Viscoelastic Surfactant/Fatty Acid Interfaces: Fluid Dynamics, Rheology, and Structure

Restricted (Penn State Only)
Niroobakhsh, Zahra
Graduate Program:
Materials Science and Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
July 31, 2017
Committee Members:
  • Andrew Leonard Belmonte, Dissertation Advisor
  • Ralph H Colby, Committee Chair
  • Robert John Hickey III, Committee Member
  • James Patrick Runt, Committee Member
  • Russell Taylor Johns, Outside Member
  • Catanionic surfactant systems
  • Interfacial materials
  • Interfacial rheology
  • SAXS
  • Viscoelastic
  • Non-Newtonian Fluid Dynamics
  • microstructure
  • Ternary phase diagram
  • Lamellar
  • Fatty acid
  • Cationic surfactant
  • Microemulsion
The present dissertation concerns an interdisciplinary subject focusing on both viscoelastic material characterizations and fluid dynamics experimental development for an immiscible amphiphilic system. In particular, a self-assembling material formed at the interface between an aqueous cationic surfactant (cetylpyridinium chloride, CPCl) solution and a fatty acid such as oleic acid (OA) is studied. The resulting interfacial layer forms owing to instantaneous self-organization of CPCl and OA molecules, which is similar to established surfactant systems including microemulsions and catanionic surfactants. First, a ternary phase diagram for CPCl, OA and water was established, to study the phase behavior of equilibrium bulk phases. The morphology and structure of bulk phases were characterized using the Small-Angle X-ray Scattering (SAXS) and oscillatory rheology. In the ternary diagram, we observed the formation of a gel phase at the equimolar mixing ratio of OA and CPCl. To characterize the interfacial layer made between CPCl solutions and OA, we reproduced the interfacial materials in small capillaries and employed SAXS. Both bulk gel and the interfacial material exhibit a domain spacing on the order of 20 nm as a lamellar phase. Interfacial rheology is applied to understand viscoelastic properties of the interfacial layer made between CPCl solution and a thin layer of OA. Both bulk rheology of the gel phase as well as the interfacial rheology of interfacial layer demonstrated a viscoelastic gel behavior (elastic modulus larger than loss modulus, and both moduli independent of frequency). Furthermore, a detailed comparison of bulk and interfacial materials characteristics are discussed. In the fluid dynamics part, two flow geometries are used to examine flow instabilities during interfacial material formation. In the first flow configuration, injection of an aqueous CPCl solution into an immiscible OA results in formation of various geometry including a columnar geometry due to development of an interfacial layer. The interfacial layer or the column wall exhibits peculiar elastic behavior such as wrinkling, buckling, or rupture. The physical properties of the interfacial material such as velocity of the interfacial layer are measured using visualization techniques (utilizing Phanton V5 high speed video camera). The second flow geometry is a quasi-2D Hele-Shaw cell, which is a classic method to study dynamics of an interface in a controlled and reproducible way. We observed that the interface dynamics and strength of self-assembled material are strongly dependent on surfactant concentration and flow conditions. The effect of oil molecular structures has been studied using various oils, including inert oils, other fatty acids, and triglycerides of oleic acid (triolein). We eventually discuss possibility of a connection between flow instabilities, phase behavior and interfacial material characteristics. In our system, both components are known to have antimicrobial properties which made them suitable for many pharmaceutical and medical applications. The results offer a good potential for generalization to a wide variety of complex fluid systems including surfactant mixtures, polymer solutions, emulsions, and colloidal domains, where the physical properties of the interfacial materials can be tuned for variety of applications such as in chemical, food, and oil industries.