RESEARCH PAPER Rheology and microstructure of dilute graphene oxide suspension Waka Tesfai • Pawan Singh • Youssef Shatilla • Muhammad Z. Iqbal • Ahmed A. Abdala Received: 20 May 2013 / Accepted: 31 August 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Graphene and graphene oxide are poten- tial candidates as nanofluids for thermal management applications. Here, we investigate the rheological properties and intrinsic viscosity of aqueous suspen- sion of graphene and use the measured intrinsic viscosity to determine the aspect ratio of graphene oxide. Dilute suspension of graphene oxide (0.05 to 0.5 mg/mL) exhibits a shear thinning behavior at low shear rates followed by a shear-independent region that starts at shear rate between 5 and 100/s depending on the concentration. This shear thinning behavior becomes more pronounced with the increase of particle loading. Moreover, AFM imaging of the dried graphene oxide indicates the evolution of irregular and thin low fractal aggregates of 0.3–1.8 nm thickness at lower concentrations to oblate compact structures of 1–18 nm thickness of nanosheets at higher concentra- tion. These observations elucidate the microstructure growth mechanisms of graphene oxide in multiphase systems, which are important for nanofluids applica- tions and for dispersing graphene and graphene oxide in composite materials. The suspension has a very high intrinsic viscosity of 1661 due to the high graphene oxide aspect ratio. Based on this intrinsic viscosity, we predict graphene oxide aspect ratio of 2445. While the classical Einstein and Batchelor models underestimate the relative viscosity of graphene oxide suspension, Krieger–Dougherty prediction is in a good agreement with the experimental measurement. Keywords Graphene oxide  Nanofluids  Rheology  Intrinsic viscosity  Suspension  Aspect ratio Introduction Graphene is a two-dimension sheet (material) of an atomic thick layer of hexagonally arranged carbon atoms. Since its first production by Novoselov et al. (2004) using the micromechanical cleavage method, graphene and graphene derivatives have attracted significant research activities on the thermal, mechan- ical, and electrical transport properties of graphene. Many industrial and scientific applications benefit from the outstanding chemical, thermal, and electrical properties of graphene and graphene oxide (GO). Among these applications polymer nanocomposites (Kim et al. 2010; Potts et al. 2011), nanofluids for W. Tesfai  P. Singh  Y. Shatilla Laboratory of Nano and Microfluidics, Masdar Institute of Science and Technology, Abu Dhabi, UAE M. Z. Iqbal  A. A. Abdala (&) Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE e-mail: aabdala@pi.ac.ae Present Address: A. A. Abdala Department of Chemical Engineering and Petroleum Refining, Faculty of Petroleum and Mining Engineering, Suez University, Suez, Egypt 123 J Nanopart Res (2013) 15:1989 DOI 10.1007/s11051-013-1989-3