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KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

MS THESIS DEFENSE BY MOHAMAD NABIZADEH MASHAD TOROGHI

 

Title: Combined Effects of Surfactant and Viscoelasticity on Drop Dynamics

 

Speaker: Mohamad Nabizadeh Mashad Toroghi

 

Time: August 16th, 2018, 10 AM

 

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

 

Thesis Committee Members:

Prof. Dr. Metin Muradoğlu (Advisor, Koç University)

Prof. Dr. Burak Erman (Koç University)

Doç. Dr. Ayşe Gül Güngör (İstanbul Technical University)

 

Abstract:

Combined effects of surfactant and viscoelasticity on droplet dynamics are studied in an  axisymmetric tube using a front-tracking method. Both rising and sedimenting droplet cases are considered. Viscoelasticity is contained in the droplet fluid while surfactant is contained in the continuous phase. The FENE-CR model is used to account for droplet liquid viscoelasticity. The incompressible Navier-Stokes equations are solved fully coupled with the surfactant evolution and the viscoelastic model equations in the entire computational domain. The surface tension is related to the interfacial surfactant concentration by a non-linear equation of state. Extensive simulations are performed to investigate the effects of both viscoelasticity and surfactant on droplet dynamics in nearly spherical, dimpled ellipsoidal cap and skirted regimes. It is found that the surfactant-induced Marangoni stresses generally counteract the viscous shear stresses to reduce mobility of the interface and thus decrease the terminal velocity of the droplet. This effect is most pronounced in the nearly spherical regime in which the interface is almost fully immobilized making the droplet behave like a solid sphere. In the dimpled ellipsoidal cap and skirted regimes, the effects of surfactant diminish significantly since the surfactant adsorbed at the leading edge is quickly convected toward the rear edge rendering the most part of the interface clean. The viscoelastic stresses are found to be nearly uniform in the nearly spherical regime and thus have negligible influence on drop dynamics. However, the viscoelastic stresses concentrate around the rear stagnation point and induce an indentation there especially in the dimpled ellipsoidal cap and skirted regimes. This effect is significantly amplified in the presence of the surfactant leading to creation of a hole at the drop centerline in extreme cases.

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