Nanosuspension Rheology Measurement using Drop Impact onto a Hard Sphere

A comprehensive understanding of the rheological and fluid mechanics properties of nanosuspensions is crucial for optimizing their formulation and processing, improving their stability and performance, and ensuring their safety and efficacy in various applications. Traditional characterization methods require a large amount of sample and are infeasible when sufficient amounts are unattainable. Therefore, there is often a need to develop new characterization methods that require no more than a single droplet. When a liquid drop impacts onto a solid substrate it generates a radially spreading flow in a thin lamella. The lamella is bounded by a rim, formed by surface tension. If the Reynolds number of an impact is much larger than the unity, the flow in the lamella is governed by the inertial effects. The viscosity becomes dominant only in an expanding viscous boundary layer. The flow quickly slows down when the thickness of the boundary layer is equal to the thickness of the lamella. Finally, the impact leads to the formation of a residual liquid film whose thickness is determined exclusively by the liquid viscosity and drop impact parameters. In this work, we perform droplet impact experiments of various Newtonian liquid drops and nanosuspensions on hard spherical targets. The impact is observed using high-speed imaging from two perpendicular angles, and the results are analyzed using digital image processing. The convex shape of the target opens the side view on the lamella, which is blocked by the rim in the case of a flat substrate. The residual film thickness is measured and the viscosity of the liquid is determined. Finally, a theoretical model has been developed to enable the measurements of viscoelastic parameters, such as the complex modulus profile and yield strength of non-Newtonian liquids. The capacity to evaluate properties using minimal sample quantities and at a wide range of strain rates is a major advantage of the method used in this study. This approach is particularly useful in industries such as medicine, pharmaceuticals, food processing, and cosmetics, where viscosity plays a key role in determining product quality and performance.