PhD defence Huanshu Tan

Evaporation and dissolution of droplets in ternary systems 

Huanshu Tan is a PhD student in the Physics of Fluids group his supervisors are prof.dr. D. Lohse and prof.dr. X. Zhang of the Faculty of Science and Technology

The evaporation and dissolution of droplets in the multi-component system are omnipresent in nature, science, and many advanced technologies. In the droplets, the two-way interactions between the fluid properties and hydrodynamics make the dynamical behaviors complicated. To have a quantitative understanding of these processes is not only a challenge but also significantly crucial for many applications, such as inkjet printing, liquid-liquid extraction, DNA mapping, emulsion formation, and so on. In this thesis, we studied the evaporation and dissolution of droplets in ternary systems with the “ouzo effect” through a combined experimental-numerical-theoretical approach.

In Part I, we explored the evaporating and dissolving processes of “ouzo” (water, ethanol, and anise oil) droplets on surfaces. Through a series of studies, we revealed microdroplet nucleation processes triggered by the evaporation or dissolution of the droplets in ternary systems and consequently induced dynamical behaviors of the droplets. The local diffusion rates of different components determines the variation of the local composition and the consequent preference of the microdroplet nucleation: at the contact line of the evaporating flat “ouzo” droplet (§2), at the top of the evaporating spherical “ouzo” droplet (§3), and at the middle of the dissolving spherical “ouzo” droplet (§4). To gain insight into the evaporation process of sessile mixture droplets, we numerically and experimentally performed a systematic investigation in §5, by successively increasing the mixture complexity from pure water over a binary water-ethanol mixture to the ternary “ouzo” mixture. We provided an in-depth discussion on several important factors to the hydrodynamics inside the evaporating droplets, including preferential evaporation, evaporative cooling effect, thermal and solutal Marangoni flows, and their interplays as well. Based on these understandings, we proposed a generalized diffusion model in §3 to predict the evaporative mass loss rate of the evaporating spherical ouzo droplets. To explore the multi-diffusion process at the interface of the dissolving multicomponent droplets, in §4 we proposed a one-dimensional model by integrating multi-diffusion process theory and the liquid-liquid equilibrium theory. Besides, there are man interesting phenomena discovered along with the investigation in Part I, including self-formed oil ring at the droplet contact line (§2), wrapping of nucleated oil (§3), suspending microdroplets rings (§4). In this part, we fully revealed the complicated dynamical behaviors existing in the evaporating and dissolving multicomponent droplets.

In Part II, we performed exploratory research on the application of the evaporating multicomponent droplets. Inspired by the interesting phenomenon observed in §2, we employed the self-formed oil ring in the evaporation-driven particle fabrication to prevent the formation of the pinning contact line in §6. That is the evaporating colloid ouzo droplets acquire “self-lubrication” ability. Through our method, the evaporation-driven particles assembly is practicable by using the commonly used hydrophobic surfaces and the fabrication of high-porosity supraparticles with controllable shapes is feasible by changing the initial ratio of oil to nanoparticles in the colloidal drops. The appearance of self-formed lubricating oil layer conduces to the complete detachment of the generated supraparticles, which allows the repeated utilization of the surfaces. We believe that our approach of particle synthesis with many advantages including, scalability, flexibility, operability, and cost-efficiency, and chemical-consumption-efficiency, will give tremendous contributions to the mass production of the advanced particles.

In the last Part III, our attention focused on the nucleated nanodroplets on the surface induced by the “ouzo effect.” We used the solvent exchange method to form the surface nanodroplets in a narrow channel with controlled flow conditions. In §7, we used this method to form surface nanodroplets in different controlled conditions and statistically analyzed the volume of the nucleated nanodroplets. Based on the idea that the growths of surface nanodroplets result from the exposure of the droplet nuclei to an oil oversaturation pulse during the exchange process, we developed a theoretical framework and revealed a 3/4 power law relationship between the volume of the nanodroplets and the Péclet number of the flow. The convection effects by density difference during the exchange process were also discussed. In §8, we have developed a comprehensive three-dimensional (3D) spherical cap fitting procedure for the accurate extraction of the morphologic characteristics of complete or truncated spherical caps from atomic force microscopy (AFM) images. Our method integrates several advanced digital image analysis techniques to construct a 3D spherical cap model. We expect the developed 3D spherical-cap fitting procedure will find many applications in imaging analysis.