Mixing and spreading of different liquids are omnipresent in nature, life, and technology, such as oil pollution on the sea, estuaries, food processing, the cosmetic and beverage industries, lab-on-a-chip, and polymer processing. Where different liquids, having different physical properties including surface tensions and viscosities, meet Marangoni and other physico-chemical hydrodynamic phenomena are important. However, the mixing and spread- ing mechanisms for miscible liquids remain poorly characterized. We observed that a deposited soluble liquid drop on a liquid surface remains as a static lens without immediately spreading and mixing, which is a counterintuitive result, when two liquids have different surface tensions. Simultaneously, a convective flow is generated, which is referred as interfacial turbulence corresponding to ‘Marangoni instability’. Once the liquids near the interface are completely mixed, the Marangoni flows stop. We develop a theoretical model to predict the finite spreading time and length scales and Marangoni-driven convection flow speed. The fundamental understanding on this solutal-Marangoni flow enables driving bulk flows and constructing an effective drug delivery and surface cleaning material without surface contamination by immiscible chemical species.
H. Kim, K. Muller, O. Shardt, S. Afkhami, H.A. Stone, "Solutal-Marangoni flows of miscible liquids drive transport without surface contamination," accepted in Nature Physics.