Solutal Marangoni flows of miscible liquids drive transport without surface contamination

The way two liquids interact depends on how miscible they are. A remarkable phenomenon involving two miscible liquids is now reported: placing a drop of isopropanol on a water surface results in a Marangoni flow, and a static lens in the middle. Mixing and spreading of different liquids are omnipresent in nature, life and technology, such as oil pollution on the sea 1 , 2 , estuaries 3 , food processing 4 , cosmetic and beverage industries 5 , 6 , lab-on-a-chip devices 7 , and polymer processing 8 . However, the mixing and spreading mechanisms for miscible liquids remain poorly characterized. Here, we show that a fully soluble liquid drop deposited on a liquid surface remains as a static lens without immediately spreading and mixing, and simultaneously a Marangoni-driven convective flow is generated, which are counterintuitive results when two liquids have different surface tensions. To understand the dynamics, we develop a theoretical model to predict the finite spreading time and length scales, the Marangoni-driven convection flow speed, and the finite timescale to establish the quasi-steady state for the Marangoni flow. The fundamental understanding of this solutal Marangoni flow may enable driving bulk flows and constructing an effective drug delivery and surface cleaning approach without causing surface contamination by immiscible chemical species.