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.