Unexpected large impact of small charges on surface frictions with similar wetting properties

Translucency and negative temperature-dependence for the slip length of water on graphene

Carbonous materials, such as graphene and carbon nanotubes, have attracted tremendous attention in the fields of nanofluidics due to the slip at the interface between solid and liquid. The dependence of slip length for water on the types of supporting substrates and thickness of the carbonous layer, which is critical for applications such as sustainable cooling of electronic devices, remains unknown. In this paper, using colloidal probe atomic force microscopy, we measured the slip length l_s of water on graphene supported by hydrophilic and hydrophobic substrates, i.e., SiO₂ and octadecyltrimethoxysilane (OTS). The l_s on single-layer graphene supported by SiO₂ is found to be 1.6 ± 1.9 nm, and that of OTS is 8.5 ± 0.9 nm. When the thickness of few-layer graphene increases to 3-4 layers, both l_s values gradually converge to the value of graphite (4.3 ± 3.5 nm). Such a thickness dependence is termed slip length translucency. Further, l_s is found to decrease by about 70% when temperature increases from 300 K to 350 K for 2-layer graphene supported by SiO₂. These observations are explained by analysis based on the Green-Kubo relation and McLachlan theory. Our results provide the first set of reference values for the slip length of water on supported few-layer graphene. They can not only serve as a direct experimental reference for solid-liquid interaction, but also provide a guideline for the design of nanofluidics-based devices, for example thermo-mechanical nanofluidic devices.

Surface Potential Driven Water Harvesting from Fog

Access to clean water is a global challenge, and fog collectors are a promising solution. Polycarbonate (PC) fibers have been used in fog collectors but with limited efficiency. In this study, we show that controlling voltage polarity and humidity during the electrospinning of PC fibers improves their surface properties for water collection capability. We experimentally measured the effect of both the surface morphology and the chemistry of PC fiber on their surface potential and mechanical properties in relation to the water collection efficiency from fog. PC fibers produced at high humidity and with negative voltage polarity show a superior water collection rate combined with the highest tensile strength. We proved that electric potential on surface and morphology are crucial, as often designed by nature, for enhancing the water collection capabilities via the single-step production of fibers without any postprocessing needs.

Atomic structure causing an obvious difference in thermal conductance at the Pd-H2O interface: a molecular dynamics simulation

Thermal transfer across solid-liquid interfaces is influenced by multiple factors such as surface wettability, interfacial water layer density, molecular structure, and mass density depletion length. However, the dominant factors in interfacial heat transport are yet to be investigated. In this work, we explore the contributions from these factors by employing the Pd-water model for water molecules forming ordered, partially ordered, and disordered structures on Pd (100), (110) and (111) surfaces, respectively. The results revealed that the ordered water layer on the (100) surface can introduce a "phonon bridge" at the solid-liquid interface to improve thermal transfer, while the partially ordered water layer on the (110) surface can further promote thermal transfer due to the enhanced interfacial friction. On the other hand, the decreased density depletion length also makes dominant contributions to the enhancement of interfacial thermal transfer. The results are explained by the interfacial friction coefficient, surface potential energy distribution and density depletion length. We also introduce an efficient technique by tuning the vacancy defects on the solid surface to tune the atomic structure as well as the thermal transfer. Our study reveals the complex relationship between the atomic structure of the crystal face, the water layer structure and the thermal boundary conductance, which will inspire more experimental and theoretical studies toward the improvement of interfacial thermal transport by tuning the structure of the water layer.