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Evaporation of simple and complex fluids at different surface wettability
Májek, Juraj ; Ryabov, Artem (advisor) ; Procházka, Marek (referee)
In this work, we investigate the evaporation dynamics of sessile droplets in two scenarios: (i) droplets evaporating on Teflon plasma polymer coated sur- faces, exhibiting anomalously long evaporation lifetime for their size, (ii) drying droplets with liposome suspensions that gradually spread while evaporating. (i) By observing water droplets evaporating on a silicon wafer coated with a Teflon thin film, we discover that the water disintegrates and penetrates the Teflon film. The droplets hide some of their volume underneath, which then does not evaporate. This leads to anomalously long evaporation times. We attribute this effect to structural instability of the Teflon layer, stemming from the contact of non-polar Teflon and polar SiO2. By using Teflon surfaces at most week old, this effect can be prevented. We further identify and describe typical features of the penetration, allowing for its early detection. (ii) We measure the spreading of liposome-laden droplets and find an increase of radius up to 30% of the initial value. During the increase, the droplet contact line spreads in bursts, or protrusions, not uniformly. Existing literature shows that liposomes adsorb on both the air/water and the water/surface interface and reduce the corresponding surface tensions γgl and γsl. We find that when the...
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Fractal growth of polyethylene nanoislands on polyethylene oxide thin films
Májek, Juraj ; Ryabov, Artem (advisor) ; Kylián, Ondřej (referee)
Plasma polymer fragments deposited from vapor on non-wetting polymer substrates are seen to aggregate into fractal nanoislands. Dependent on conditions of the experiment, the islands attain diverse shapes ranging from dendritic snowflakes, branching seaweed to twisting snakes. In our work, we identify dominant kinetic processes responsible for this diversity and relate them to physical characteristics of the experiment. We review and implement basic computer models of deposition and ag- gregation of diffusing particles: The Diffusion-Limited Aggregation (DLA), both on a lattice and without a lattice, and the Cluster-Cluster Aggregation (CCA). The off-lattice DLA yields isotropic random fractals. The lattice DLA fractals are influenced by the properties of the lattice itself, which can be chosen to represent the symmetry of the substrate layer on which the islands grow. Fractals generated in the CCA model are more linear. Com- petition between diffusion and deposition rates gives a transition between off-lattice DLA and CCA fractals. Each of these models comprises a mechanism that we conjecture to be dominant during growth of distinct observed polyethylene nanoislands. Thus the multiple observed fractal shapes allow us to draw conclusions on micro- scopic kinetics of the surface diffusion of deposited...
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