Universal contact-line dynamics at the nanoscale.

Area of Science:

• Surface science
• Nanotechnology
• Fluid dynamics

Background:

• Understanding nanoscale liquid behavior is crucial for applications in microfluidics and materials science.
• The dynamics of liquid-air interfaces and contact lines at the nanoscale are complex and not fully understood.
• Non-equilibrium conditions can significantly influence liquid flow and interface evolution.

Purpose of the Study:

• To investigate the relaxation dynamics of the contact angle for viscous liquids on smooth substrates at the nanoscale.
• To simultaneously monitor the liquid-air interface evolution and contact line movement.
• To identify and characterize the different regimes governing this relaxation process.

Main Methods:

• Atomic force microscopy (AFM) was employed to measure polystyrene nanostripes.
• Simultaneous monitoring of the temporal evolution of the liquid-air interface and contact line position.
• Comparison of experimental results with numerical calculations from a lubrication model.

Main Results:

• Three successive dynamic regimes were identified: stationary contact line leveling, receding contact line dewetting, and front collapse.
• Self-similar evolution of the liquid interface was observed in the initial leveling regime, matching lubrication model predictions.
• A universal critical contact angle and dimensionless time for the transition to dewetting were identified across varying viscosities and film thicknesses.

Conclusions:

• The study elucidates the fundamental nanoscale relaxation dynamics of viscous liquids on substrates.
• The observed self-similar behavior and universal dewetting transition provide key insights into nanoscale fluid mechanics.
• These findings have implications for controlling liquid behavior in nanotechnological applications.