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

The highly dense suspensions of soft micro-hydrogels with a narrow size distribution, which form a regular lattice structure, exhibit a simple feature in the yielding behavior: the yield strain $\gamma_{\mathrm{c}}$ [ca. 2.5{\%} and ca. 4.8{\%} for PNIPMA) and PNIPA hydrogel particles, respectively] is nearly independent of the cross-link concentration, particle diameter, and particle concentration (c) in the limited c range examined here, and $\gamma_{\mathrm{c}}$ is almost constant in a wide range of equilibrium shear moduli over two orders of magnitude. Further, no appreciable difference in $\gamma_{\mathrm{c}}$ is observed in the dense pastes with crystalline and glassy structures which are formed by mono- and bidisperse microgels, respectively. In addition, the highly dense suspensions of NIPA core\textbf{--}NIPMA shell microgels are similar in $\gamma_{\mathrm{c}}$ to those of NIPMA microgels. These results indicate that $\gamma_{\mathrm{c}}$ for the highly dense suspensions of soft micro-hydrogels depends primarily on the kind of constituent polymer near the particle surface. The yield strain $\gamma_{\mathrm{c}}$ is expected to be governed by short-range interactions such as adhesion and friction. [Reference] K. Urayama, T. Saeki, S. Cong, S. Uratani, T. Takigawa, M. Murai, Suzuki, \textit{Soft Matter}, DOI: 10.1039/c4sm01841a.