Superspreading mechanisms: An overview

An aqueous solution of trisiloxane-ethoxylate surfactants (superspreaders) has fascinating surface properties that promote rapid spreading over hydrophobic substrates and efficiently reduce the surface tension at the air/solution interface to 21–22 mN/m. Superspreaders have a variety of commercial and industrial applications, and can be used as adjuvants, surface modifiers for fabrics, cleaners and much more. Since their discovery in the 1960s, and despite their significant technological applications, the phenomenon that drives superspreading is still not well understood and is under continuous discussion. The goal of the presented review is to discuss and analyze the data presented in the literature and then to elucidate the concepts and mechanisms to explain what drives the fast rate of spreading. Two concepts are presented (and then excluded) for elucidating the understanding of the fast spreading rate over hydrophobic surfaces: the first concept concludes that the spreading is driven by the contact angle dynamics due to the reduction in the surface tension and/or interfacial tension of the solution/substrate leading to a decreased contact angle during spreading and the value of the spreading coefficient S ≥ 0, while the second concept attempts to show that the spreading is driven by the Marangoni flow over a stretching surface of a spreading drop or at the precursor film. However, neither the spreading coefficient, S ≥ 0, nor the Marangoni flow satisfactorily explains why the rate of spreading vs. the degree of surface wettability has a maximum. This review will help readers gain insight on superspreading and stimulate researchers to explore the superspreading phenomenon for novel applications.