Many natural superhydrophobic structures have hierarchical two-tier roughness which is empirically known to promote robust superhydrophobicity. We report the wetting and dewetting properties of two-tier roughness as a function of the wettability of the working fluid, where the surface tension of water/ethanol drops is tuned by the mixing ratio, and compare the results to one-tier roughness. When the ethanol concentration of deposited drops is gradually increased on one-tier control samples, the impalement of the microtier-only surface occurs at a lower ethanol concentration compared to the nanotier-only surface. The corresponding two-tier surface exhibits a two-stage wetting transition, first for the impalement of the microscale texture and then for the nanoscale one. The impaled drops are subsequently subjected to vibration-induced dewetting. Drops impaling one-tier surfaces could not be dewetted; neither could drops impaling both tiers of the two-tier roughness. However, on the two-tier surface, drops impaling only the microscale roughness exhibited a full dewetting transition upon vibration. Our work suggests that two-tier roughness is essential for preventing catastrophic, irreversible wetting of superhydrophobic surfaces.

We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 -1636 nm grown via sputter deposition. While exhibiting a constant value up to ∼100 nm, the thermal conductivity increases with film thickness thereafter. This trend is in stark contrast with previous thermal conductivity measurements of amorphous systems, which have shown thickness-independent thermal conductivities. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of ∼1.8 THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.

The continuous wavelet transform is employed to analyze the dynamics and time-dependent energy distribution of phonon wave-packet propagation and scattering in molecular dynamics simulations. The equations of the one-dimensional continuous wavelet transform are presented and then discretized for implementation. Practical aspects and limitations of the transform are discussed, with attention to its application in the analysis of molecular dynamics simulations. The transform is demonstrated using three examples that are relevant to nanoscale thermal transport. First, a system of wave packets that interfere in both the spatial and Fourier domains are separated by the wavelet transform, allowing the measurement of each packet's contribution to the system energy. Second, the wavelet transform is applied to a multiple wave-packet simulation of a silicon, heavy-silicon interface. The wavelet-based calculation of mode-dependent transmission is validated through comparison to literature results and theoretical predictions. Third, the dynamic scattering of a large amplitude wave packet is studied using the wavelet transform. The transform reveals a transition in the structure of the energy distribution. Unlike current techniques, the wavelet transform can be used to determine how the energy of a simulated system is distributed in time, in space, and among wave numbers, simultaneously. The ability to resolve phonon motion and energy from a vibrating ensemble of atoms in a molecular dynamics simulation makes the wavelet transform a promising technique for probing the physical mechanisms of nanoscale thermal transport.

A wide range of modern technological devices utilize materials structured at the nanoscale to improve performance. The efficiencies of many of these devices depend on their thermal transport properties; whether a high or low conductivity is desirable, control over thermal transport is crucial to the continued development of device performance. Here we review recent experimental, computational, and theoretical studies that have highlighted potential methods for controlling phonon-mediated heat transfer. We discuss those parameters that affect thermal boundary conductance, such as interface morphology and material composition, as well as the emergent effects due to several interfaces in close proximity, as in a multilayered structure or superlattice. Furthermore, we explore future research directions as well as some of the challenges related to improving device thermal performance through the implementation of phonon engineering techniques.

Although thermal transport in silicon is dominated by phonons in the solid state, electrons also participate as the system approaches, and exceeds, its melting point. Thus, the contribution from both phonons and electrons must be considered in any model for the thermal conductivity, k, of silicon near the melting point. In this paper, equilibrium molecular dynamics simulations measure the vibration mediated thermal conductivity in Stillinger-Weber silicon at temperatures ranging from 1400 to 2000 K -encompassing the solid-liquid phase transition. Non-equilibrium molecular dynamics is also employed as a confirmatory study. The electron contribution may then be estimated by comparing these results to experimental measurements of k. The resulting relationship may provide a guide for the modeling of heat transport under conditions realized in high temperature applications, such as laser irradiation or rapid thermal processing of silicon substrates.