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Numerical and Experimental Study of the Effects of Surfactant on Droplet Motion and Deformation

Salehi, Moloud Sadat | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53721 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Firoozabadi, Bahar; Afshin, Hossein
  7. Abstract:
  8. Surface Active Agents/surfactant have a strong tendency to adsorb at the interface between two immiscible fluids, due to the existence of hydrophobic portions in their molecules’ structure. Accumulation of these molecules on the liquid interface changes the intermolecular forces and causes the interfacial tension to decrease. Production of drops of the same size at a specified rate and controlling their movement’s speed are among the most important factors effective in the efficiency of processes associated with liquid drops, which could profoundly be influenced by the presence of a small amount of a surface active agent. In this study, the effects of surface active agents on the growth, formation, and movement of liquid drops are investigated using experimental and numerical methods.In order to understand the drop behavior in the presence of a surface active agent, firstly, the dynamics of a clean drop (without any impurity and surfactant) should be considered and discussed. Accordingly, the present work is conducted in four sections: experimental investigation of the growth and formation of the Newtonian and shear-thinning liquid drops in the clean state, numerical simulation of the dynamics of a clean drop’s movement, experimental investigation of the influences of a surface active agent on the drop formation process, and studying the movement of liquid drops in the presence of surface active agent using both numerical and experimental techniques.According to the results of the first section, the reduction of the liquid thread length and duration of the necking process are the major effects of a rise in the shear-thinning property of clean liquids’ drops. In the second section, the movement of a clean drop is simulated by using the Lattice Boltzmann Method, developing a code, and making some modifications in the colour gradient model. In contrast with common multiphase models, this model enables us to predict the behavior of a drop in the oscillating regime with reasonable accuracy. Also, it has been shown that the limiting Reynolds number and the final shape of drops in all deformation regimes conform well with our experimental results.In the third section, reduction of the drops’ diameter and increment of the drop length and the probability of satellite drop formation could be observed by increment of the concentration of surface active agent. Based on our findings, Surfactant particles with smaller size and higher diffusion coefficient have more effects on the variations of the dynamics of drop growth.In the final stage of the present study, dynamics of the drop movement in the presence of a surfactant is studied by numerically simulating the problem in OpenFOAM, which is open-source software, and using the existing laboratory instruments. Our results showed that the limiting velocity of a drop containing surfactant is lower compared to that of a clean drop, and as the surfactant concentration rises, this difference increases, and the limiting velocity gets close to that of a rigid sphere. Moreover, if the distribution of surfactant density on the liquid interface is almost uniform, the deformation of the interface does not show any decrease, and the drop behaves like a clean drop with a lower interfacial tension
  9. Keywords:
  10. Image Processing ; Numerical Simulation ; Surfactants ; Droplet Formation ; Shear Thining Fluid ; Drop Movement ; Droplet Dynamics

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