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Non-Equilibrium Interfacial Behavior of Dynamic Interfaces in Presence of Surfactants and Nanoparticles; Experimental and Computational Fluid Dynamic Investigations

Fayzi, Pouyan | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51583 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Bastani, Dariush; Lotfi, Marzieh
  7. Abstract:
  8. The present study has been performed for better understanding about the dynamic behavior of fluid interfaces in presence of surfactants, nanoparticles and their interactions in gas-liquid dispersion systems. One of the main purposes of this work is to investigate the influence of surface modified nanoparticles on the dynamic behavior of gas-liquid interfaces. For this purpose, the rising bubble experiment was used as one of the most conventional procedures. Local velocities of bubbles rising in nanoparticle solutions were determined experimentally. Influences of silica nanoparticles which were modified via three approaches were investigated in these experiments. Heat treatment was applied to dehydrated nanosilica to enhance the hydrophobicity of the silica particle surfaces. Chemical functionalization was done to obtain environmentally responsive silica nanoparticles. Nanosilica particles are coated with hydrophilic and hydrophobic agents. It was found that the addition of nanoparticles to the liquid phase affected the rising bubble hydrodynamics. The local velocities of rising bubbles decreased with the concentration of nanoparticles. For solutions of fumed and dehydrated silica nanoparticles, after initial acceleration the local velocity diminished gradually. The friction drag of the bubbles increased due to the accumulation of nanoparticles at the gas-liquid interface. However, environmentally responsive nanosilica particles behaved like surfactant molecules due to the interfacial activity of hydrophilic and hydrophobic chains. Silica nanoparticles coated with both hydrophilic and hydrophobic agents had a stronger effect on the interfacial properties than those coated only by the hydrophobic agent. Also, local velocities and aspect ratios of rising bubbles were measured to investigate the effects of bubble detachment shape on rising bubble hydrodynamics. Two types of capillary were employed to generate bubbles of identical volume: one glassy nozzle aligned vertically and the other stainless steel needle aligned horizontally. Horizontally injected bubbles had spherical initial shape and their values of aspect ratio slightly fluctuated around unity. However, vertically injected bubbles had surface-stretched initial shape and their values of aspect ratios decreased sharply from 1.1 to 0.65. There is a notable correspondence between the variation of local velocities and aspect ratios which reflects the relevance of the detachment shape of the bubbles to their surface energy. This work also presents the results of numerical simulation of liquid flow on a static bubble. Velocity and surfactant concentration profiles of the liquid around a bubble exposed in flow field were estimated via unsteady CFD solution. Governing equations are the Navier-Stokes and conservation of mass in the liquid bulk and at the interface. Resulted profiles of surfactant concentration and velocity in the liquid bulk are used to determine the dynamic surface tension data via two adsorption isotherms of Langmuir and Frumkin. Data of dynamic surface tension were validated via experimental data of “bubble in the flow field” protocol. Surface concentrations of surfactants were also numerically estimated. At longer flow times, surface concentration increases due to adsorption and accumulation of surfactants at the interface. The rate of mass transfer is increased by the development of velocity profile around the bubble surface. Steady state was reached about 36 sec after flow starts
  9. Keywords:
  10. Surfactants ; Silica Nanoparticles ; Surface Tension ; Local Velocity ; Computational Fluid Dynamics (CFD) ; Aspect Ratio

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