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...

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.