Binary Water Droplet Impact Dynamics on Flat and Micropillared Hydrophobic Substrates

Area of Science:

• Fluid Dynamics
• Surface Science
• Materials Science

Background:

• Understanding droplet impact dynamics is crucial for various applications.
• Surface wettability and morphology significantly influence droplet behavior.
• The Cassie-Baxter and Wenzel states describe droplet interactions with textured surfaces.

Purpose of the Study:

• To experimentally investigate binary droplet impact dynamics on flat and micropillared hydrophobic surfaces.
• To analyze the influence of surface wettability and impact velocity (Weber number) on droplet interaction regimes.
• To develop energy-based models and empirical correlations for droplet spreading and height.

Main Methods:

• Experimental investigation of binary droplet impact on polydimethylsiloxane (PDMS) substrates with varying solid fractions.
• Utilizing high-speed visualization to capture transient droplet evolution during vertical head-on collisions.
• Employing energy-based analysis to model droplet spreading ratio and axial height.

Main Results:

• Micropillared substrates increase droplet lift-off probability compared to flat surfaces.
• Observed interaction regimes include no lift-off, lift-off, partial lift-off, direct coalescence, and conglutination with breakup.
• In the Cassie-Baxter state, maximum droplet height is governed by Weber number, not solid fraction.

Conclusions:

• Surface morphology and impact conditions dictate binary droplet interaction outcomes.
• Simplified empirical correlations were proposed for droplet spreading and height prediction.
• Findings can guide surface design for applications in spray cooling, self-cleaning, and energy harvesting.