Dynamics of drop spreading on fibrous porous media

Wetting and spreading phenomena on solid surfaces and in porous media are being investigated for various applications including composite processing. Liquid drop spreading on porous media is governed by the inertial, gravity, viscous and capillary forces. These capillary forces are due to the capillaries formed by the porous bed. The influence of the nature of porous media, and therefore, the capillaries on the overall imbibition process is very significant. In this study, drop-spreading experiments were carried out with silicone oil and polyvinyl alcohol water solution (PVA) on glass, and two different kinds of glass fiber mats used in composite processing. These glass fiber mats form heterogeneous and anisotropic porous fibrous media. Different drop sizes were chosen to understand the varying effect of driving forces. The drop-spreading behavior was examined by measuring the contact radius and height of the drop with time. It was observed that contact radius for a drop spreading on porous media increases initially and then decreases. Height of the drop, on the other hand, decreases monotonically. Though we observed similar overall behavior for both silicon oil and polyvinyl alcohol water solution, there were qualitative and quantitative differences. Our experimental data further show that volume imbibed into porous media varies linearly with time for polyvinyl alcohol and varies with square root of time for silicon oil. Moreover, it is shown that the time required for drop imbibition into both the porous media increases with initial drop size. Finally, experimental results were compared with numerical predictions from the literature, showing that spreading dynamics of silicon oil on unidirectional mat (UDMAT) is similar to spreading on membranes. However, the spreading dynamics of polyvinyl alcohol water solution on all the substrates is substantially different than all the results reported in the literature.