Water flow in graphene nanochannels driven by imposed thermal gradients: the role of flexural phonons

Accurate control of fluid transport in nanoscale structures is key to enable the design of foreseeable nanofluidic devices with applications in many fields such as chip cooling, energy conversion, drug delivery and medical diagnosis. Here, inspired by the experimental observation of intrinsic thermal ripples in graphene and by recent advances in the manipulation of 2D nanomaterials, we introduce a graphene-based thermal nanopump which produces controlled and continuous liquid flow in nanoslit channels. We investigate the performance of this thermal nanopump employing large scale molecular dynamics simulations. Upon systematically imposing thermal gradients, a net water flow towards the low-temperature zone is observed, achieving flow velocities up to 4 m s^-1. We observe that water flow rates increase monotonically due to larger ripple fluctuations on the graphene layers as higher thermal gradients are applied. Moreover, we find that the out-of-plane flexural phonons in graphene are responsible for flow generation wherein lower frequency phonon branches are activated with higher imposed thermal gradients. Furthermore, by modifying the wettability of the channel walls, an increase of 50% in the water flow rates is observed, showing that the efficiency of the proposed thermal pump can be enhanced by tuning the channel wall hydrophobicity. Our results indicate that thermal gradients can be employed to drive continuous water flow in graphene nanoslit channels with potential applications in nanofluidic devices.

Intercalation leads to inverse layer dependence of friction on chemically doped MoS2

We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS₂). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS₂exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined withab initiocalculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.

Tribological Properties of 2D Materials and Composites-A Review of Recent Advances

This paper aims to provide a theoretical and experimental understanding of the importance of novel 2D materials in solid-film lubrication, along with modulating strategies adopted so far to improve their performance for spacecraft and industrial applications. The mechanisms and the underlying physics of 2D materials are reviewed with experimental results. This paper covers some of the widely investigated solid lubricants such as MoS2, graphene, and boron compounds, namely h-BN and boric acid. Solid lubricants such as black phosphorus that have gained research prominence are also discussed regarding their application as additives in polymeric materials. The effects of process conditions, film deposition parameters, and dopants concentration on friction and wear rate are discussed with a qualitative and quantitative emphasis that are supported with adequate examples and application areas and summarized in the form of graphs and tables for easy readability. The use of advanced manufacturing methods such as powder metallurgy and sintering to produce solid lubricants of superior tribological performance and the subsequent economic gain from their development as a substitute for liquid lubricant are also evaluated.

Grain size and hydroxyl-coverage dependent tribology of polycrystalline graphene

Functional groups and grain boundaries of polycrystalline graphenes play important roles in their tribological behaviors but the mechanism is still elusive. Here, we have investigated the influences of hydroxyl groups, coverage, and grain size on the surface corrugation, friction, and motion behavior of polycrystalline graphene using molecular dynamics simulations. The results show that the corrugation of polycrystalline graphene increases with respect to an increase in grain size. The introduction of hydroxyl groups suppresses the corrugation. The friction between carbon nanotube (CNT) and polycrystalline graphene increases the formation of hydrogen bonds when the interfaces are grafted with hydroxyl groups. The highest amount of friction appears when the ratio of hydroxyl groups on CNT, and polycrystalline graphene, is about 15%-5%. This is due to the balance between the interface space and the formed hydrogen bonds. Furthermore, polycrystalline slides following the movement of CNT owing to high friction. In addition, the energy dissipation as a result of the vibration of the hydroxyl groups plays a more important role as the ratio of hydroxyl groups increases.

Aqueous Ion Trapping and Transport in Graphene-Embedded 18-Crown-6 Ether Pores

Using extensive room-temperature molecular dynamics simulations, we investigate selective aqueous cation trapping and permeation in graphene-embedded 18-crown-6 ether pores. We show that in the presence of suspended water-immersed crown-porous graphene, K⁺ ions rapidly organize and trap stably within the pores, in contrast with Na⁺ ions. As a result, significant qualitative differences in permeation between ionic species arise. The trapped ion occupancy and permeation behaviors are shown to be highly voltage-tunable. Interestingly, we demonstrate the possibility of performing conceptually straightforward ion-based logical operations resulting from controllable membrane charging by the trapped ions. In addition, we show that ionic transistors based on crown-porous graphene are possible, suggesting utility in cascaded ion-based logic circuitry. Our results indicate that in addition to numerous possible applications of graphene-embedded crown ether nanopores, including deionization, ion sensing/sieving, and energy storage, simple ion-based logical elements may prove promising as building blocks for reliable nanofluidic computational devices.

Approaches for Achieving Superlubricity in Two-Dimensional Materials

Controlling friction and reducing wear of moving mechanical systems is important in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to design materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and other 2D materials opened an era for conformal, atomically thin solid lubricants. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating friction and wear by introducing 2D materials at the tribological interface. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values-superlubricity-across scales from nano- up to macroscale contacts.

Engineering-scale superlubricity of the fingerprint-like carbon films based on high power pulsed plasma enhanced chemical vapor deposition

Engineering scale superlubricity was realized by the fingerprint-like carbon films, which offer exciting application opportunity in vehicles, turbines, and manufacturing equipment.