Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 61

interfacial dilation at the apex due to the outward flow. Chengara et al. [118] also
provided considerable evidence of surface tension gradient being an important driv-
ing force in superspreading. Kumar et al. [119] pointed out that bilayer adsorption
may account for the ability of superspreaders to maintain the low apex concentra-
tion necessary to drive Marangoni spreading, or a high concentration at the contact
line to maintain a low contact angle and force spreading.
A number of hypotheses are based on the formation of a precursor film on a
solid surface. Zhu et al. [108] noted the influence of the water vapour pressure in
the surrounding atmosphere on the trisiloxane wetting as superspreading was ob-
served only in saturated or supersaturated water vapour. This has given rise to the
suggestion that fast spreading may be caused by surface flow of a thin precursor film
formed from the vapour phase. Churaev et al. [120] showed that fast spreading and
climbing of trisiloxane surfactant solutions over hydrophobic surfaces, are caused
by formation of extremely thick wetting films, stabilized by mutual repulsion of
vesicles, and emphasized the important role of disjoining pressure. Other attempts
to explain trisiloxane wetting included the idea of the surfactant molecule transfer
on a bare hydrophilic solid in front of the moving contact line—the autophilic phe-
nomenon [32, 78]. This theory was used to explain the second slow stage of wetting
found by Ivanova et al. [100], while the first fast stage was related to disintegration
of aggregates in the vicinity of the three-phase contact line.
Ananthapadmanabhan et al. [101] first postulated that the rapid spreading is a
consequence of the unusual shape of the hydrophobe group. However, Hill claimed
that 'T' ('umbrella-like') shape of superspreader molecule is not the explanation
for unusual wetting properties and emphasised the importance of the end-capping
groups [116]. Kumar et al. [119] compared superspreaders to n -alkyl polyoxyethy-
lene surfactants, which have n -alkyl chain instead of trisiloxane backbone. The
hydrophobicity of the alkyl chain and the trisiloxane backbone is approximately
the same, but their size is very different, which served as a reason for the different
spreading power of polyethoxylates with alkyl groups and trisiloxanes. Radulovic et
al. [105, 106] confirmed the superiority of trisiloxanes over conventional nonionic
surfactant. They compared Silwet ® L-77 with Triton ® X-100, surfactants which
have the same chemical composition and similar length of poly(ethyleneoxide)
hydrophilic tails, but very different structure, size and hydrophobicity of the hy-
drophobic head (Fig. 17). It was concluded that the larger area of the trisiloxane
hydrophobic head undoubtedly contributes towards a more efficient (and quicker)
decrease in interfacial tensions; hence, the higher wetting potential. Svitova et al.
[102] showed that the rate of trisiloxane spreading depends on surfactant nature, its
structure, and concentration and the subphase nature. Generally, it was concluded
that E 8 has one of the best wetting properties. The increase in surfactant tail length
( n 12, 16) is known to suppress the superspreading ability of siloxane surfactants

[108].
Trisiloxane molecules with short chain lengths also exhibited poorer wetting
ability than those with medium lengths [67]. This is in agreement with the recent

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