2D materials deserved an interest in material science and other disciplines due to their exciting electronic, mechanical, and optical properties. The unique behavior of these materials originate from very strong in-plane atoms interactions and very weak van der Waals out-of-plane interactions. Graphene, a single layer of carbon atoms, is the first 2D material that was discovered in 2004 by Andre Geim and Konstantin Novoselov (they received the Nobel Prize “for groundbreaking experiments regarding the two-dimensional material graphene” for their discovery). Since then the number of discovered two-dimensional (2D) materials has grown steadily and each new 2D material exhibits unique physical and chemical properties.
2D materials received a well deserved attention in the field of tribology as well. Graphene, thanks to its remarkable mechanical, thermal and chemical properties, was shown to reduce friction to vanishingly small values (so called superlubricity state) at nano and macroscales, wear and in general may be used as a protective coating. Graphene oxide was also shown to reduce friction and wear when used as an additive in lubricants. Besides, graphene is impermeable to liquids and gases (water or oxygen) and consequently reduces corrosion and oxidation, which are unusually causing severe damage on the surfaces.
While 2D materials attracted the attention of tribologists as coatings to reduce friction and wear primarily as solid lubricants, lately the frictional behavior at the 2D-materials-liquid interfaces has become an intriguing research area. The solid-liquid interaction is of importance from the point of view of fundamental science, but also from a point of view of a number of applications, including development of more efficient nanofluidic devices (membranes, nanotubes, filters and even energy harvesting devices). The efficiency of such devices is highly affected by the friction at the liquid-solid interface.
In the classical macroscopic hydrodynamic theory (continuum fluid dynamics) it is typically assumed that the molecules of a liquid stick to the solid walls. This behavior is attributed to the friction between the liquid and solid molecules. However, the classical picture changes when a liquid flow is considered at the scale of a few tens of nanometers. At this scale, liquid molecules start to slip over the solid molecules reducing friction. The reduction of friction is characterized by a slip length: the larger is the slip length, the lower is friction. For example, carbon nanotubes were shown to exhibit a slip length of over of several hundreds of nanometers, which allows water to flow almost frictionless through the CNT channels.
A recent review of the interaction of 2D materials with liquids combined by researchers from University of Illinois at Urbana-Champaign explores recent developments in the area of interaction of 2D materials with liquids. In the review wettability, electrochemical properties and friction at the 2D-materisl-solid interfaces were addressed. It appears that there are several factors impacting friction of liquid molecules on 2D surfaces: wettability, energy corrugation, electrostatic interaction and electrolyte ions. Hexagonal boron nitride (h-BN) and graphene have a similar wettability behavior, but h-BN generates 3 times larger friction with water molecules as compared to graphene (the slip length is one third). This behavior is attributed to the heterogeneous structure of h-BN that creates a larger corrugation in the potential energy. Electrostatic interactions between h-BN and polar water molecules were shown to have a large impact on the friction force based on molecular dynamics simulations results. Finally, the slip of water molecules on graphene surfaces was shown to bre reduced in the presence of ions in the liquid.
Further information: Snapp, P., Kim, J.M., Cho, C. et al. Interaction of 2D materials with liquids: wettability, electrochemical properties, friction, and emerging directions. NPG Asia Mater 12, 22 (2020). https://doi.org/10.1038/s41427-020-0203-1