Correlations Between Dynamical Heterogeneities and Viscoelasticity of Confined Colloidal Thin Films
Yingxi Elaine Zhu and Prasad Sarangapani. Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556
Our recent study on confined hard-sphere colloidal suspensions demonstrates that glass transition can be observed `sooner' as film thickness approaches a critical value while volume fraction remains constant. In this work, we present a new study of the rheological properties of strongly confined colloidal thin films by using a home-designed micro-rheometer interfaced with a confocal microscope. The system we focus on is the model “hard-sphere” poly-(methyl methacrylate) (PMMA) particles of 1.2 um in diameter suspended in the density and refractive index matched nonpolar solvent. We visualize the shear-induced structural relaxation at a single particle level and measure the rheological properties of confined PMMA suspensions, whose bulk particle volume fraction ranges from 0.30~0.57, confined between two surfaces at narrow gap spacing ranging from 50 um to 2-3 um. The application of shear excitation greatly accelerates structural relaxation compared to quiescent colloidal fluids and we visualize particle displacements during the “bond breakage” process in strongly confined thin films. Additionally, we characterize their patterns, size and lifetimes under varied shear rates, and correlate their behaviors to the measured visco-elastic and visco-plastic properties of confined colloidal thin films.