Dynamics of an impacting emulsion droplet

Area of Science:

• Fluid dynamics
• Surface science
• Materials science

Background:

• Emulsions, particularly oil-based pesticides, are crucial in agriculture but pose environmental risks due to droplet bouncing off hydrophobic plant surfaces.
• Bouncing droplets lead to reduced pesticide efficacy and widespread environmental contamination of soil and water sources.

Purpose of the Study:

• To investigate the droplet impact dynamics of oil-in-water emulsions on hydrophobic surfaces.
• To elucidate the underlying physics governing the transition from bouncing to sticking and back to bouncing.
• To develop strategies for controlling droplet adhesion and surface coverage.

Main Methods:

• Experimental analysis of droplet impact dynamics at varying speeds.
• Investigation of the role of in situ surface lubrication and suction forces.
• Characterization of key time scales: droplet contact, oil impregnation, and oil ridge formation.

Main Results:

• Observed an unexpected transition from bouncing to sticking, followed by bouncing again, as droplet impact velocity increased.
• Identified self-generated surface lubrication and suction forces from nascent oil layers as the governing physical mechanism.
• Demonstrated that controlling droplet behavior relies on balancing three critical time scales.

Conclusions:

• The study reveals a novel adhesion mechanism for emulsion droplets on hydrophobic surfaces.
• Precise control over droplet bouncing and surface oil coverage is achievable by manipulating time scales.
• Findings offer significant potential for optimizing applications in agriculture, cooling sprays, combustion, and additive manufacturing.