Surfactant dynamics: hidden variables controlling fluid flows

Surfactants – molecules and particles that preferentially adsorb to fluid interfaces – play a ubiquitous role in the fluids of industry, of nature and of life. Since most surfactants cannot be seen directly, their behaviour must be inferred from their impact on observed flows, like the buoyant rise...

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Bibliographic Details
Published in:Journal of fluid mechanics Vol. 892
Main Authors: Manikantan, Harishankar, Squires, Todd M.
Format: Journal Article
Language:English
Published: Cambridge, UK Cambridge University Press 10.06.2020
Subjects:
Adsorption
Aquatic reptiles
Boundary conditions
Bubbles
Capillary waves
Coexistence
Damping
Experiments
Fluid dynamics
Fluid flow
Fluid mechanics
Fluids
Influence
Interfaces
Interfacial stresses
JFM Perspectives
Monomolecular films
Phase transitions
Pressure
Rheological properties
Rheology
Surface chemistry
Surface diffusion
Surfactants
Thermodynamic equilibrium
Thickness
Thin films
Velocity
Wave damping
complex fluids
non-Newtonian flows
interfacial flows (free surface)
ISSN:0022-1120, 1469-7645
Online Access:Get full text
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Summary:Surfactants – molecules and particles that preferentially adsorb to fluid interfaces – play a ubiquitous role in the fluids of industry, of nature and of life. Since most surfactants cannot be seen directly, their behaviour must be inferred from their impact on observed flows, like the buoyant rise of a bubble, or the thickness of a coating film. In so doing, however, a difficulty arises: physically distinct surfactant processes can affect measurable flows in qualitatively identical ways, raising the spectre of confusion or even misinterpretation. This Perspective describes, in one coherent piece, both the equilibrium properties and dynamic processes of surfactants, to better enable the fluid mechanics community to understand, interpret and design surfactant/fluid systems. Specifically, we treat the equilibrium thermodynamics of surfactants at interfaces, including surface pressure, isotherms of soluble and insoluble surfactants and surface dilatational moduli (Gibbs and Marangoni). We describe surfactant dynamics in fluid systems, including surfactant transport and interfacial stress boundary conditions, the competition between surface diffusion, advection and adsorption/desorption, Marangoni stresses and flows and surface-excess rheology. We discuss paradigmatic problems from fluid mechanics that are impacted by surfactants, including translating drops and bubbles, surfactant adsorption to clean and oscillating interfaces; capillary wave damping, thin-film dynamics, foam drainage and the dynamics of particles and probes at surfactant-laden interfaces. Finally, we discuss the additional richness and complexity that frequently arise in ‘real’ surfactants, including phase transitions, phase coexistence and polycrystalline phases within surfactant monolayers, and their impact on non-Newtonian surface rheology.
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current address: Department of Chemical Engineering, University of California, Davis, CA 95616, USA.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2020.170