A simple feature of yielding behavior of highly dense suspensions of soft micro-hydrogel particles

Abstract

The highly dense suspensions of soft micro-hydrogels with a narrow size distribution (typically ϕ_eff > 0.9 where ϕ_eff is the apparent volume fraction of the particle), which form a regular lattice structure, exhibit a simple feature in the yielding behavior: the yield strain γ_c [ca. 2.5% and ca. 4.8% for poly(N-isopropylmethacrylamide) (PNIPMA) and poly(N-isopropylacrylamide) (PNIPA) hydrogel particles, respectively] is nearly insensitive to the cross-link concentration (c_x), particle diameter (D_h), and particle concentration (c) in the limited c range examined here, and γ_c is almost constant in a wide range of equilibrium shear moduli over two orders of magnitude. In addition, no appreciable difference in γ_c is observed in the dense pastes with crystalline and glassy structures which are formed by mono- and bidisperse microgels, respectively. This is in contrast to a finite difference in γ_c for the crystal and glass formed by the hard sphere reported by Koumakis et al. [Soft Matter, 4, 2008 (2008)]. Furthermore, the highly dense suspensions of NIPA core–NIPMA shell microgels are similar in γ_c to those of NIPMA microgels. These results indicate that γ_c for the highly dense suspensions of soft micro-hydrogels depends primarily on the kind of constituent polymer near the particle surface. The yield strain γ_c is expected to be governed by short-range interactions such as adhesion and friction.