Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 50

Figure 10. 'Phase diagram' of the behaviour of C 12 E 10 in ethylene glycol as a function of the surfac-
tant concentration relative top CMC, and the RH [71].
hydrophobic barrier, which increases with the surfactant concentration and viscous
solvent. Because of this barrier, the lateral extension of fingers is limited.
They assumed that the mobility of the surfactant on the solid controls the emer-
gence of a surface-tension gradient at the free liquid surface. Once born, this gra-
dient gives rise to a Marangoni flow, which dominates the spreading process. For a
given surfactant, the mobility increases with increasing RH.
This explains why RH controls the start of the Marangoni flow, even on surfaces
where adsorbed water is not mobile. This also explains the change in the dendritic
pattern with concentration, surfactant or solvent.
Subsequent experiments [31] have been performed to study of the role of the
thickness of a precursor wetting film in the amplification of the fingering spreading.
For this purpose, clean silicon wafers were deposited with thin films of solution of
ethylene glycol in methanol. The surfactant C 12 E 10 was chosen at the fixed CMC
concentration, the humidity was kept around 60% and the thickness of film was var-
ied from 30 to 250 nm. Under these conditions, the advancing line of the surfactant
precursor on the pre-deposited film was clearly visible. It has allowed revealing that
instabilities appear behind the contact line of solvent film. When the thickness of
pre-deposited film was increased, the branching of fingers and the thickness of their
tips were decreased. The spreading dynamics, namely: the radii of fingers R f from
the centre of drop to their tips, and the length of surfactant leading film, R 0 ,were
found to scale as t 1 / 2 , but the drop radius, R c , changed with time as t α ,where α
took on values from 0.3 to 0.45 with increasing thickness of the pre-deposited film.
In [75], the influence of surfactant concentration at the fixed thickness of solvent
pre-deposited film was studied. At a concentration below CMC, the thickness and
base width of fingers were observed. The profile of the spreading drop is smooth
and no boundary between the drop and the fingers was observed. For a concen-
tration near CMC, fingers became more straight. At higher concentrations, fingers
disappeared and a hollow part appeared at the tip of the fingers.
Many authors investigated the effect of surfactant solubility on the unstable
spreading of aqueous solutions. In the case of sparingly soluble surfactants [75],