Polymodal rheological behaviors induced by self-assembly of surfactants confined in nanotubes

Abstract

Self-assembly is currently one of the most studied branches of materials chemistry, and has attracted a lot of attention due to its diverse potential applications in electronics, engineering, biomedical, and optical fields. Thus far, many previous studies have reported on the self-assembly of surfactants for developing various functional materials. In this study, we focus on the chemical nature of the wall surface as a critical parameter which can drastically change the self-assembled structure. We have studied the rheological properties of surfactant solutions in chemically distinct nanotubes using a computer simulation. Graphs of the Weissenberg number (Wi) versus the viscosity (η) in hydrophilic, hydroneutral, and hydrophobic nanotubes at different concentrations of surfactant were derived. We found that differences in the viscosity can be obtained depending on the chemical nature of the wall surface, even in systems without surfactant molecules. With the surfactant molecules, characteristic viscosity behaviors were observed with rich steady-state morphologies. For example, although the concentration of the surfactant is the same (c = 30%), completely different viscosity behaviors were observed in hydrophilic (shear-thinning) and hydrophobic (shear-thickening) nanotubes. Our simulations offer a guide to controlling the rheological properties of surfactant aqueous solutions by altering the chemical nature of the wall surface and elucidating the effects of confinement, the concentration of the surfactant aqueous solution, and the self-assembled structure.