Starbursts and wispy drops: surfactants spreading on gels.

The spreading dynamics of surfactant-laden droplets have been studied for both solid and thin liquid substrates. In general, the presence of a surfactant modifies the contact line dynamics due to the Marangoni effect, which creates a surface tension gradient and thus generates pattern-forming instabilities. To determine the effects of varying substrate mobility from a liquid to a solid, we examine the intermediate case of a viscoelastic substrate. We observe novel instabilities of the surfactant-laden drop influenced by both the substrate fluidity and the surfactant concentration. The experimental apparatus consists of a petri dish containing a gel substrate composed of 0.04% to 0.16% agar by weight in FIG. 1. Phase diagram for droplets of nonionic surfactant solution Triton instability; blue images: wispy drops; green images: uniform spreading.