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.

 


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.

H. Kim, J. Lee J, T-H Kim, and H-Y Kim, "Spontaneous Marangoni mixing of miscible liquids at a liquid-liquid-air interface," Langmuir 31(31), 8726 (2014)

 

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:

 

The movie presents the self-healing behavior by the surface tension difference between the film and the droplet. The DI water droplet falls toward the soap film having a relatively low surface tension. If the droplet inertia is large enough compared to the surface energy of the soap film, after the droplet contacts the thin film, the film deforms and drags by the water droplet penetration by the gravity effect. However, the thin film can not be ruptured by the impact because the Marangoni effect retracts the soap film to the center of the impact location. Then, the droplet can pass the film without the rupture and the soap film can be healed. This move is taken with a high-speed camera (Phantom V7.3) @9900 fps.

Relevant literatures:
L. Courbin and H.A. Howard, "Impact, puncturing, and the self-healing of soap films," Phys. Fluids 18.9, 91105 (2006)
T. Gilet and J. Bush, "The fluid trampoline: droplets bouncing on a soap film," J. Fluid Mech. 625, 167 (2009)