Microrheology of pH-Responsive Nanoparticle Monolayers at Fluid Interfaces

The effect of shear on the structure and dynamics of polyelectrolyte-grafted nanoparticles (PGNP) straddling at a water–oil interface was investigated through mesoscale simulations. Using electrostatic dissipative particle dynamics, long-range electrostatic forces are solved in a triclinic simulation box, which allows us to apply continuous shear deformation to the monolayer. The monolayers with different particle coverages were examined under shear and degree of ionization of PGNPs were set to be high. While undisturbed, the particles arrange themselves on a hexagonal lattice due to the long-range electrostatic interactions. As the monolayer with low particle concentration is subjected to the shear flow, the free voids allow particles to move in the shear direction. However, strong inter-particle forces at high particle concentration result in the collective motion of domains to repair the adjacent defect. The in-plane structure of monolayer is analyzed by structure factor and Voronoi diagram. The dynamics of local domains are observed through snapshots and averaged velocity contours. The rheology measurements of the monolayer were also performed under small-amplitude oscillatory shear and the results were correlated with the detailed structure evolution.