The spreading of shear-thinning nanosuspension droplets on a smooth surface

Understanding and predicting the spreading of droplets impacting on smooth surfaces is crucial in many applications such as printed electronics and spray coating where the fluid is a suspension and in general non-Newtonian. A key quantity is the maximum spreading diameter which has been extensively studied for Newtonian fluids and is determined by the impact velocity, Reynolds number, Weber number and wetting properties of the surface. The crucial question that emerges for non-Newtonian fluids is how to account for the viscous dissipation given the shear rate dependence of the viscosity and the possible presence of a yield stress. We address this question by studying the impact of graphene oxide suspension droplets over a wide range of concentrations where the rheology changes from viscous fluid to shear-thinning yield stress fluid with increasing concentration. We identify the apparent viscosity during the spreading process that allows us to define a corresponding Reynolds number. This enables us to formulate an energy balance model that successfully predicts the maximum spreading diameter for both shear-thinning yield stress fluids and Newtonian fluids and therefore provides a unified description of this problem.