Molecular dynamics investigation of surface roughness scale effect on interfacial thermal conductance at solid-liquid interfaces

Computing the Work of Solid-Liquid Adhesion in Systems with Damped Coulomb Interactions via Molecular Dynamics: Approaches and Insights

Recently, the dry-surface method [Langmuir2016, 31, 8335−8345] has been developed to compute the work of adhesion of solid–liquid and other interfaces using molecular dynamics via thermodynamic integration. Unfortunately, when long-range Coulombic interactions are present in the interface, a special treatment is required, such as solving additional Poisson equations, which is usually not implemented in generic molecular dynamics software, or as fixing some groups of atoms in place, which is undesirable most of the time. In this work, we replace the long-range Coulombic interactions with damped Coulomb interactions, and explore several thermal integration paths. We demonstrate that regardless of the integration path, the same work of adhesion values are obtained as long as the path is reversible, but the numerical efficiency differs vastly. Simple scaling of the interactions is most efficient, requiring as little as 8 sampling points, followed by changing the Coulomb damping parameter, while modifying the Coulomb interaction cutoff length performs worst. We also demonstrate that switching long-range Coulombic interactions to damped ones results in a higher work of adhesion by about 10 mJ/m² because of slightly different liquid molecule orientation at the solid–liquid interface, and this value is mostly unchanged for surfaces with substantially different Coulombic interactions at the solid–liquid interface. Finally, even though it is possible to split the work of adhesion into van der Waals and Coulomb components, it is known that the specific per-component values are highly dependent on the integration path. We obtain an extreme case, which demonstrates that caution should be taken even when restricting to qualitative comparison.

Study on Mercury Adsorption and Desorption on Different Modified Biochars

To develop high-performance biochar adsorbents, the adsorption ability and rate of untreated (BC-CK) and six modified biochars with amino (BC-NH), epoxy (BC-C₂H₂O), ethoxy (BC-C₂H₅O), hydrosulfuryl (BC-SH), selenium (BC-Se), and chitosan (BC-Chitosan) on Hg(II) and MeHg were investigated by simulated experiment. The results indicated that the some modified biochars (BC-NH, BC-C₂H₂O, BC-C₂H₅O and BC-Chitosan) showed lower adsorption capacity than the untreated, possibly due to the decreased specific surface area and pore volume. Whereas, BC-SH and BC-Se was improved immensely by forming stable -SH-Hg and Hg-Se with the adsorption capacity 1.26 and 1.51 times as much as BC-CK, respectively. In spite of that, Hg desorption capacities and rates of all biochars were extremely low, exhibiting great adsorption stability of biochars on Hg in another way. In addition, BC-Chitosan performed the highest adsorption speed. These provided insights on the adsorption effectiveness for Hg in the aqueous solution that was critical for evaluating the application of modified biochars.

Quasi-Casimir coupling can induce thermal resonance of adsorbed liquid layers in a nanogap

In a vacuum nanogap, phonon heat transfer can be induced by quasi-Casimir coupling in the absence of electromagnetic fields. However, it is unknown whether phonons can be transmitted across a...

Molecular Insight into Bubble Nucleation on the Surface with Wettability Transition at Controlled Temperatures

A surface with a smart wettability transition has recently been proposed to enhance the boiling heat transfer in either macro- or microscale systems. This work explores the mechanisms of bubble nucleation on surfaces with wettability transitions at controlled temperatures by molecular simulations. The results of the interaction energy at the interface and potential energy distribution of water molecules show that the nanostructure promotes nucleation over the copper surface and causes lower absolute potential energy to provide fixed nucleation sites for the initial generation of the bubble nucleus and shortens the incipient nucleation time, as compared to the mixed-wettability or hydrophilic nanostructure surface. An investigation on more nanostructured surfaces shows that a surface (F) with a wettability transition temperature of 620.0 K has the shortest average incipient nucleation time at 1672 ps with a wall temperature of 634.3 K. The surface with tunable wettability has also a high interfacial thermal conductance at low superheats, but it may not promote the critical heat flux at high superheats. The heat-transfer performance of the smart surface is better than the plate, the hydrophobic nanostructure, and the mixed-wettability surfaces, while it is lower than the hydrophilic nanostructure surface. This proposes a new method and provides insight for promoting bubble nucleation on a surface with temperature-dependent wettability.

