Impact and spreading dynamics of a drop of fiber suspension on a hydrophilic solid substrate

The presence of non-Brownian spherical particles dispersed in a liquid modifies the impact and spreading dynamics of a drop on a hydrophilic substrate. The difference in spreading dynamics is attributed to the increase in the viscosity of the suspension caused by the presence of the particles. Similarly, the presence of anisotropic non-Brownian particles, such as fibers, also increases the bulk viscosity of the suspension. In addition to the diameter D, of the fiber, the length L, which determines the aspect ratio A=L/D, is crucial in controlling the viscosity of fiber suspension. Therefore, we hypothesize that the drop impact of fiber suspensions with different volume fractions will result in a similar modification of the spreading dynamics.

To investigate the impact and spreading dynamics, we prepare suspensions of fibers with an aspect ratio A=12 at different volume fractions. These volume fractions span the dilute, semi-dilute, and dense concentration regimes. Additionally, we conduct a subset of experiments with aspect ratios A=4 and A=20. Furthermore, we characterize the thickness of the resulting droplet film, as well as the coating and orientation of fibers after the spreading dynamics reach a steady state.

The presence of fibers significantly influences the spreading dynamics and final size of the droplet on the hydrophilic substrate. Notably, the resulting droplet size after spreading decreases as the volume fraction of fibers in the suspension increases. To rationalize these results, we use a modified equation, originally developed for spherical particles, which incorporates the viscosity of the suspension. Additionally, we observe an increase in the splashing of the droplet during spreading when increasing the Weber number and the volume fraction. Furthermore, we show that as the volume fraction increases, the final thickness of the droplet increases, and the resulting fiber coating becomes less uniform. We also highlight the secondary influence of fiber geometry on the coatings, such as the overlap of fibers, which further affects the coating uniformity. Despite these geometry-induced modifications, the radial orientation of the fibers remains isotropic across all volume fractions considered in this study.