Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 59

Figure 16. Left: Photograph of the spreading trisiloxane solution droplet at the highest surfactant
concentration used. Right: power n from the evolution of the radius R = C · t n for the 'superspreading'
of the trisiloxane solutions versus concentration [112].

increase in spreading area and decrease in contact angle making it an exceptionally
effective wetting agent. Nevertheless, the debate continues about the nature of the
driving force for superspreading and interpretation of the observed dynamics.
Svitova et al. [109] established the essential parameters of trisiloxane wetting:
the critical wetting concentration, CWC, and the critical aggregation concentration,
CAC; the latter being very similar to CMC. CWC is considered to be the con-
centration above which spreading occurs over a (moderately hydrophobic) solid
and liquid substrates. Hence, it is associated with the beginning of superspreading.
CWC, which is reported to be independent of the substrate surface energy, repre-
sents the maximal spreading potential of a trisiloxane surfactant [109, 110]. The
occurrence of the transition from partial wetting to complete spreading at CWC
was first mentioned in [111]. Recently, three regimes of spreading were identified:
(i) complete non-wetting during the spreading process at low concentrations, (ii) a
transition from initial non-wetting to partial wetting at the end of the spreading
process at intermediate concentrations, and (iii) partial wetting both at the begin-
ning and at the end of the spreading process at higher concentrations. Transitions
between different regimes flawlessly correspond to relevant critical concentrations:
CAC or CMC, and CWC [100].
Both the extent and the dynamics of trisiloxane spreading attracted significant
attention over the years. Rafai and Bonn [112, 113] suggested a R t n power law

to describe trisiloxane spreading with a power n larger than 0.1 for concentrations
both above and below CMC (Fig. 16). For very high bulk concentrations, a linear
relation R

∝

1) was reported.
In [111], three regimes of the spreading dynamics were observed: early stages
where wetting diameter is proportional to t n with n in the range 0.12-0.22; dur-
ing the second stage the exponent increases to 0.38-0.58; during the last stage of
spreading the surface roughness and local tension gradients lead to an asymmetric
drop shape and formation of fingers and dendrites. Nikolov et al. [77] also reported
finger instabilities during spreading of the drop of aqueous Silwet ® L-77 solution
on a hydrophobic plate. Stoebe et al. [66] observed the small fingers appeared at the
edges of droplets of aqueous trisiloxane solutions, but found the length of fingers

∝ t ( n =