Evaporation-Driven Crumpling and Assembling of Two-Dimensional Materials

Two-dimensional (2D) materials, such as graphene, boron nitride and molybdenum disulfide, have attracted tremendous attention over the past few years for their exceptional electronic, mechanical and thermal properties underpinned by their extremely large specific surface area, often demonstrated at the single layer level. However, a bulk form of these 2D materials, either as powders or a monolith is necessary for most applications such as high-performance electrodes in energy storage, filters for waste water/gas treatments in environmental systems, and lightweight structures. Crumping of 2D materials by droplet evaporation creates a new form of aggregation-resistant ultrafine particles with more scalable properties such as high specific surface areas. However, the underpinned fundamental mechanics theory that addresses large deformation, severe instability and self-assembly of 2D sheets under dynamic solid-liquid interactions during liquid evaporation is lacking. In the present study, I will propose a systematical theoretical mechanics framework to study self-folding, crumpling and self-assembly of 2D nanomaterials and their assembly into 3D structures. First, I will establish the energy-based continuum theoretical mechanics model to describe the liquid evaporation-driven self-folding and crumpling of a single graphene sheet by taking into account material geometric shape, dimension and surface wettability. Both the critical elastocapillary and self-folding lengths of graphene that lead to a stable folded pattern by van der Waals energy after the complete evaporation of liquid are formulated. Then, I will build a rotational spring-mechanical slider mechanics model to reveal the simultaneous process of crumpling and self-assembly of multiple graphene sheets and their competition during droplet evaporation, where the rotational spring is developed to describe the out-of-plane deformation of crumpling sheets, and the mechanical slider is implemented to describe the interactive binding energy in self-folding of a single sheet or overlapping of neighboring sheets. This rotational spring-mechanical slider mechanics model is calibrated with the analysis on self-folding and crumpling of a single graphene sheet. Afterward, I will develop a coarse-grained computational model to simulate the crumpling and assembling process of graphene by liquid evaporation. The simulation results show remarkable agreement with theoretical predictions, from crumpling and assembling energies of graphene during liquid evaporation to overall size and accessible area of the crumpled particles after the complete evaporation of liquid. Besides, both theoretical predictions and simulation results agree well with independent experiments. These findings are expected to offer immediate and quantitative application guidance to control the crumpling and self-assembling process of 2D materials by solution evaporation such as aerosol processing to fine tune particle size and morphology. More importantly, the mechanics theories and coarse-grained modeling established here could be extended to elucidate fundamental mechanism of large deformation, instability, and self-assembly of a broad scope of other low-dimensional nanomaterials such as lipid membranes, nanowires, nanotubes, nanofibers and nanoparticles, for their emerging applications including ultrafine particle manufacturing and various printing processes.

Related Publications:

[1] Liu, Qingchang, Jiaxing Huang, and Baoxing Xu. "Evaporation-driven crumpling and assembling of two-dimensional (2D) materials: A rotational spring–mechanical slider model." Journal of the Mechanics and Physics of Solids133 (2019): 103722. doi.org/10.1016/j.jmps.2019.103722

[2] Liu, Qingchang, and Baoxing Xu. "Two-and three-dimensional self-folding of free-standing graphene by liquid evaporation." Soft matter 14.29 (2018): 5968-5976. DOI: 10.1039/C8SM00873F