Superspreaders versus "cousin" non-superspreaders: Disjoining pressure in gravitational film drainage

Gravitational drainage of vertical films supported on a wire frame of two superspreaders SILWET L-77 and BREAK-THRU S 278 and their respective "cousin" non-superspreaders SILWET L-7607 and BREAK-THRU S 233 revealed drastic differences. The superspreader films showed complicated dynamic "turbulent"-like interferometric patterns in distinction from the ordered color bands of the "cousin" non-superspreaders, which resembled those of the ordinary surfactants. Nevertheless, the superspreader films stabilized themselves at the thickness about 35 nm and revealed an order of magnitude longer lifetime before bursting compared to that of the "cousin" non-superspreaders. Notably, the superspreaders revealed drastic differences from the non-superspreaders in aqueous solutions with no contact with any solid hydrophobic surface. The self-stabilization of the superspreader films is attributed to significant disjoining pressure probably related to long superspreader bilayers hanging from the free surfaces. The scaling law for the disjoining pressure was found as p_disj(h) ∼ h^-m (with m ≈ 9-11) for the sufficiently concentrated superspreader solutions, and as p_disj(h) ∼ h^-s (with s ≈ 6) for more dilute solutions (in both cases, concentrations were above the critical micelle concentration). The non-superspreaders do not possess any significant disjoining pressure even in the films with thicknesses in the 35-100 nm range. The results show that gravitational drainage of vertical films is a useful simple tool for measuring disjoining pressure.