Moving contact lines in dynamic wetting phenomena: Wetting failure, elastocapillarity, and droplet evaporation
2021-05
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Moving contact lines in dynamic wetting phenomena: Wetting failure, elastocapillarity, and droplet evaporation
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2021-05
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Dynamic wetting refers to a system where a fluid displaces another fluid (usually air), while remaining in contact with a solid. This displacement involves a moving fluid-solid-fluid junction, referred to as the dynamic contact line. While dynamic wetting phenomena are encountered in an everyday setting (e.g., rain sliding on car windows, coffee droplets evaporating on the table), the physics of moving contact lines is quite complex as their motion is governed by several physical factors. In this thesis we study four different model problems to advance our fundamental understanding of moving contact lines in dynamic wetting: (i) dynamic wetting failure of non-Newtonian liquids, (ii) droplet spreading on soft solids, (iii) droplet evaporation on inclined substrates and (iv) droplet evaporation on soft solids. The diversity of problems studied in this thesis, emphasizes the importance of moving contact lines in dynamic wetting phenomena. Motivated by the use of non-Newtonian liquids in coating processes, we study the effect of two non-Newtonian rheologies, shear thinning and shear thickening, on the onset of dynamic wetting failure. Flow-visualization experiments using a curtain coating geometry and a hydrodynamic model of liquid displacing air between two parallel plates are developed. We find that shear thinning postpones the onset of wetting failure while shear thickening promotes it. Our results provide insight into how rheological properties of non-Newtonian liquids can be fine-tuned to postpone wetting failure as much as possible. We then investigate the spreading of liquid droplets on soft viscoelastic substrates. A theoretical model is developed to study the effect of various solid properties (e.g., softness, thickness, wettability) on the spreading of perfectly wetting and partially wetting droplets. Our simulations show that softer substrates speed up droplet spreading for perfectly wetting droplets but slow down spreading for partially wetting droplets. Our findings can provide insight into the design of soft substrates for desired applications. The effect of substrate inclination on droplet evaporation is studied in the third problem. We develop a mathematical model based on lubrication theory and investigate pure-solvent and particle-laden droplet evaporation on smooth and rough inclined substrates. We find that on smooth substrates, steeper inclination speeds up evaporation. On rough substrates, the effect of substrate inclination depends on the Bond number Bo, measuring the relative importance of surface-tension forces to gravity forces. At low Bo, a steeper substrate inclination slows down evaporation whereas at high Bo it speeds up evaporation. Additionally, we investigate the effect of substrate inclination and initial particle loading on the final particle deposition patterns. Lastly, we develop a lubrication-theory-based model to study droplet evaporation on soft solid substrates. Our simulations show that on softer substrates droplets exhibit pinning, leading to faster evaporation. Results from our model qualitatively reproduce similar trends observed in experiments. We believe results of this work can provide guidelines toward engineering of soft solid substrates, designed for droplet-evaporation-related applications.
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University of Minnesota Ph.D. dissertation. May 2021. Major: Chemical Engineering. Advisor: Satish Kumar. 1 computer file (PDF); x, 160 pages.
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Charitatos, Vasileios. (2021). Moving contact lines in dynamic wetting phenomena: Wetting failure, elastocapillarity, and droplet evaporation. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/226424.
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