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How does a porous solid such as the cereal "Wheatena" manage to tentatively sit on water for a while before sinking? I assume it initially somehow evades capillary action, perhaps through some sort of surface tension or geometrical structural property. But Wheatena's initial dryness, I would think, would tend to accelerate absorption of water. Perhaps the kernels have air in them and so are tentatively lighter than water as a whole, perhaps reinforced by less penetrable outer wall. Is this much different much different from a floating bog ecosystem, in which gas pockets promote buoyancy? Wheatena poured onto water

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For something to be wettable by water requires its surface to be introduced to the water and then get friendly with it. This initial equilibration process requires the surface to become fully populated with adsorbed water molecules and can take anywhere from seconds to minutes. Since water vapor is being given off continually by the water in the vicinity of the surface, this initial wetout process can take place a short distance in front of the advancing water as the vapor penetrates the surface. This gives rise to the phenomenon of a porous surface being apparently unwettable for a few seconds after making the acquaintance of the water, and then rapidly wetting out completely and soaking into the porous material all at once.

Exactly the same initiatory time lag process occurs when inkjet ink is squirted onto a sheet of plain paper. The droplets first reside as tiny ink igloos on the surface of the paper for ~milliseconds and then rapidly disappear into the paper.

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  • $\begingroup$ Thank you; that's quite helpful. $\endgroup$
    – Sketcher
    Mar 6, 2022 at 0:44

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