Solutal Marangoni flows of miscible liquids drive transport without surface contamination

Abstract

Mixing and spreading of di erent 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 di erent 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 e ective drug delivery and surface cleaning approach without causing surface contamination by immiscible chemical species.When a sessile oil drop is released on top of a water surface, it spreads until a monolayer is achieved 9 , because the liquids are immiscible, as shown in Fig. 1a. In contrast, if a water drop is placed on a water surface, it shows a cascade of coalescence events and the liquids are rapidly mixed (Fig. 1b) 10 . In contrast with these two configurations, we captured unexpected mixing and spreading features between fully miscible liquids. When a drop of alcoholfor example, isopropanol (IPA)-is placed on a water surface, it spontaneously generates a Marangoni convective flow along the outward radial direction and we observed that there is a static liquid lens in the middle ( Fig. 1c and Supplementary Fig. 1), even though these two liquids are infinitely miscible. Here, we discuss solutal Marangoni effects in fully miscible liquids to explain the finite size lens and the associated flow (more details are provided in Supplementary Videos 1-3).To visualize the spreading and mixing pattern of a miscible liquid drop, IPA (volume V = 7.2 ± 0.2 µl), placed on a water bath (400 ml deionized (DI) water in an 196-mm-diameter Petri dish with depth H = 14 mm), we used time-resolved particle tracking velocimetry (PTV) and high-speed schlieren measurement techniques (Supplementary Information). For PTV experiments, we seeded polystyrene particles (diameter = 100 µm) in solution and recorded the particle motion from top and side views. IPA is less dense than water and therefore the sessile drop floats on the surface. The drop initially spreads out and quickly achieves a static central lens with a near constant diameter 2R (see Fig. 1c and Supplementary Figs 5-7) during which the IPA continuously leaks at the boundary (Fig. 1c, Supplementary Fig. 1 and Supplementary Videos 3 and 4). The inset of Fig. 2a shows a top view of a schlieren pattern, that is, an interfacial turbulence structure representing the mass transfer between the...

Hydrodynamic response of a surfactant-laden interface to a radial flow

“…The solution that satisfies both the boundary conditioñ v (1) z (q, 0) = 0 while vanishing far away from the interface, lim z→−∞ṽ…”

“…Taking the inverse Hankel transform then leads to the velocity components of the counterflow The streamlines corresponding to the total velocity field v = v (0) + v (1) are plotted in Fig. 3.…”

“…Streamlines of the total velocity field v = v (0) +v(1) in the low-compressibility regime 0 < β < 1. The point source is marked by a purple dot.…”

Convection-dominated dissolution for single and multiple immersed sessile droplets

“…For a multi-component drop dissolving in a host liquid, there is formation of Marangoni flow caused by the variation of surface tension over the droplet surface, which in addition can influence the behaviour of emulsification (Tan et al 2019). Similarly, the Marangoni flow also plays a crucial role in multi-component sessile Convection-dominated dissolution for immersed sessile drops 892 A21-3 droplet evaporation (Scriven & Sternling 1960;Tan et al 2016;Diddens et al 2017;Kim et al 2017;Edwards et al 2018;Li et al 2018bLi et al , 2019.…”

Directional and velocity control of active droplets using a rigid-frame

“…Self-propelled oil droplets locomote in water due to the effect of solutal Marangoni convection. [31][32][33][34] Fig. 1 shows the propulsion mechanism for a droplet in water, together with the chemical formula of oleic acid.…”