Drying path dependence in microrheological characteristics of cellulose nanofiber dispersion revealed by single particle tracking

Abstract

Abstract Rheological characterization of cellulose nanofiber (CNF) dispersion is important for the process design of nanopaper fabrication. However, the existing macroscopic approach is difficult to reveal microscopic signal, or nonequilibrium nature of drying, distinct from the concentration dependence in equilibrium. We report the microscopic characterization in the drying process of CNF dispersion by the microscopy movie analysis, based on the single particle tracking (SPT) of probe particles. Since SPT does not require invasive shear flow to the system of interest, the rheological characterization in the drying process can be realised. We focused on the role of initial CNF concentration to examine the nonequilibrium effect of drying. The path of drying for a higher initial CNF concentration is not simply a time compression of the path of the lower initial concentration. It is revealed by the time-evolution curves of the generalised diffusion coefficient and the scaling exponent for probe particles. One of the origins of this path-wise dependence on the initial CNF concentration is the macroscopic spatial nonuniformity of the drying process. Thus, the effect of macroscopic condition is microscopically determined.