Branching random graph model of rough surfaces describes thermal properties of the effective molecular potential

Fluid properties near rough surfaces are crucial in describing fundamental surface phenomena and modern industrial material design implementations. One of the most powerful approaches to model real rough materials is based on the surface representation in terms of random geometry. Understanding the influence of random solid geometry on the low-temperature fluid thermodynamics is a cutting-edge problem. Therefore, this work extends recent studies bypassing high-temperature expansion and small heterogeneity scale. We introduce random branching trees whose topology reflects the hierarchical properties of a random solid geometry. This mathematical representation allows us to obtain averaged free energy using a statistical model of virtual clusters interacting through random ultrametric pairwise potentials. Our results demonstrate that a significant impact to fluid-solid interface energy is induced by the hierarchical structure of random geometry at low temperature. These calculations coincide with direct Monte Carlo simulations. Due to the study's interdisciplinary nature, the developed approach can be applied to a wide range of quenched disorder systems on random graphs.

Measurement of effective wetting area at hydrophobic solid-liquid interface

HYPOTHESES

The effective wetting area, a parameter somewhat different from the apparent contact area at solid-liquid interfaces, plays a significant role in surface wettability. However, determination of the effective wetting area for hydrophobic surfaces remains an open question. In the present study, we developed an electrochemical impedance method to evaluate the effective wetting area at a hydrophobic solid-liquid interface.

EXPERIMENTS

Patterned Si surfaces were prepared using the anisotropic wet etching method, and the water contact angle and electrochemical impedance were measured experimentally. The effective wetting area at the solid-liquid interface was examined based on the wettability and impedance results.

FINDINGS

The electrochemical impedance for the patterned Si surfaces increased with increasing surface hydrophobicity, whereas the effective wetting area decreased. The intermediate wetting state (i.e. partial wetting model) was confirmed at the patterned Si surfaces, and the effective wetting area was theoretically estimated. The effective wetting area predicted from the electrochemical impedance agreed well with that predicted from the partial wetting model, thereby demonstrating the validity of the electrochemical impedance method for evaluating the effective wetting area at the hydrophobic solid-liquid interface.

Cross-Plane and In-Plane Heat Conductions in Layer-by-Layer Membrane: Molecular Dynamics Study

A material with anisotropic heat conduction characteristics, which is determined by molecular scale structure, provides a way of controlling heat flow in nanoscale spaces. As such, here, we consider layer-by-layer (LbL) membranes, which are an electrostatic assembly of polyelectrolyte multilayers and are expected to have different heat conduction characteristics between cross-plane and in-plane directions. We constructed models of a poly(acrylic acid)/polyethylenimine (PAA/PEI) LbL membrane sandwiched by charged solid walls and investigated their anisotropic heat conduction using molecular dynamics simulations. In the cross-plane direction, the thermal boundary resistance between the solid wall and the LbL membrane and that between the constituent PAA and PEI layers decrease with increasing degree of ionization (solid surface charge density and the number of electric charges per PAA/PEI molecule). When the degree of ionization is low, the cross-plane thermal conductivity of a constituent layer is higher than that of the bulk state. As the degree of ionization increases, however, the cross-plane thermal conductivity of PAA, a linear polymer, decreases because of the increase in the number of in-plane oriented polymer chains. In the in-plane direction, we investigated the heat conduction of each layer and found the enhancement of effective in-plane thermal conductivity again due to the in-plane oriented chain alignment. The heat conduction in the LbL membrane is three-dimensionally enhanced compared to those in the bulk states of the constituent polymers because of the electrostatic interactions in the cross-plane direction and the molecular alignment in the in-plane direction.

