Water transport and absorption in pharmaceutical tablets – a numerical study

The quality of a coated pharmaceutical tablet can be strongly affected by the interactions of water droplets with the porous substrate during processes such as coating process. Three different mechanisms co-exist in the coating process: water spreading, absorption and evaporation. Disentangling the fundamental understanding of these phenomena can therefore be crucial for achieving a higher quality of the products (e.g. a longer shelf-life of the tablets) and for controlling the efficiency of the process. This paper aims to investigate the spreading and absorption mechanisms after droplet impingement on a tablet using a Lattice-Boltzmann methodology. Our numerical results (droplet height and spreading, penetration depth and absorbed volume) are in a good agreement with experimental data and numerical simulations available in the literature. In particular, the spreading phase is characterised by the capillary spreading time scale, as confirmed by previous studies. In contrast to previous studies, we find that the absorption process begins at times shorter than the capillary spreading time but with a different power-law in the absorbed volume. We explain this behaviour through a modified Washburn law that takes into account three-dimensional effects. Our data can be used as a benchmark to test novel mathematical models.

Fluctuation assisted spreading of a fluid filled elastic blister

In this theoretical and numerical study, we show how spatially extended fluctuations can influence and dominate the dynamics of a fluid filled elastic blister as it deforms onto a pre-wetted solid substrate. To describe the blister dynamics, we develop a stochastic elastohydrodynamic framework that couples the viscous flow, the elastic bending of the interface and the noise from the environment. We deploy a scaling analysis to find the elastohydrodynamic spreading law $\hat{R}\sim \hat{t}^{1/11}$, where $\hat{R}$ is the spreading radius and $\hat{t}$ is time, a direct analogue to the capillary spreading of drops, while the inclusion of noise in our model highlights that the dynamics speeds up significantly $\hat{R}\sim \hat{t}^{1/6}$ as local changes in curvature at the spreading front enhance the peeling of the elastic interface from the substrate. These fluctuations have a pronounced influence on the shape of the deforming blister and lead to the formation of a precursor film similar to a perfectly wetting droplet. Moreover, our analysis identifies a distinct criterion for the transition between the deterministic and the stochastic spreading regime, which is further illustrated by numerical simulations.