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Why do aqueous surfactant solutions spread over hydrophobic substrates?
Victor Starov1, Natalia Ivanova, Ramon G Rubio
1Department of Chemical Engineering, Loughborough University, Loughborough, LE11 3TU, UK. V.M.Starov@lboro.ac.uk
Advances in Colloid and Interface Science
|May 14, 2010
Summary
Surfactant concentration dictates droplet spreading on hydrophobic surfaces. Below a critical level, spreading is slow. Above it, a fast initial stage driven by liquid-vapor adsorption is followed by a slower stage of substrate adsorption.
Area of Science:
- Physical Chemistry
- Surface Science
- Fluid Dynamics
Background:
- Droplet spreading on hydrophobic substrates is crucial in various applications.
- The role of surfactant concentration in controlling spreading kinetics is not fully understood.
Purpose of the Study:
- To investigate the influence of surfactant concentration on the stages of aqueous droplet spreading over hydrophobic surfaces.
- To elucidate the underlying mechanisms driving the observed spreading behaviors.
Main Methods:
- Experimental observation of droplet spreading dynamics.
- Analysis of spreading kinetics at different surfactant concentrations.
- Theoretical interpretation based on interfacial phenomena and molecular adsorption.
Main Results:
- Spreading occurs in one slow stage below a critical surfactant concentration.
- Above the critical concentration, spreading proceeds in two stages: a fast initial stage and a subsequent slow stage.
- The slow stage, in both cases, is governed by surfactant transfer to the bare substrate (autophilic phenomenon).
- The fast stage, observed at higher concentrations, is driven by surfactant adsorption at the liquid-vapor interface.
Conclusions:
- Surfactant adsorption on the substrate in front of the contact line is a spontaneous process that drives slow spreading.
- The transition from one-stage to two-stage spreading is linked to the surfactant concentration relative to a critical value.
- Adsorption at the liquid-vapor interface is identified as the key mechanism for the initial fast spreading stage at concentrations above the critical value.
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