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Nonequilibrium interfacial diffusivity resolves anomalies in monolayer hydrodynamics.
Tyler J Mucci1, Bruce L Liu1, Joe A Adam1
1Rensselaer Polytechnic Institute, Department of Mechanical, Aerospace and Nuclear Engineering, Troy, New York 12180, USA.
Researchers found that using equilibrium surface diffusivity in non-equilibrium systems causes anomalies in surface dilatational viscosity. Enhanced diffusivity, possibly due to phase domains, explains these issues in surfactant monolayer hydrodynamics.
Area of Science:
- Physical Chemistry
- Fluid Dynamics
- Surface Science
Background:
- Hydrodynamics of surfactant monolayers depend on surface tension, shear viscosity, dilatational viscosity, and diffusivity.
- Surface dilatational viscosity studies show anomalies, including apparent negative values, suggesting a gap in understanding.
- Existing models often assume equilibrium conditions, which may not apply to dynamic interfacial phenomena.
Purpose of the Study:
- To investigate the anomalies observed in surface dilatational viscosity measurements.
- To identify the factors contributing to discrepancies in surfactant monolayer hydrodynamics.
- To determine the role of surface diffusivity in non-equilibrium interfacial flow systems.
Main Methods:
- Employed full-field interfacial velocity measurements to capture dynamic surface behavior.
- Utilized Navier-Stokes equations to model and analyze the interfacial flow fields.
- Compared measurements in non-equilibrium systems with predictions based on equilibrium surface diffusivity.
Main Results:
- Revealed that using equilibrium surface diffusivity for non-equilibrium systems is a source of anomalies.
- Observed surface diffusivity values over seven orders of magnitude larger than equilibrium values in nonsolenoidal flow fields.
- Identified micron-sized phase domains within the monolayer as the probable cause for this enhanced diffusivity.
Conclusions:
- The study highlights the critical importance of considering non-equilibrium effects in interfacial hydrodynamics.
- Apparent anomalies in surface dilatational viscosity can be explained by the significant enhancement of surface diffusivity in dynamic systems.
- Micron-sized phase domains are implicated as a key structural feature driving enhanced diffusivity and influencing monolayer hydrodynamics.

