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Controlled spreading of complex droplets

University of British Columbia

The current thesis investigates the controlled spreading of droplets of complex fluids. This thesis makes four primary scientific contributions. Firstly, we provide detailed theoretical analysis on spreading of yield stress fluids. We employ lubrication theory, asymptotic solutions, and numerical simulations to explain the dynamics and final static shape of a viscoplastic droplet on a solid horizontal surface. We show that the final radius of the droplet becomes smaller with increasing the yield stress. Secondly, we provide experimental data to verify our theoretical solutions. In our experiments, we first provide a method to eliminate the apparent slip of the yield stress fluid. The method uses a chemical modification of glass surfaces to generate permanent positive charges, resulting in a no-slip boundary condition. We directly observe the slip and no-slip of the Carbopol droplets, using a visualization method based on confocal microscopy. We then perform shadowgraphy experiments to measure the final radius of the droplets under different conditions such as extruding and impacting droplets. We compare the theoretical and experimental results and discuss the similarities and differences. Briefly, the asymptotic solutions overestimates the experimental results (most likely due to the assumption of a shallow layer), while numerical solutions are much closer to the experimental outcomes. Thirdly, we provide a comprehensive rheological characterization of a particular thermo-responsive fluid, Pluronic F127. We show that the aqueous solution of the polymer undergoes a sol(Newtonian)-gel(yield stress) transition upon heating. We further characterize the properties of the gel in detail. Finally, we show one can thermally trigger a thermo-responsive droplet to externally control the final shape of the droplet on a surface. In short, the final radius of the droplet can be controlled by heating the surface; for a given concentration, the larger the surface temperature, the smaller the final shape of a droplet. In the same part of the thesis, we introduce a novel experimental method based on optical coherence tomography to identify the solidified region inside a droplet.

Text

2017-01-21

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/60120

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/