Complex Fluids


We study about complex fluids including binary mixtures, partially miscible mixtures, emulsion liquids, and non-newtonian fluids. Using flow visualization techniques, we try to discover new features from the fluid mechanics point of view. We endeavor to achieve real applications using these complex fluids.



An evaporating liquid drop containing solutes or particulates leaves a deposit whose form is determined by the flow field and interactions between suspended particles and a solid substrate, which are crucial for coating processes. We discover that a whisky drop creates a relatively uniform pattern after drying, in contrast to the well-known "coffee-ring stain". Based on measurements of the flow field, we show there are two key features to produce the uniform deposit pattern: First, spontaneous multiple sequential Marangoni flows continuously mix suspended materials and prevent non-uniform coating. Second, surface-adsorbable macromolecules in whisky contribute to a uniform deposit of the suspended materials. From this understanding of the pattern formation, we design a model liquid to achieve a nearly uniform surface deposit, which offers a new physicochemical avenue for control of coatings.

H. Kim, F. Boulogne, E. Um, I. Jacobi, E. Button, and H.A. Stone, "Controlled uniform coating induced by multiple Marangoni flows and surface-adsorbable macromolecules," Phys. Rev. Lett. 116, 124501 (2016)
(selected as a PRL cover and PRL Editors' Suggestion and highlighted in Physics and Nature physics)



In an evaporating millimeter-sized drop, the flow field can be visualized by particles using conventional flow field measurement methods. However, to measure small-scale features in a relatively large system – such as the chemical distribution of molecules in a millimeter-sized droplet – while maintaining the global perspective needed to understand physical phenomena, is difficult but often important. Here, using physicochemical effects, we directly visualize the selective evaporation of a binary mixture droplet where the chemical distribution of a volatile component is not constant in time and space. We find that a mixture consisting of an organic solvent and deionized water dissolves suspended fluorescent polystyrene particles at a critical concentration, i.e. 90 wt. % of the organic solvent in water. Using this idea, we show that during evaporation the particles begin to disappear from near the contact line of a binary mixture droplet. Also, we develop a diffusion-dominated model to predict the experimental observations. For the low concentration of water in the mixture, the model shows good agreement with experimental measurements. We expect that the experimental idea introduced and demonstrated here will enable visualization of hidden features that have not been explored before. 


Related publication:
H. Kim and H. A. Stone, "Direct measurement of selective evaporation of binary mixtures by dissolving materials," Accepted in Journal of Fluid Mechanics.