Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 51

a drop of AOT solution was deposited on the water layer and it was found that the
spreading had stable or fingering behaviour. At surfactant concentrations lower than
0.4 CMC, the spreading process was stable for the all ranges of thicknesses: no fin-
gers are observed. For concentrations in the vicinity of 1 CMC up to 2 CMC, the
spreading edge of an AOT drop is uniform for several seconds (from 3 to 10 s), de-
fined by the concentration and the film thickness, then the indistinct, round-tipped
and straight fingers appear behind the thickened rim. The fingers become wider
when the concentration decreases and the thickness of water layer increases. At the
4 CMC on the 25 µm and 50 µm thick films, spreading is stable and uniform with a
distinct surfactant covered disk of liquid in the centre of the drop, which is in agree-
ment with the results of Troian et al. [63]. However, on the 100 µm film, fingering
spreading behaviour occurs again.
In [45] sparingly and highly soluble anionic surfactants were compared in a wide
range of surfactant concentrations with water films ranging from 25 µm to 100 µm
in thickness. Sodium dodecyl sulphate (SDS) was used as a soluble surfactant. The
spreading behaviour of SDS solutions shows many similarities with some notable
differences from the AOT solution [75]. The spreading rates were of the same order
of magnitude for both the SDS and AOT, but for the SDS the rate did not vary with
the water film thickness. Authors related this fact to fast desorption of surfactant
and relatively significant gravity force for the high film thickness. The fingers oc-
curred at the SDS concentration of 0.4 CMC, which is two times less than in the
AOT case (0.8 CMC), and almost ten times earlier. The shape of fingers was more
pronounced and branched in the case of SDS drop deposition. At the CMC and high
film thickness, the fingering was observed in both surfactants. However, at the thick
film in the SDS case, the disk remained in the centre after drop deposition, exhibits
the protrusion instabilities that extend from the edge of disk, while in the AOT case
the disk is stable.
Nikolov et al. [77] reported the finger instabilities during the spreading of drop
of aqueous trisiloxane solution (Silwet ® L-77) on a hydrophobic plate. A droplet of
aqueous trisiloxane solution was placed on the cap of a water drop and immediately
started to spread at a high rate. In one second, advanced front fingers were generated
and became more pronounced during spreading.
Stoebe et al. [66] observed that small fingers appeared at the edges of droplets
of aqueous trisiloxane solutions spreading on the surfaces of various surface ener-
gies. The length of fingers was found to be less than 10% of the dynamic droplet
radius, which served as a reason not to consider the fingering in the analysis of their
experiment.
Troian et al. [63] investigated theoretically the mechanism responsible for the
instability during the spreading of surfactant-laden drop. Their physical model rep-
resents a hemispherical liquid drop covered with insoluble surfactant spreading on
a thin layer of the same liquid.
Considerable efforts were invested in the investigation of instabilities in the
course of spreading and progress was achieved in the understanding of the nature of