Rheological Behavior and Roll Coating Properties of PDMS Enhanced with Multi-Walled Carbon Nanotubes and Fumed Silica

Scalable manufacturing of micro- and nanoscale textured surfaces from polymer composites is desirable in many applications from drag reduction in ship applications to energy-efficient radiative cooling of infrastructure. Creation of such surfaces, however, remains a challenge. By exploiting the ribbing phenomena that arise when viscous forces dominate over surface tension forces, we can create topographic patterns using roll-to-roll manufacturing techniques. In this work, we analyze how the rheology of yield stress fluids impacts the morphology of roll-coated surfaces using polydimethylsiloxane (PDMS) samples enhanced to varying degrees with multiwalled carbon nanotubes (CNTs) and fumed silica. We observe that CNTs increasingly dominate the large amplitude oscillatory shear response of PDMS composites. However, their impact is modified by the presence of fumed silica, which introduces a transition from intracycle strain softening to hardening behavior. The roll coating behavior of these PDMS composites is examined using image processing to link the rheological properties with the resulting surface morphologies, specifically focusing on two parameters defining surface morphology-ribbing wavenumber and branching patterns. While both types of PDMS composites display comparable wavenumbers, they exhibit different degrees of branching. The deviation in branching can be attributed to the intracycle strain hardening behavior seen at low CNT loadings in PDMS composites containing fumed silica. The study provides insights into the interactions occurring between CNTs and fumed silica in PDMS composites and highlights the significance of analyzing rheological parameters that are relevant at the high strains and strain rates experienced during roll coating, advancing our understanding of ribbing stability in yield stress fluids.