Effects of a surfactant monolayer on the measurement of equilibrium interfacial tension of a drop in extensional flow

The effect of surfactant monolayer concentration on the measurement of interfacial surface tension using transient drop deformation methods is studied using the Boundary Integral Method. Emulsion droplets with a surfactant monolayer modeled with the Langmuir equation of state initially in equilibriu...

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Veröffentlicht in:Journal of colloid and interface science Jg. 333; H. 2; S. 570 - 578
Hauptverfasser: González-Mancera, Andrés, Gupta, Vijay Kumar, Jamal, Mustapha, Eggleton, Charles D.
Format: Journal Article
Sprache:Englisch
Veröffentlicht: Amsterdam Elsevier Inc 15.05.2009
Elsevier
Schlagworte:
Chemistry
Colloidal state and disperse state
Computer Simulation
Emulsions - chemistry
Emulsions. Microemulsions. Foams
Exact sciences and technology
General and physical chemistry
Interfacial tension
Marangoni stress
Models, Chemical
Surface physical chemistry
Surface Tension
Surface-Active Agents - chemistry
Surfactant
Tensiometry
Viscosity
Marangoni stress
Tensiometry
Surfactant
Interfacial tension
Viscosity
Monolayer
Deformation
Spherical shape
Retraction
Interface tension
Theory
Equilibrium shape
Relaxation time
Concentration measurement
Emulsion
Packing
Dynamics
Transient response
Droplet
Equilibrium
Steady state
Transient method
Drop
Equations of state
Response time
Distribution
Models
Surface tension
Physicochemical properties
Rheological properties
ISSN:0021-9797, 1095-7103, 1095-7103
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Zusammenfassung:The effect of surfactant monolayer concentration on the measurement of interfacial surface tension using transient drop deformation methods is studied using the Boundary Integral Method. Emulsion droplets with a surfactant monolayer modeled with the Langmuir equation of state initially in equilibrium are suddenly subjected to axisymmetric extensional flows until a steady state deformation is reached. The external flow is then removed and the retraction of the drops to a spherical equilibrium shape in a quiescent state is simulated. The transient response of the drop to the imposed flow is analyzed to obtain a characteristic response time, τ s ∗ . Neglecting the initial and final stages, the retraction process can be closely approximated by an exponential decay with a characteristic time, τ r ∗ . The strength of the external flow on each model drop is increased in order to investigate the coupled effect of deformation and surfactant distribution on the characteristic relaxation time. Different model drops are considered by varying the internal viscosity and the equilibrium surfactant concentrations from a surfactant free state (clean) to high concentrations approaching the maximum packing limit. The characteristic times obtained from the simulated drop dynamics both in extension and retraction are used to determine an apparent surface tension employing linear theory. In extension the apparent surface tension under predicts the prescribed equilibrium surface tension. The error increases monotonically with the equilibrium surfactant concentration and diverges as the maximum packing limit is approached. In retraction the apparent surface tension under predicts the prescribed equilibrium surface tension depends non-monotonically on the equilibrium surfactant concentration. The error is highest for moderate surfactant concentrations and decreases as the maximum packing limit is approached. It was found that the difference between the prescribed surface tension and the apparent surface tension increased as the viscosity ratio decreased. Differences as large as 40% were seen between the prescribed surface tension and the apparent surface tension predicted by the linear theory. Effects of surfactant surface concentration at equilibrium, Γ eq , extensional flow rate, G, and internal viscosity, λμ, on the measurement of interfacial surface tension are investigated through simulation.
Bibliographie:ObjectType-Article-1
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Current address: Departamento de Ingenieria Mecánica, Universidad de los Andes, Bogotá, Colombia
ISSN:0021-9797
1095-7103
1095-7103
DOI:10.1016/j.jcis.2009.02.004