Interfacial Phenomena

 

An interface of liquid is omnipresent in nature, life, and industry. We focus on surface tension dominant problems including 1) Marangoni effects, 2) wetting and dewetting, and 3) interfacial instabilities. We utilize various direct optical measurement techniques, for instance 2D/3D Particle Image Velocimetry, interferometric analysis, Schlieren method, and high-speed and fluorescent imaging, to study these problems.

 

 

 

 

We investigate and control vapor-mediated solutal Marangoni flow inside a sessile droplet. In this study, we show that not only the surface tension of volatile liquid but also the vapor concentration can vary a local surface tension along the interface, which changes the internal flow speed. To predict the flow speed and oscillatory frequency of the solutal Marangoni flow, we develop a theoretical model based on scaling analysis that shows a good agreement with experimental results. By varying the number of vapor sources, we generated and controlled multiple vortices. We also performed mixing experiments that present vapor-driven solutal Marangoni effect can be used as a promising flow actuator and mixer for the sessile-droplet-microfluidics device. We believe that the current study will motivate low-cost and portable sample flow control and mixing system for the near future.

 

• Park J; Ryu J; Sung HJ; and Kim H, "Control of solutal Marangoni-driven vortical flows and enhancement of mixing efficiency," accepted, JOURNAL OF COLLOID AND INTERFACE SCIENCE (IF: 6.361) 


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.

Related publication:
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.

 

We perform a quantitative analysis of a spontaneous vortex caused by Marangoni effects. When a drop of volatile liquid with a lower surface tension than that of water is deposited on a free water surface, it creates surface tension gradients leading to a Marangoni vortex at the contact line, i.e. liquid-liquid-air contact point. We report that the vorticity sign is determined by the gradient of surface tension between two liquids and the vortex strength is mainly proportional to the Marangoni force. Based on energy conservation, we develop an analytical solution to estimate the vortex strength. This figure can show one of the Marangoni effect examples.

Related publication: