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PhD Defence José Encarnación Escobar | Topological transitions of surface microdroplets

topological transitions of surface microdroplets

José Encarnación Escobar is a PhD student in the research group Physics of Fluids (POF). His supervisor is prof.dr. D. Lohse from the Faculty of Science and Technology.

We humans, characterize for our pursuit of a deep understanding of nature and also for our way of using this knowledge in order to alter our environment (sometimes also to protect and preserve it) in the most convenient way for us. Droplets are present in a great variety of natural processes and also in a wide collection of human applications, and thus they have captured the attention of the scientific community already for very long. In particular, the study of some animals and plants strategies, have shown us how to control the adhesion of liquids like oils and water, and these strategies are now used and adapted to many industrial processes and manufactured tools. These strategies that in animals and plants are a result from the evolution of the species, have led to microscopic and intricate geometries and these materials have been taken as inspiration and reproduced by the scientific community.

In this thesis, we provide a collection of experimental studies of the fundamentals of the droplet behavior on substrates with different geometries and materials. In all of these studies, the droplet’s volume evolves in time by means of natural diffusion and convection and is forced by the properties of the materials in which they sit to adopt changes in topology to minimize their surface free energy. The knowledge acquired through these experiments allows us to understand the behavior of droplets and their contact lines under different conditions imposed by the topology of the substrates.

In particular, we have studied the behavior of drying and dissolving drops on substrates engraved with concentric rings and spiral patterns. These experiments show the special motion of the contact line between the rings and spiraling groves that we name zipping-dewetting, as it opens the gap between the grooves with its advance. This zipping-dewetting is shown to be a succession of stable states controlled by the diffusion process and the geometry of the substrates. Additionally, we have studied the effect of chemical heterogeneities of the substrate with elliptical shape. The experimental results compared to the computational calculations show the strengths and weaknesses of our computational methods to predict the behavior of the droplets on real experimental conditions.

Finally, we studied a very eye-catching effect that an entrapped bubble causes in some dissolving drops. Its heartbeat-like motion and the waves expelled by the system, besides being amusing to see, highly foment mixing and dissolution. This phenomenon that we call puffing, and that only happens for some combination of liquids, is just the previous step for a convective dissolution of the drops driven by Marangoni tensions which dramatically enhance mixing.

The studies in this book offer a step forward in the fundamental understanding of droplets that can open the door to the optimization of processes as, for example, microfluidics mixing components, nano-architecture, advanced printing and 3D-printing technology or droplet-based diagnostics.