Instability development of a viscous liquid drop impacting a smooth substrate.

Area of Science:

• Fluid dynamics
• Surface physics
• Rheology

Background:

• Liquid drop impact dynamics are crucial in processes like printing and coating.
• Understanding instability onset is key to controlling these phenomena.
• Previous studies identified inviscid and viscous regimes but lacked a unified model.

Purpose of the Study:

• To investigate the instability development in viscous liquid drops impacting a smooth substrate.
• To determine the influence of surrounding air pressure and liquid viscosity on instability onset time.
• To develop a model explaining the balance of stresses at instability onset.

Main Methods:

• High-speed photography was employed to capture real-time dynamics.
• Experimental measurements of instability onset time were correlated with air pressure and liquid viscosity.
• A quantitative model was constructed comparing destabilizing and stabilizing stresses.

Main Results:

• Instability onset time decreases with air pressure (to the power of -2) and increases linearly with liquid viscosity.
• Experimental data supports a model where destabilizing air stress and stabilizing viscous stress balance at onset.
• The model successfully differentiates between inviscid (surface tension stabilized) and viscous (viscosity stabilized) regimes.

Conclusions:

• A unified stress-balance model explains instability in viscous liquid drop impacts.
• Air pressure and liquid viscosity are critical parameters controlling instability onset.
• The findings provide insights into controlling drop impact dynamics in various applications.