, the rough hydrophobic surface leads to heat transfer deterioration instead. It is revealed that the underlying mechanism of regulatory effects resulting from surface physicochemical properties is originated from the variation of interfacial thermal resistance. It is available to reduce the overall interfacial resistance and further improve the heat and mass transfer efficiency through increasing surface roughness, enhancing surface wettability, and increasing the area proportion of the hydrophilic region. This work provides guidelines to achieve rapid and efficient thin-liquid-film boiling and serves as a reference for the optimized design of surfaces utilized for high-heat flux removal through vaporization processes.

Enhanced Water Nucleation and Growth Based on Microdroplet Mobility on Lubricant-Infused Surfaces

Lubricant-infused surfaces (LISs) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation free-energy barrier and high droplet mobility. Recent studies showed that oil menisci surrounding condensate microdroplets form distinct oil-rich and oil-poor regions. These topographical differences in the oil surface cause water microdroplets to rigorously self-propel long distances, continuously redistributing the oil film and potentially refreshing the surface for re-nucleation. However, the dynamic interplay between oil film redistribution, microdroplet self-propulsion, and droplet nucleation and growth is not yet understood. Using high-speed microscopy, we reveal that during water condensation on LISs, the smallest visible droplets (diameter: ∼1 μm, qualitatively representing nucleation) predominantly emerge in oil-poor regions due to a lower nucleation free-energy barrier. Considering the significant heat transfer performance of microdroplets (<10 μm) and transient characteristic of microdroplet movement, we compare the apparent nucleation rate density and water collection rate for LISs with oils of different viscosities and a solid hydrophobic surface at a wide range of subcooling temperatures. Generally, the lowest lubricant viscosity leads to the highest nucleation rate density. We characterize the length and frequency of microdroplet movement and attribute the nucleation enhancement primarily to higher droplet mobility and surface refreshing frequency. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed nonlinearity between LISs and the solid hydrophobic surface, we introduce two dominant regimes that influence the condensation efficiency: mobility-limited and coalescence-limited. We compare these regimes based on droplet growth rates and water collection rates on the different surfaces. Our findings advance the understanding of dynamic water-lubricant interactions and provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies.

Head-on Collision of Two Nanodroplets on a Solid Surface: A Molecular Dynamics Simulation Study

Most researchers focus on the collision of a single droplet with a solid surface, while it is common for a droplet to collide with a sessile droplet on a solid surface in reality. This study performed the head-on collision of two nanodroplets on a solid surface using the molecular dynamics simulation method. The effects of impact velocity, interaction intensity between solid and liquid atoms, and the solid fraction of the surface on the collision process are studied with independent simulation cases. The maximum spreading factor and the dimensionless maximum spreading time are recorded and calculated to describe the collision process quantitatively. The simulation results indicate that the maximum spreading factor depends more on the solid fraction than the interaction intensity since it does not fundamentally change the wetting state of the droplet at its maximum spreading state. Because of two different effects, the maximum dimensionless spreading time decreases first and then increases with the interaction intensity, and both effects weaken with the increase of impact velocity. As the solid fraction increases, the maximum spreading factor increases significantly at high impact velocity, and the maximum dimensionless spreading time first decreases and then increases because the wetting state of the coalescent droplet at the maximum spreading moment gradually changes from the Wenzel state to the Cassie state. In general, the initial wetting state of the sessile droplet and the wetting state of the coalescent droplet at the maximum spreading moment have important effects on the maximum spreading factor and the maximum spreading time. We establish a theoretical prediction model for the maximum spreading factor on a smooth surface based on energy conservation with quite good accuracy. This research has improved our understanding of the head-on collision process of two nanodroplets on a solid surface.

Droplet Nucleation and Growth in the Presence of Noncondensable Gas: A Molecular Dynamics Study

The presence of noncondensable gas (NCG) followed by undesirable heat transfer deterioration cannot be avoided in some situations. In this work, droplet nucleation and growth for the Ar-Ne mixed system are investigated using molecular dynamics simulation. Different droplet state transition modes corresponding to the subcooling degree or NCG content are obtained. The interaction between NCG and a droplet caused by gas enrichment near the solid surface is considered to explain the droplet wetting state during the condensation process. Finally, the disappearance mechanism of the flooding mode on the nanostructured surface under a large amount of NCG is clarified from the nanoscale, which could encourage a clear understanding of the NCG effect on dropwise condensation heat transfer on nanostructured superhydrophobic surfaces.

Cloaked Droplets on Lubricant-Infused Surfaces: Union of Constant Mean Curvature Interfaces Dictated by Thin-Film Tension

It has been recently shown that small-volume droplets on lubricant-infused surfaces (LISs) can be analytically modeled using rotationally symmetric constant mean curvature (CMC) surfaces. While such an approach is available for noncloaked droplets, a similar approach is missing for cloaked droplets that are ubiquitous in a number of LIS-related applications. The presence of a thin cloaking film on the top spherical cap portion and its gradual transition to a bulk meniscus remain unaddressed for its implications on the interfacial profile of cloaked droplets. Here, we take into account the cloaking film tension and the disjoining pressure to present a mean curvature-based framework for modeling cloaked droplets on LISs. The transition of the bulk meniscus to a thin film is formulated as a transition regime, which is then modeled as a single imaginary point akin to the Neumann point of noncloaked droplets. We next show that the shape of a small droplet on a LIS essentially obeys a simple fundamental mean curvature relation that changes forms depending on the regimes of lubrication and whether the droplet is cloaked or noncloaked. We validate our framework with the droplet profiles recorded during the evaporation of cloaked droplets in our experiments, as well as those published in the literature. In addition, we also demonstrate the ability to model the shapes of floating droplets on LISs reported in the literature. In addition to quantifying the effect of disjoining pressure on interfacial profiles, we importantly unmask the behavior of the contact line, which is optically covered by the lubricant meniscus around the droplets on LISs.

A comparison between superhydrophobic surfaces (SHS) and slippery liquid-infused porous surfaces (SLIPS) in application

Slippery liquid-infused porous surfaces inspired by the Nepenthes pitcher plant exhibit excellent performances and are known for their extremely low contact angle hysteresis (<5°) and smooth surface. In contrast, superhydrophobic surfaces (SHS) exhibit poor pressure stability, difficulty in self-healing, and difficulty in removing low surface tension liquids or organic solvents, which can affect the stable air layer. Thus, these issues can be avoided through the replacement of SHS with slippery liquid infused porous surfaces (SLIPS). In this review, the theoretical models of SHS and SLIPS are classified initially, and several design standards for the preparation of SLIPS are briefly described. Then, we focus on comparing the differences in the application of SHS and SLIPS, such as pressure stability, transparency, and droplet manipulation. However, there are still some problems that need to be improved during the preparation of SLIPS, such as the evaporation of the lubricant layer, the use of a lubricant layer of toxic perfluoropolyether and other substances, and easily lost nanostructured lubricant layer. Accordingly, several new improved methods are proposed in this review, and finally, the potential applications and development prospects of SLIPS are presented.

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.