Liquid exfoliation of multilayer graphene in sheared solvents: A molecular dynamics investigation

Abstract

Liquid-phase exfoliation, the use of a sheared liquid to delaminate graphite into few-layer graphene, is a promising technique for the large-scale production of graphene. But the micro and nanoscale fluid-structure processes controlling the exfoliation are not fully understood. Here we perform non-equilibrium molecular dynamics simulations of a defect-free graphite nanoplatelet suspended in a shear flow and measure the critical shear rateγ c needed for the exfoliation to occur. We compareγ c for different solvents including water and NMP, and nanoplatelets of different lengths. Using a theoretical model based on a balance between the work done by viscous shearing forces and the change in interfacial energies upon layer sliding, we are able to predict the critical shear ratesγ c measured in simulations. We find that an accurate prediction of the exfoliation of short graphite nanoplatelets is possible only if both hydrodynamic slip and the fluid forces on the graphene edges are considered, and if an accurate value of the solid-liquid surface energy is used. The commonly used"geometric-mean" approximation for the solid-liquid energy leads to grossly incorrect predictions. arXiv:1912.03179v1 [cond-mat.soft]

Violations of Jeffery's theory in the dynamics of nanographene in shear flow

“…From this expression, the Pe number corresponding tȯ γ * is seen to be independent of the fluid viscosity and to scale as Pe * ∼ a 2 E f /k B T . For HBCs in water, using f = 1 and E = 60 mJ/m 2 , we get Pe * = 9 [52]. In the case of HBCs in NMP, E = 4 mJ/m 2 and Pe * = 0.6 [52].…”

“…, where E is the change in surface energy associated with the separation in a liquid of two solid molecular layers, and f is a nondimensional function that is typically O(1) [52]. From this expression, the Pe number corresponding tȯ γ * is seen to be independent of the fluid viscosity and to scale as Pe * ∼ a 2 E f /k B T .…”

Effect of hydrodynamic slip on the rotational dynamics of a thin Brownian platelet in shear flow

“…Finally, alignment is an essential ingredient to impart superior properties to nanocomposite materials. Surface modifications producing substantial slip could be used to align particles produced from the exfoliation of 2-D layered materials, which can be easily produced on mass scales by liquid-phase exfoliation (Botto 2019;Gravelle et al 2020;Salussolia et al 2020) and have therefore potential for applications. Because the ideal infinite Péclet number regime may not be achievable in practice, our results provide theoretical guidelines for deciding in which cases slip will have a dominant effect on the alignment of plate-like nanoparticles in the presence of Brownian motion.…”

“…We add to figure 15 red dash-dot lines corresponding to the Pe number for three shear stress values:γ η = 10 7 Pa,γ η = 2 × 10 4 Pa,γ η = 10 Pa (the Péclet number scales proportionally to the stress according to Pe ≈ c 1γ ηa 3 /(k B T), where 'c 1 = 6.29 ± 0.02' was calculated from a boundary integral simulation of the hydrodynamic torque on a particle rotating with assigned velocity in a still fluid, see § 4.3). These characteristic shear stresses are, respectively, typical of: exfoliation processes in a low viscosity fluid (Paton et al 2014); mixing in very viscous fluids (Huang & Terentjev 2012) and dispersion by microfluidisation (Karagiannidis et al 2017;Paton et al 2017); fast lubrication processes (Jonsson & Bhushan 1995), high-speed blade coating (Willenbacher, Hanciogullari & Wagner 1997) and MD simulations (Voeltzel et al 2018;Gravelle et al 2020;Gravelle, Kamal & Botto 2021). The effects of slip for each of these characteristic shear stresses can be assessed by identifying, for a given value of a, the region where the corresponding Pe lies.…”

“…In the case of NMP or ethanol, the slip length of graphene is also relatively large () (Gravelle et al. 2020). Ionic liquid (IL) can also give large slip lengths, but in this case has been found to depend strongly on the shear rate (Voeltzel et al.…”