Shear rheology of graphene oxide dispersions
Introduction
Around 10 years ago, the experimental isolation of the first single sheet of graphene [1, 2], a two-dimensional monolayer of carbon atoms, raised great excitement in the material science community, because its physical existence had been debated for 70 years [3]. Since then graphene has been considered as a “rising star” and countless applications have been introduced. Many of these are related to a chemically modified version of graphene, called graphene oxide (G-O). In particular, G-O contains a variety of reactive oxygen functional groups, making it versatile through different chemical modifications [4]. Owing to their excellent mechanical, thermal, and electric properties, G-O sheets have been used in various applications such as nanocomposites [5], inks [6], and coating for supercapacitor electrodes [7, 8]. Rheology of G-O dispersions depends on parameters such as G-O concentration and sheet size, which are critical for the design of ad hoc materials. When dispersed, G-O sheets tend to aggregate [9, 10, 11••], similar to suspensions containing carbon nanotubes [12] or rod-like micelles [13], because attractive forces are dominant. Despite the extensive literature in the rheology of carbon nanotube based materials (both in dispersion [14] and composite [15] forms), much less has been reported on the rheology of graphene oxide dispersions.
In this review, we focus on existing findings on the shear rheology of aqueous G-O dispersions and G-O-composites. Owing to space limitations, results for G-O interfacial rheology can be found in Imperiali et al. [16], G-O electorheology is reviewed by Zhang et al. [7]. We also refer to the review of Fan et al. [17] for energy-related applications of G-O sheets.
Section snippets
Shear rheology of aqueous graphene oxide dispersions
Shear rheological characterization is generally fulfilled through measurements in oscillatory linear shear flow, steady shear flow and transient shear flow. We have organized this review into sections based on flow types, in the order commonly adopted to describe G-O rheology. A schematic representation of the findings is summarized in Figure 1.
Shear rheology G-O composites
Graphene oxide has been widely used to improve mechanical properties of a variety of materials, such as polydimethylsiloxane (PDMS) [29, 30••, 31], hydrogels [32•, 33, 34, 35], polycarbonate [5, 36], polymethyl methacrylate [37], polyurethane [38], and cellulose [39]. El Achaby and Qaiss [40] compared the rheological behavior of polyethylene filled with G-O and carbon nanotubes. They found that at the same filler concentration, G-O sheets performed better than carbon nanotubes in terms of
Conclusions and outlook
In this review, the rheology of aqueous and composite graphene oxide dispersion is presented. The shear rheology of G-O dispersion can be summarized as follows. Firstly, a critical concentration ϕc exists, below which G-O sheets are dispersed, and above which they self-organize. Secondly, at ϕ/ϕc > 1, G-O aggregates are formed or dismantled, depending upon whether the dominant mechanism is Brownian diffusion or flow convection, quantified by the Peclet number. At low Pe, G-O sheets
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors thank Prof Pier Luca Maffettone for helpful discussion and comments. The authors thank also Dr Steve Aird for careful proof reading. We also acknowledge financial support from the OIST Graduate University with subsidy funding from the Cabinet Office, Government of Japan.
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