Precursory flow in the formation of cellulose nanofiber films revealed by multiscale image analysis

Abstract

Film materials made of cellulose nanofibers (CNFs), i.e., so-called nanopapers, are fabricated from aqueous dispersions of CNFs. The drying in the container causes the phase transition from sol to gel and final state of transparent film. The nonequilibrium and nonuniform nature of the drying calls for the least invasive in-process measurement to understand the process. In this paper, we focus on the transient state of nanopaper formation through the time-series image analysis from optical measurements on the multiple spatiotemporal scales and resolutions. The combination of single-particle tracking and particle image velocimetry reveals the intermittent flow with a low symmetry of space pattern in the middle stage of drying process. The small portion of CNF is sufficient for suppression of coffee-ring phenomena, and the flow is in combination with the emergence of partial structural order. Furthermore, we find that the flow pattern depends on the sidewall material of the sample container. We characterize the three stages of drying process through the flow patterns by the correlation length of velocity autocorrelation function as well as the mean and standard deviation of the flow speed in the containers.