Capillary Crumpling of 2D Nanomaterials Toward 3D Architected Structures

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 suspended 2D nanomaterials by the 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, we proposed a theoretical mechanics framework to quantitatively describe the simultaneous process of crumpling and self-assembly of 2D materials during the droplet evaporation and their assembly into 3D architected structures. A rotational spring is developed to describe the out-of-plane deformation of crumpling sheets, and a mechanical slider is implemented to describe the interactive binding energy in self-folding of a single sheet or overlapping of neighboring sheets in assembling. This rotational spring-mechanical slider mechanics model is calibrated with the energy-based continuum mechanics analysis by crumpling a single 2D sheet, and is further extended to a network model to characterize the crumpling and assembling of multiple sheets in the droplet. An equivalent pressure model is developed to unify the resultant forces associated with the liquid evaporation including capillary force, vapor pressure, gas pressure, vapor recoil pressure and capillary flow-induced shear force. A coarse-grained model of 2D materials is developed and its dynamic interaction with liquid molecules during evaporation is mimicked by proposing a controllable virtual van der Waals force field. 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. The effect of concentration, size, shape, number and size distribution of 2D graphene sheets in liquid droplets on crumpling and self-assembling energy and shape, size and surface morphology of crumpled particles is also discussed. 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