Interfacial thermal resonance in an SiC-SiC nanogap with various atomic surface terminations.

Quasi-Casimir coupling can induce phonon heat transfer across a sub-nanometer vacuum gap between monoatomic solid walls without electromagnetic fields. However, it remains unclear how the atomic surface terminations in diatomic molecules contribute to phonon transmission across a nanogap. Herein, we study the thermal energy transport across an SiC-SiC nanogap with four pairs of atomic surface terminations using classical nonequilibrium molecular dynamics simulations. In the case of identical atomic surface terminations, the net heat flux and thermal gap conductance are much greater than those in the nonidentical cases. Thermal resonance occurs between identical atomic terminated layers, whereas it vanishes between nonidentical ones. A notable heat transfer enhancement in the identical case of C-C is due to optical phonon transmission, with thermal resonance between the C-terminated layers. Our findings deepen the understanding of phonon heat transfer across a nanogap and provide insights into thermal management in nanoscale SiC power devices.

Estimation of surface free energy at microstructured surface to investigate intermediate wetting state for partial wetting model.

While partial wetting at nano-/microstructured surfaces can be described using the intermediate wetting state between the Cassie-Baxter and Wenzel states, the limitations of the partial wetting model remain unclear. In this study, we performed surface free energy analysis at a microstructured Si-water interface from both theoretical and experimental viewpoints. We experimentally measured the water contact angle on microstructured Si surfaces with square holes and compared the measured values with theoretical predictions. Furthermore, the surface free energy was analyzed using the effective wetting area estimated from the measured contact angle and electrochemical impedance spectroscopy results. We verified the validity of the partial wetting model for fabricated Si surfaces with a hole aperture a less than 230 µm and a hole height h of 12 µm, and for a < 400 µm, h = 40 µm. The model was found to be applicable to microstructured Si surfaces with a/h < 10.

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 nanogap via solid-like liquid layers adsorbed on solid surfaces. Here, we elucidate that phonon transmission across a nanogap can be induced by quasi-Casimir coupling via adsorbed liquid layers using classical nonequilibrium molecular dynamics simulation. We modulated the gap distance to verify the existence of quasi-Casimir coupling between interfacial solid-liquid or liquid-liquid layers. Thermal resonance can be induced between two liquid layers by quasi-Casimir coupling, agitating the co-occurrence of thermal resonance between interfacial solid layers, while a liquid monolayer disturbs the resonance. The thermal resonance between liquid layers results in a larger heat flux and thermal gap conductance compared with those in the vacuum gap case, while the liquid layer limits the acoustic phonon transmission in the interfacial solid layers. The fundamental understanding of quasi-Casimir heat transfer at the solid-liquid interface could pave the way for future nanoscale energy transport and thermal management.

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.

Intermediate wetting state at nano/microstructured surfaces.

A general partial wetting model to describe an intermediate wetting state is proposed in this study to explain the deviations between the experimental results and classical theoretical wetting models for hydrophobic surfaces. We derived a theoretical partial wetting model for the static intermediate wetting state based on the thermodynamic energy minimization method. The contact angle based on the partial wetting model is a function of structural parameters and effective wetting ratio f, which agrees with the classical Wenzel and Cassie-Baxter models at f = 1 and 0, respectively. Si samples including porous surfaces, patterned surfaces and hierarchical nano/microstructured surfaces were prepared experimentally, having the same chemical composition but different physical morphology. We found that the experimental water contact angles deviate significantly from the classical Wenzel and Cassie-Baxter models but show good agreement with the proposed partial wetting model.

Effective Wetting Area Based on Electrochemical Impedance Analysis: Hydrophilic Structured Surface.

Wettability on nano/microstructured surfaces is gaining remarkable interest for a wide range of applications; however, little is known about the effective wetting area of the solid-liquid interface. In this study, the effect of wettability on electrochemical impedance was experimentally investigated to obtain a better understanding of the effective wetting area. We demonstrate that the water contact angle decreases significantly at hydrophilic surfaces with denser nano/microstructures. Based on the analysis of equivalent electrical circuits, we found that the electrochemical impedance decreases with reducing the water contact angle, showing a dependence on the effective wetting area, i.e., the real solid-liquid contact area. Also, the charge transfer resistance at low frequency was found to be the dominant parameter to estimate the effective wetting area at the solid-liquid interface.

Scale effect of slip boundary condition at solid-liquid interface.

Rapid advances in microelectromechanical systems have stimulated the development of compact devices, which require effective cooling technologies (e.g., microchannel cooling). However, the inconsistencies between experimental and classical theoretical predictions for the liquid flow in microchannel remain unclarified. Given the larger surface/volume ratio of microchannel, the surface effects increase as channel scale decreases. Here we show the scale effect of the boundary condition at the solid-liquid interface on single-phase convective heat transfer characteristics in microchannels. We demonstrate that the deviation from classical theory with a reduction in hydraulic diameters is due to the breakdown of the continuum solid-liquid boundary condition. The forced convective heat transfer characteristics of single-phase laminar flow in a parallel-plate microchannel are investigated. Using the theoretical Poiseuille and Nusselt numbers derived under the slip boundary condition at the solid-liquid interface, we estimate the slip length and thermal slip length at the interface.

Molecular dynamics study on condensation/evaporation coefficients of chain molecules at liquid-vapor interface.

The structure and thermodynamic properties of the liquid-vapor interface are of fundamental interest for numerous technological implications. For simple molecules, e.g., argon and water, the molecular condensation/evaporation behavior depends strongly on their translational motion and the system temperature. Existing molecular dynamics (MD) results are consistent with the theoretical predictions based on the assumption that the liquid and vapor states in the vicinity of the liquid-vapor interface are isotropic. Additionally, similar molecular condensation/evaporation characteristics have been found for long-chain molecules, e.g., dodecane. It is unclear, however, whether the isotropic assumption is valid and whether the molecular orientation or the chain length of the molecules affects the condensation/evaporation behavior at the liquid-vapor interface. In this study, MD simulations were performed to study the molecular condensation/evaporation behavior of the straight-chain alkanes, i.e., butane, octane, and dodecane, at the liquid-vapor interface, and the effects of the molecular orientation and chain length were investigated in equilibrium systems. The results showed that the condensation/evaporation behavior of chain molecules primarily depends on the molecular translational energy and the surface temperature and is independent of the molecular chain length. Furthermore, the orientation at the liquid-vapor interface was disordered when the surface temperature was sufficiently higher than the triple point and had no significant effect on the molecular condensation/evaporation behavior. The validity of the isotropic assumption was confirmed, and we conclude that the condensation/evaporation coefficients can be predicted by the liquid-to-vapor translational length ratio, even for chain molecules.