Comparative Study of the Heat and Mass Transfer Characteristics between Counter-Flow and Cross-Flow Heat Source Towers

The heat source tower (HST), as a cleaner energy production, which can absorb the low-grade energy from ambient air to drive the heat pump unit without emissions has attracted more and more interest. In addition, HST has excellent economic applicability by using cooling tower equipment, which was idle in winter. However, there are few studies on comparative analysis of thermal behavior between counter-flow and cross-flow HST. A mathematical model suitable for both HST types was developed to identify the performance discrepancies between them. Then a parametric study was carried out in order to investigate the impact of solution and air as well as packing material properties on energy transfer of HSTs. Finally, the characteristics of solution dilution and dehumidification were investigated. As the inlet solution temperature increases, increases first, then decreases gradually, but a transition point occurs in the solution at −5 °C. Moreover, the transition section of moisture transfer direction for counter-flow HST was located in the 0.78 m and 0.26–1.56 m of packing material height, under the condition that the air relative humidity was 50%. In summary, this work intuitively indicates the thermal performance difference between counter-flow and cross-flow HST, also could assist the selection of proper operating conditions in HSTs.

Optimization Method for Grooved Surface Structures Regarding the Evaporation Heat Transfer of Ultrathin Liquid Films at the Nanoscale

Rough nanostructured surfaces can enhance evaporation heat transfer. Most studies artificially optimized the geometry and size during the design of nanostructured surfaces. Instead of the empirical design of nanostructured surfaces, this paper proposes a mathematical optimization method of the grooved nanostructured surface design. This method is inspired by the molecular dynamics simulations of grooved nanostructured surfaces. The results show that the heat transfer performance exhibits a positive correlation with the defined sectional area of the grooved nanostructured surface; thus, this method is developed to convert the maximum heat transfer and evaporation rate to a mathematical conditional extremum solution. The mathematical description of the optimization method is to solve the surface structure with the maximum sectional area when the heat transfer area is constant. Comparing the molecular dynamics (MD) simulation results of the optimal surface and the existing ones under the same simulation conditions indicates that the optimal surface has the best heat transfer performance compared with the other ones. Additionally, discussions on the types of grooved nanostructured surfaces, the materials of solid and liquid, and the wettability of grooved surfaces verify the generality of the calculation results and the optimization method. The explanation of the method is that different nanostructured surfaces have a similar potential energy per liquid atom, which affects the latent heat of the evaporation process. However, the maximum sectional area corresponds to the minimum interfacial thermal resistance and the maximum interaction energy per unit area, which will enhance the heat transfer at the solid-liquid interface. Moreover, a nanostructured surface with the maximum sectional area also obtains the maximum area of the liquid-vapor interface and thus enhances the evaporation heat transfer process.

Heat transfer across a fractal surface

The effects of surface irregularities and imperfections on the thermal resistance at a solid-liquid interface have been investigated using molecular dynamics. The molecular model comprises liquid argon confined between silver walls. The surface roughness was designed using fractal theory, introducing stochastic patterns of multiple scales that resemble realistic surface geometries. In agreement with most previous studies, we find that increasing the strength of the solid-liquid interactions monotonically reduces the thermal resistance across smooth interfaces. Yet, the behavior of the thermal resistance across rough surfaces is more complex. Following the initially anticipated decrease, the thermal resistance starts to increase once the strength of solid-liquid interaction increases past a threshold. We attribute the above behavior to two competing phenomena, namely, the area of the solid-liquid interface and the introduction of vibrational anharmonicities and localization of phonons resulting from the surface roughness. Finally, we demonstrate that, for the same fractal dimension and depth of surface roughness, different surfaces practically have the same thermal resistance, solid-liquid radial distribution function, and liquid density profiles. We conclude that the above fractal parameters are useful in deriving reduced models for properties related to the surface geometry.