Interaction of 2D materials with liquids: wettability, electrochemical properties, friction, and emerging directions

The emergence of two-dimensional (2D) materials as functional surfaces for sensing, electronics, mechanics, and other myriad applications underscores the importance of understanding 2D material–liquid interactions. The thinness and environmental sensitivity of 2D materials induce novel surface forces that drive liquid interactions. This complexity makes fundamental 2D material–liquid interactions variable. In this review, we discuss the (1) wettability, (2) electrical double layer (EDL) structure, and (3) frictional interactions originating from 2D material–liquid interactions. While many 2D materials are inherently hydrophilic, their wettability is perturbed by their substrate and contaminants, which can shift the contact angle. This modulation of the wetting behavior enables templating, filtration, and actuation. Similarly, the inherent EDL at 2D material–liquid interfaces is easily perturbed. This EDL modulation partially explains the wettability modulation and enables distinctive electrofluidic systems, including supercapacitors, energy harvesters, microfluidic sensors, and nanojunction gating devices. Furthermore, nanoconfinement of liquid molecules at 2D material surfaces arising from a perturbed liquid structure results in distinctive hydrofrictional behavior, influencing the use of 2D materials in microchannels. We expect 2D material–liquid interactions to inform future fields of study, including modulation of the chemical reactivity of 2D materials via tuning 2D material–liquid interactions. Overall, 2D material–liquid interactions are a rich area for research that enables the unique tuning of surface properties, electrical and mechanical interactions, and chemistry. Materials made from single layers of atoms have unique interactions with liquids that can unlock new chemical and biological applications. Peter Snapp and SungWoo Nam from the University of Illinois at Urbana-Champaign in the United States review how studies of water on graphene films have revealed methods to control how tightly droplets make contact with two-dimensional (2D) materials. For example, by using chemical modifications to make 2D surfaces more or less water-repelling, researchers can direct cellular and crystal growth, or produce nanoscale membranes capable of separating oil from water. Combining surface engineering with external electric fields enables droplets to form into charged layers capable of storing energy like a capacitor. The charged layers have been applied for biomolecular sensing, and to modify nanoscale frictional behavior in fluid channels. 2D materials’ thinness and environmental sensitivity induce novel surface forces which render fundamental 2D material–liquid interactions variable. 2D material wettability is perturbed by substrates and contaminants enabling templating, filtration, and actuation. Fluid structure at 2D material–liquid interfaces is similarly perturbed, partially explaining wettability modulation and enabling distinctive electro-fluidics applications including supercapacitors, energy harvesters, and sensors. Finally, nanoconfinement of molecules arising from perturbed liquid structure modified hydro-frictional behavior, influencing 2D materials’ use in microchannels. 2D material–liquid interactions will inform future fields of study, including modulation of 2D materials’ chemical reactivity, offering a rich area for research on tuned surface fluid interactions.