Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 272

Figure 4. Average dimensionless retraction rate of shear-thinning drops as a function of the consis-
tency coefficient [16].

zero) would be expected to exhibit lower local viscosities during inertial expansion
and therefore an increased maximum spreading diameter.
Measurements shown in Fig. 3 however show the opposite trend: the maximum
spreading diameter is higher for less shear-thinning fluids. Together with Fig. 1 this
suggests that whilst shear-thinning effects affect the impact behaviour, the influence
of the consistency coefficient K , which gives an indication of the maximum viscos-
ity of the fluid for 0 hence determines the average apparent viscosity during

the process, appears to dominate.
Drop retraction on the hydrophobic substrate can be studied by measuring the
retraction rate, defined as:

γ ˙ →

max β(t)
β m

.

v r

χ ˙ =

R max =−

(1)

When this quantity is nondimensionalized with the inertial timescale given by:

4

ρR 0
σ

3 π

T I = (2)

one finds it decreases linearly with the consistency coefficient, as shown in Fig. 4.
This trend is qualitatively similar to the retraction rate dependence on viscosity in
Newtonian drops, and further confirms the dominant influence of the consistency
coefficient with respect to the power-law exponent.

C. Impact of Yield-Stress Drops
Despite yield-stress fluids have been studied for about one century, and one of
the most important spray applications, painting, makes use of a yield stress fluid
(paint), the first investigation of yield-stress drops was published only recently [18].