State-and-rate friction in contact-line dynamics.

Area of Science:

• Fluid dynamics
• Surface science
• Tribology

Background:

• Understanding the dynamics of contact lines is crucial for various applications, including coating, printing, and microfluidics.
• The behavior of sessile drops during removal involves complex interactions at the liquid-solid interface.

Purpose of the Study:

• To probe the macroscopic properties and dynamics of contact-line motion during droplet aspiration.
• To identify the forces opposing contact-line movement and their dependencies.

Main Methods:

• Simultaneous measurement of contact angle and drop width during droplet removal from horizontal surfaces.
• Development and application of an overdamped dynamical model for contact-line motion.
• Experimental investigation across multiple liquid-substrate pairs.

Main Results:

• A force opposing contact-line motion was identified, dependent on contact time and withdrawal rate.
• For water on silanized glass, an overdamped dynamical model accurately captured the behavior.
• For other liquid-substrate pairs, maximum static friction was found to be important alongside damping.

Conclusions:

• Contact-line motion is governed by a drag force influenced by contact history and withdrawal speed.
• Rate-and-state friction principles offer a qualitative explanation for observed phenomena across different liquid-substrate combinations.
• The findings contribute to a deeper understanding of fluid-surface interactions and friction dynamics.