Biomedical Engineering Reference
In-Depth Information
Drop rebound can be observed either with or without emission of satellite
droplets, the latter case being often referred to as 'dry rebound'. Conventionally,
one defines a 'dynamic Leidenfrost temperature' as the smallest temperature for
which the drop bounces off the wall without breaking-up or scattering secondary
droplets [53]. Its value has been shown to increase with the Weber number [54].
The impact morphology of viscoelastic drops can be significantly different from
that of Newtonian drops, even when viscoelasticity is obtained through the addi-
tion of small amounts of polymer to a Newtonian solvent [55]. In particular, the
fluid viscoelasticity has three major effects, which are illustrated in Fig. 15: (i) in-
hibition of drop splashing (or, equivalently, translation of the splashing threshold to
higher Weber numbers); (ii) suppression of secondary atomization in sessile drops;
(iii) suppression of secondary atomization during drop bouncing, which in turn af-
fects the dynamic Leidenfrost temperature [56].
It should be observed that qualitatively similar effects could be achieved by sim-
ply increasing the fluid viscosity, which reduces the mechanical energy available
for drop break-up or for the ejection of satellite droplets by increasing the overall
energy dissipation. However, since the viscosity of dilute polymer solutions is al-
most identical to that of the pure solvent, these effects can be interpreted solely in
terms of the polymer chains elasticity.
In particular, a high elongational viscosity is known to change substantially the
breakup dynamics of free-surface flows and their decay into drops [57]: thus, elon-
gational viscosity opposes to the scattering of secondary droplets from the free
surface of the impacting drop.
However, one can speculate about at least two other effects of the polymer addi-
tive that contribute simultaneously to suppress secondary atomization. First of all,
it is likely that the additive improves not only the stability of the surface between
the drop and the surrounding atmosphere, but also that of the surface in contact
with the vapor cushion that separates the drop from the hot wall: this reduces the
chances that the liquid may locally touch the wall and start boiling. Second, even if
the liquid makes contact with the wall, the presence of the polymer can significantly
affect the process of growth, detachment, and rise of vapor bubbles [58, 59], hence
prevent their bursting on the drop free surface.
Elongational viscosity can also be invoked to explain the inhibition of drop
splashing observed in dilute polymer solutions, both because the fluid elasticity
improves the stability of the liquid rim during drop spreading, counter-acting the
growth of perturbations, and because it prevents the break-up of liquid bridges
which may form between different parts of the drop in case of large deformations
[57].
The transition temperature where drop bouncing occurs without secondary at-
omization, which conventionally defines the dynamic Leidenfrost temperature, is
plotted with respect to the impact Weber number in Fig. 16, for both Newtonian
and viscoelastic drops impacting on a polished aluminum surface.
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