Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 60

was less than 10% of the dynamic droplet radius, which served as a reason not to
consider the fingering in the analysis of their experiment. Recently, Radulovic et
al. [114] argued that the power law was not a suitable model to describe trisiloxane
dynamics and proposed an exponential relationship, R t/τ) , for solutions

above CWC. Their model gave a satisfactory fit for both the evolution of the contact
angle and drop radius with time, introducing the time constant, τ , as the main pa-
rameter of trisiloxane wetting which is directly related to the underlying diffusion
of superspreader molecules. Values of the time constant were found to decrease
with increasing concentration; hence, the faster wetting. Ivanova et al. [100] also
dismissed the power law and suggested that partial wetting of trisiloxanes proceeds
in two stages: the fast short first stage, which is followed by a much slower second
stage. They found that the characteristic time for the second stage decreased with
increasing concentrations. Certain discrepancies from the proposed model were ob-
served during the first wetting stage, which is understandable as the characteristic
time scale for the first stage is an order of magnitude smaller than that for the second
stage.
Initial attempts to shed light on the wetting ability of trisiloxanes related the
phenomenon to its ability to adsorb at the interface and significantly reduce ten-
sions at the moving liquid front, creating a positive spreading coefficient. However,
the positive value of the spreading coefficient during spreading could not be either
proved or theoretically explained, because the adsorption cannot keep pace with the
expansion of the drop due to diluted solutions at the perimeter and the surface con-
centrations becomes reduced at the contact line. The main drawback of the latter
explanation is that it can be equally applied to any aqueous surfactant solution and
unfortunately is not specific to trisiloxanes. Subsequent theories included solution
turbidity, the presence of a disperse phase, as a vital parameter in superspreading.
It was first mentioned in [115, 116] that at concentrations < 0.1 wt%, trisiloxane
solutions contain small aggregates, such as vesicles. These vesicles and/or other
aggregates disintegrate efficiently to transfer surfactant molecules to the contact
surfaces and enhance spreading. Zhu et al. [108] confirmed that superspreading oc-
curs only when dispersed particles (vesicles) are present. Ruckenstein [117] argued
bilayer adsorption on hydrophobic surfaces and the Marangoni effect it triggers are
responsible for superwetting. Stoebe et al. [67] also concluded that the aggregate
disintegration plays an important part as it results in sudden increases in surface
tension gradients and a corresponding increase in Marangoni flow. Svitova et al.
[111] gave further insight into surfactant self-assembly and aggregate structure as
main factor responsible for transition between partial to complete wetting.
Nikolov et al. [77] suggested that although aggregates are present in the solu-
tion, they do not play a crucial role in initiating the Marangoni effect which they
believed to contribute to the rapid spreading. The surface concentrations at the drop
apex are assumed to remain high compared to those at the perimeter, resulting in
the drop being pulled out by the higher tension at the perimeter than at the apex. To
maintain a high apex concentration, surfactant adsorption must exceed the rate of

∝ exp ( −