Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 276

without being conclusive since it was not possible to disentangle the effects of wall
slip and surface wettability during experiments. It is speculated that at low inertia,
where a gravitational subsidence is observed, the creeping movement amplitude is
governed by interfacial effects rather than wall slip, while at high impact velocities
wall slip effects become appreciable only in the last moments of the recoil, when
shear rates become very low.
When the drop radius is much larger than the capillary length, a = √ σ/ρg ,

surface tension effects can be neglected in comparison with those of gravity; fur-
thermore, large diameters also imply large Weber numbers, so that impacts are
dominated by inertia and by the rheological properties of the fluid only. Such exper-
imental conditions are explored in a recent work [27], which describes the impact
of relatively large bits (characteristic sizes between 10 and 30 mm) of various vis-
coplastic fluids, with yield stress magnitudes ranging from 4 to 124 Pa, and capillary
lengths of the order of a few millimeters. Although these fluids include many aque-
ous Carbopol dispersions, it must be observed that their yield stress magnitudes are
significantly smaller than the values reported in the open literature for similar fluids
[28].
By comparing impacts on a glass surface and on a super-hydrophobic surface
(contact angle of nearly 180 ◦ ), these experiments confirm that the maximum spread-
ing diameter of viscoplastic drops is weakly dependent on the surface wettability,
and smaller than the capillary limit as defined in Ref. [24]; unfortunately, the latter
result can also be obtained with high-viscosity Newtonian fluids [11], so that it is
not possible to establish whether the yield stress has an independent influence.
The most interesting finding of this work is the strong and rapid recoil, which
may even be followed by a complete rebound, observed after the spreading phase
of Carbopol drops impacting on the super-hydrophobic surface. Since both a recoil
driven by surface tension and a purely elastic rebound (the flow threshold corre-
sponds to a shear deformation of about 25%, whereas deformations during impact
vary between 100% and 500%) must be ruled out, it is suggested that at such high
velocity gradients (We 1400) Carbopol solutions may exhibit a viscoelastic be-

haviour: during the rapid spreading phase, flow is faster than the fluid relaxation
time, resulting into giant elastic deformations on short time scales. This conjecture
is supported by the comparison of experimental results with a minimal model of
elasto-viscoplastic inertial spreading, where elasticity is tentatively accounted for
by the storage modulus measured below the flow threshold (indeed, a very rough
approximation). However, it appears that in order to obtain independent evidence
in support of this picture, dynamic rheometric tests with characteristic frequency
comparable with the inverse of the impact time scale are necessary.

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