<p>The influence of surfactant type on the aggregation structure and rheological behavior of multi-walled carbon nanotube (MWCNT) suspensions in N-methyl-2-pyrrolidone (NMP) was investigated through combined rheological measurements and scaling analysis. The MWCNT concentration was fixed at 1 wt%, which is above percolation threshold, and four types of suspensions were prepared: a surfactant-free suspension, and suspensions containing the nonionic surfactant Tween 80, the cationic surfactant CTAB, and the anionic surfactant SDBS. The rheological properties of the suspensions were characterized under both steady as well as oscillatory shear measurement. All the surfactants acted to reduce the suspension viscosity and to increase the storage modulus. The extent of viscosity reduction and modulus enhancement was relatively larger for CTAB and SDBS than for Tween 80. Shear viscosity and storage modulus were measured while varying the MWCNT concentration from 0.8 to 1.2 wt% to estimate the intrinsic viscosity and fractal dimension. Both parameters were lower for the Tween 80 system compared with those containing CTAB or SDBS. Microscopic observations of diluted suspensions showed that aggregates formed with Tween 80 were relatively small, whereas those formed with CTAB and SDBS were larger and exhibited more interconnected network structures. These structural differences are consistent with the lower intrinsic viscosity, which scales with the average aggregate size, and the smaller fractal dimension, which indicates a morphology closer to a string-like rather than a plane-like network. Overall, the rheological analyses, including fractal dimension and microscopic observations, indicate that Tween 80 provides the most effective dispersion among the surfactants examined.</p> Graphical abstract <p></p>