Two-dimensional (2D) materials like graphene, assembled into van der Waals (vdW) materials, offer exciting properties for various applications. Twisted bilayer graphene (tBLG), in particular, displays unique moiré patterns and correlated electron behaviors. However, the high surface-to-volume ratio of 2D materials makes them susceptible to contamination during transfer processes. Interfacial vdW interactions can trap contaminants, forming nanopockets, a phenomenon previously described as "self-cleaning." While experiments show nanopockets can merge spontaneously, the driving mechanism remained unclear, with previous explanations focusing solely on Ostwald ripening (capillarity-driven mass diffusion). This study investigates the elastocapillary cleaning mechanism in tBLG, considering the previously unappreciated role of sheet elasticity.

Previous research has explored similar scenarios of thin solids confining liquids at larger scales, including liquid blisters and lenses. The unique aspect of vdW materials lies in the mobility of the confined liquids. Experiments have demonstrated that nanopockets can aggregate and merge spontaneously, either over time or after annealing. This coarsening behavior is valuable as larger pockets are more easily removed. Existing explanations invoke Ostwald ripening, a capillarity-driven mass diffusion process. However, this mechanism overlooks the elasticity of the thin sheets and fails to explain the merging of distant pockets without substance transport.

The study employed a suspended micro-nano bubble device to investigate nanopocket morphology and merging in tBLG. Monolayer graphene sheets were prepared via mechanical exfoliation and transferred using a water droplet-assisted method onto a microhole-patterned SiO₂/Si substrate. Twist angles were introduced to create moiré patterns. Atomic force microscopy (AFM) was used to visualize nanopockets and their three-dimensional geometry, including their height and radius. The motion and merging of nanopockets were observed under contact-mode AFM scanning. The influence of twist angles on nanopocket aspect ratios was examined. To study the effect of pretension, the tBLG drumhead was pressurized, creating an equibiaxial pretension. The aspect ratios of nanopockets were measured under varying pretension levels. The study also involved experiments with ethanol nanopockets. Molecular dynamics (MD) simulations complemented the experimental work, simulating the coalescence of water-filled nanopockets in a graphene bilayer with and without pretension.

The study revealed that nanopockets of water or ethanol readily move and leave no residue, restoring the initial moiré patterns. Nanopockets merge spontaneously at room temperature, influenced by mechanical stimuli. AFM observations showed elastic deformations in the graphene sheet, including bulging and wrinkling. The aspect ratio (h/a) of nanopockets was found to be insensitive to the radius but dependent on the twist angle of the tBLG. A force balance equation at the contact line was derived, considering elastic sheet tension and interfacial energies. The aspect ratio was shown to be related to an elastocapillary parameter Δγ, representing the change in interfacial energies relative to the sheet stiffness. The application of pretension, controlled by pressurizing the tBLG drumhead, flattened the nanopocket profile, suppressing wrinkling according to a derived geometric criterion. Experiments showed that the merging of nanopockets was influenced by pretension; wrinkled nanopockets showed an attractive force leading to merging, while unwrinkled nanopockets (under pretension) moved along the direction of mechanical stimuli without merging. MD simulations corroborated the experimental observations, showing an elastocapillary-driven merging process with a two-stage mechanism: an attraction stage mediated by elastic restoration forces and a spontaneous coalescence stage after initial contact. Pretension reduced the wrinkling and the driving force for merging.

The findings demonstrate the self-cleaning ability of vdW material interfaces, showing how contaminants aggregate into mobile nanopockets and the recovery of pristine interfacial structures. The proposed elastocapillary mechanism, involving both capillary and elastic forces, explains the driving force behind nanopocket coalescence. This mechanism highlights the importance of considering both capillary and elastic forces in understanding the behavior of confined liquids in 2D materials. This is a significant advancement from previous models that focused solely on capillarity-driven Ostwald ripening. The results suggest that mechanical treatments can be used to fabricate clean vdW devices. The understanding of elastocapillarity is also relevant to applications like liquid cell electron microscopy, where controlled confinement conditions are crucial.

This study demonstrated the self-cleaning ability of vdW interfaces, where contaminants aggregate into removable nanopockets, restoring the interfacial moiré patterns. An elastocapillary mechanism was proposed and validated, explaining the coalescence of nanopockets. Mechanical treatments offer a route to fabricate clean vdW devices. Future studies could explore the effect of different contaminants, substrate properties, and more complex vdW heterostructures.

The experiments focused primarily on water and ethanol nanopockets. The applicability of the elastocapillary mechanism to other contaminants and 2D materials requires further investigation. The MD simulations were limited by computational resources, restricting the size of the simulated systems. A more comprehensive model accounting for the initial pretension, non-uniform wrinkling, and curvature effects could improve the accuracy of the theoretical predictions.