Impact of aqueous suspension drops onto non-wettable porous membranes: Hydrodynamic focusing and penetration of nanoparticles

The impacts and dynamic penetration of nanoparticle suspension drops into porous filter membranes are studied experimentally and theoretically. This type of penetration is associated with hydrodynamic focusing and is radically different from the wettability-driven imbibition. In the case of hydrodynamic focusing water can penetrate into a non-wettable porous medium at very low values of the dynamic pressure associated with drop impact. Two types of membranes are used in the experiments: (i) glass fiber filter membrane wettable by the carrier fluid (water), and (ii) polytetrafluoroethylene (PTFE) depth filter membrane, non-wettable by the carrier fluid. The nanoparticle entrainment and deposition inside the membrane bulk is used to mostly visualize the ultimate penetration fronts by observing the cut cross-sections of the membranes, albeit also provide an insight into innovative applications like circuit printing on nonwovens. The deposition patterns inside the membranes are also linked to the drop splashing patterns at their front surfaces. The experimental results confirm that during the dynamic focusing water can penetrate into a non-wettable porous medium (PTFE). Water also penetrates by the same hydrodynamic focusing mechanism into the wettable glass fiber membrane, where it additionally spreads on a much longer time scale due to the wettability-driven flow. A theory explaining hydrodynamic focusing penetration of liquid into porous medium after drop impact is proposed. It is used to explain and predict water penetration into the non-wettable filter medium after drop impact, and the results are compared with the experimental data. Also the critical thickness of non-wettable membranes determined by dissipation of the kinetic energy in flow inside membrane is evaluated and compared with the experimental data.