Drop Impact onto Dry Surfaces with Complex Morphology

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Surface texture, e.g. roughness, porosity, wettability and chemical composition can significantly affect the outcome of drop impact. Section 5.1 deals with the splashing threshold on rough, textured and also porous solid surfaces. In Section 5.2 an impact of a single Newtonian drop near a hole in a flat substrate is considered as a simplified model of drop spreading on a porous substrate. The experiments described in Section 5.3 deal with drop impacts of such different liquids as water and oily Fluorinerts onto suspended thin membranes with microscopic pores of different wettability. They reveal that liquid penetration is possible even through a non-wettable porous medium if the impact velocity is high enough. A similar conclusion stems from the experiments with water drop impacts onto membranes coated with much less permeable nanofiber layers discussed in Section 5.4. In the case of nanofiber mats deposited onto impermeable surfaces, drop splashing and bouncing after impact can be fully suppressed, as the experiments of Section 5.5 show. The reason for the phenomena observed in Sections 5.3–5.5 is the hydrodynamic focusing of liquid brought by a millimeter-sized drop into micron-sized pores. The theory of the hydrodynamic focusing phenomenon is given in Section 5.6, and the results are illustrated experimentally by the amazing fact that liquid velocity in the jets which penetrated through the entire porous medium thickness is higher than that in the impacting drop, even though the viscous dissipation in flow through porous medium is extremely high. Liquid penetration following drop impact onto a nonwettable porous medium is also visualized in the experiments with the entrained seeding particles in Section 5.7, which also contains the evaluation of the critical filter thickness which can be fully penetrated in spite of the viscous dissipation in the pores. Drop impacts onto hot surfaces covered with nanofiber mats also reveal significant enhancement of surface cooling due to the hydrodynamic focusing. The latter sustains the contact of liquid coolant with the hot surface underneath and thus facilitates complete liquid vaporization and significant heat removal in the form of latent heat of evaporation (Section 5.8).