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Phase dependence of effective surface diffusivity in surfactant monolayers dilatated far from equilibrium
Tyler J Mucci1, Joe A Adam1, Amir H Hirsa1,2
1Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, 12180-3590, New York, USA. hirsaa@rpi.edu.
Soft Matter
|June 19, 2026
Summary
Large effective surface diffusivity accurately models insoluble surfactant monolayer flow, extending beyond coexisting phases to all non-gaseous states. This finding challenges previous assumptions and opens new avenues for interfacial fluid dynamics research.
Area of Science:
- Fluid dynamics
- Surface science
- Materials science
Background:
- Modeling insoluble surfactant monolayer flow during rapid compression/expansion is a hydrodynamics challenge.
- Previous work attributed large effective surface diffusivity to phase coexistence in DPPC monolayers.
Purpose of the Study:
- Investigate the applicability of large effective surface diffusivity beyond coexisting phases.
- Determine effective surface diffusivity and viscosity for DPPC and vitamin K1 monolayers across various concentrations.
Main Methods:
- Used Navier-Stokes bulk flow coupled with Boussinesq-Scriven interface models.
- Incorporated monolayer advection-diffusion dynamics.
- Fitted numerical simulations to spatio-temporal surface velocity measurements.
Main Results:
- Large effective surface diffusivity accurately describes flow in all non-gaseous monolayer phases, not just coexisting ones.
- The model failed for gaseous monolayers, likely due to extreme compressibility.
- Effective surface diffusivity and viscosity were quantified for DPPC and vitamin K1.
Conclusions:
- Large effective surface diffusivity is a broadly applicable concept for non-gaseous surfactant monolayers.
- Extreme compressibility of gaseous monolayers limits the applicability of current models.
- Future research should explore non-Newtonian and non-Fickian interfacial models.
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