Moving contact lines and Langevin formalism

Area of Science:

• Physics
• Materials Science
• Surface Science

Background:

• Previous molecular dynamics (MD) simulations modeled equilibrium contact line thermal oscillations as a 1-D Langevin harmonic oscillator.
• This model successfully determined contact line friction (ζ) and predicted wetting dynamics.

Purpose of the Study:

• To investigate if the Langevin harmonic oscillator model can be applied to moving contact lines.
• To analyze the fluctuations of dynamic contact angles and positions for advancing and receding contact lines.

Main Methods:

• Utilized an MD system of a liquid bridge between two solid plates.
• Moved plates at a steady velocity (U_plate) to create advancing/receding contact lines.
• Recorded and analyzed contact line position and dynamic contact angle fluctuations.

Main Results:

• Confirmed that moving contact line fluctuations fit a 1-D harmonic oscillator model.
• Derived a Langevin expression for non-equilibrium conditions, with the harmonic term centered on the dynamic contact line position (x^d).
• Established relationships between fluctuation variance, time decay, and friction (ζ), independent of U_plate.
• Demonstrated a new relationship between the distance from equilibrium (x^d-x⁰) and the out-of-equilibrium capillary force.
• Obtained ζ values consistent with simulations of spreading drops, indicating a universal dissipation mechanism.

Conclusions:

• The Langevin harmonic oscillator model is applicable to moving contact lines.
• Contact line friction (ζ) can be determined from fluctuation analysis, regardless of plate velocity.
• The findings support a common dissipation mechanism across different wetting scenarios.