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This .gif was posted on reddit /r/gifs, with title "A water droplet evaporating." I'm not actually sure if that is actually the case here.

Regardless, if we were to look at the process we can see the bulge protruding outwards, with flat surface in contact with the leaf. Now as the droplet evaporates away, the entire bulge shrinks while kinda conserving its shape, and therefore the flat surface area in contact with the leaf also shrinks. But when most of the water evaporates, the bulge loses it shape instead. The flat surface area is maintained, while the bulge becomes flatter and flatter, until all water is evaporated. But why? Why doesn't the process maintain its pattern of either 1) constant shape or 2) constant surface area?

I posted a very crude picture of the process below.

Fig 1

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  • $\begingroup$ The process is controlled by the simple rule that the surface energy should be minimal. $\endgroup$ – Weijun Zhou Jan 22 '18 at 13:15
  • $\begingroup$ Surface tension forces control the shape of the droplet on the surface, and, as the volume of the droplet decreases, the balance of gravitational and surface forces changes. $\endgroup$ – Chet Miller Jan 22 '18 at 13:43
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after the droplet has resided on the flat surface for a while, it manages to wet it slightly in the region of its contact. this means that the contact angle of the droplet on the patch of surface which it has been touching will be less than that present at the triple junction around its periphery. as the droplet loses volume to evaporation, the wetted contact patch prevents the droplet from contracting as one would otherwise expect, and the droplet collapses until the triple junction angle equals the wetted contact angle and only then would you see the contact patch itself begin to lose area.

This phenomenon is called wettability hysteresis and is responsible for the difference in contact angle between the advancing side and retreating side of a water droplet sliding down an incline.

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