Marangoni Enhanced Contact Line Advancement in Surfactant-Driven Superspreading: Going Beyond the deGennes-Tanner Paradigm

Since Rafai et al. (Langmuir 18, 10486, 2002) reported that a drop of a trisiloxane surfactant solution can spread in radius growing linearly with time, the origin of this superspreading phenomenon is still not fully understood. Here we crack this longstanding puzzle beyond the common deGennes-Tanner paradigm. We show that the curious linear spreading law can be resolved by a distinctive contact line structure with a tiny surfactant leak to the substrate, capable of driving the contact line at a constant speed with a locally concentrated Marangoni force. The criterion for superspreading is also derived, explaining why superspreading occurs in a range of surfactant concentration and why it is merely limited to a certain class of surfactants. We further show that the late time spreading can be governed by either the 1/6 or 1/2 power law, depending on the ability of interfacial surfactant to transfer onto the substrate. All these results can account for a variety of findings seen in experiments. Analogy to thermocapillary spreading is also made, reverberating the ubiquitous role of Marangoni effect in dynamic wetting driven by non-uniform surface tensions.