Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 275

Figure 6. Influence of the yield-stress magnitude on the maximum spreading diameter of viscoplastic
drops, for different impact Weber numbers [16].
Figure 7. Dimensionless drop diameter, β , plotted against time for drops of a viscoplastic fluid
( K = 7 . 936 Pas n , n = 0 . 3727, τ c = 26 . 1 Pa) impacting from fall heights of 10, 50 and 200 mm
on glass [G] (solid symbols) and parafilm-M [P] (open symbols) solid substrates [16].
tion phases similar to those observed for high viscosity Newtonian fluids, whereas
impacts on glass show no significant retraction phase, and slow capillary-driven
spreading follows directly on from the fast spreading of inertial expansion for low
impact velocities. At higher Weber numbers, drop diameters remain nearly constant
after maximum spreading [16, 17].
These results are substantially confirmed by a more detailed study of the effect
of surface wettability and roughness on viscoplastic drop impacts [24], which com-
pares two smooth substrates with distinct surface energies and three substrates with
similar surface energy but different roughness. The same work also attempts at a
quantification of the effects of apparent wall slip [19, 26] on drop impact, however