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Surfactant-assisted spreading of a liquid drop on a smooth solid surface
1Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA.
Journal of Colloid and Interface Science
|May 26, 2005
Summary
Insoluble surfactant monolayers enhance liquid drop spreading on solid substrates by reducing contact angles and increasing radial pressure gradients. This study numerically investigates these effects, finding overall enhanced spreading rates for most surfactants.
Area of Science:
- Fluid dynamics
- Surface science
- Materials science
Background:
- Liquid drop spreading is influenced by surface tension gradients (Marangoni stresses) caused by surfactant redistribution.
- Surfactant accumulation at the contact line alters the balance of forces, impacting spreading dynamics.
Purpose of the Study:
- To numerically investigate the axisymmetric spreading of a liquid drop with an insoluble surfactant monolayer on a smooth solid substrate.
- To determine the influence of surfactant properties and Marangoni stresses on the spreading rate.
Main Methods:
- Numerical investigation of coupled evolution equations for drop shape and monolayer concentration.
- Utilized a pseudospectral method for integration.
- Employed two models for the moving contact line and the Frumkin adsorption framework for surface equation of state.
Main Results:
- Insoluble monolayers enhance spreading via reduced equilibrium contact angle and increased radial pressure gradients.
- Surfactant accumulation at the contact line, driven by surface convection, is key.
- Marangoni stresses initially retard spreading but are overcome by favorable mechanisms, leading to overall enhancement.
- Spreading rate increases with bulkier surfactants and stronger repulsive interactions.
Conclusions:
- Insoluble surfactant monolayers generally increase liquid drop spreading rates.
- The primary mechanisms are contact angle reduction and increased radial pressure gradients.
- Simulation results align with experimental observations for small Bond numbers.
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