Figure 7.

Two modes of droplet drying. On the left-hand side, the three-phase line is free to move, keeping the contact angle constant. On the right-hand side, the three-phase line is pinned. This latter scenario leads to maxima in −Δf and ΔΓ, when the height of the drop is about a quarter of the wavelength of the shear sound. The wavelength of the shear sound is close to the penetration depth of the shear wave, which is around 100 nm, depending on the viscosity and overtone order. Maxima in −Δf and ΔΓ are observed in the kinetics of drying of fluorescein in water (Figure 4G).