The new Mumford & Sons single “If I Say” features a fluid-dynamical music video. It’s full of dendritic fingers and flowing colors – likely from combinations of inks, paints, and other fluids. Although the fingers are reminiscent of the viscosity-dependent Saffman-Taylor instability, these appear to be driven by variations in surface tension between the different fluids. That’s a major feature throughout the video; although some of the flow is caused by the syringes depositing fluids, much of it seems to be a Marangoni effect, where flow moves away from areas of low surface tension to ones with higher surface tension. (Video credit: Mumford & Sons; filmed by P. Hofstede; via Katie M.)