Rheo-optical Analysis of Functionalized Graphene Suspensions

Abstract

Wet processing of graphene sheets is a potentially interesting route for the economically viable creation of graphene-based composites. In the present work, flow dichroism and small-angle light scattering are used to investigate the dispersion of functionalized graphene sheets in a suspension and their response to shear flow. In line with expectations from scaling theory at rest, the functionalized graphene sheets are present as Brownian flat sheets, and there is no evidence of significant crumpling. More surprisingly, we find that the rate-dependent orientation of these molecularly thin sheets can be described by numerical predictions for hard spheroidal sheets, making quantitative predictions of the flow-induced orientation possible. Further comparison of the flow-induced orientation of thick gold decahedra with the thin graphene sheets shows that, except for effects of polydispersity, the flow-induced orientation is predicted well quantitatively. Adequate prediction of the effects of flow on the orientation of graphene sheets makes it possible to design wet processed graphene-based composite materials.

Hydrodynamic slip can align thin nanoplatelets in shear flow

“…The validity of the theory discussed here could be tested by measuring experimental observables that are sensitive to secondorder statistical moments, such as the 'degree of orientation'. The 'degree of orientation' of the particles, can be assessed by rheooptics experiments [64][65][66] . Contrarily, the average particles orientation angle may not be ideally suitable for discriminating between rotating and aligned particles because highly elongated plate-like particles are expected to align with the flow in a timeaverage sense regardless of the hydrodynamic slip 16,67 .…”

Orientation dynamics of dilute functionalized graphene suspensions in oscillatory flow

“…One interesting observation we found from this study is the efficacy of the oscillatory flow to create alignment or misalignment as compared to steady shear flow [51], provided strain amplitudes are chosen carefully relative to the tumbling periods. In Figs.…”

“…1 warn us of the strong effect of polydispersity on the orientation dynamics of the system. The effect of polydispersity has been discussed in detail in another work [51]. Specifically, the dichroic response in simple steady shear was found to be dominated by the larger particles in the samples.…”

“…Details on the atomic force microscopy characterization of the FGSs can be found elsewhere [21,49]. Five different concentrations of graphene sheets were prepared in the dilute regime [51]. The effective volume fractions (volume fraction of equivalent spheres with diameter equal to the average diameter of the FGSs) ranged between 2.3 × 10 −7 and 2.33 × 10 −6 .…”

“…For the graphene particles in this work we approximate the sheets by very flat spheroidal objects with λ = −1. The polydispersity of the system had a negligible impact on λ given its quadratic dependence on r. For the polydisperse graphene suspension the average rotational diffusion coefficient D r = 0.04 s −1 was calculated from the dichroism relaxation measurements [51]. The rotational Brownian motion coefficient for thin disks in dilute regime scales as D r D −3 , where D is the diameter of the disk.…”