Droplet Mobility Manipulation on Porous Media Using Backpressure

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Wetting phenomena on hydrophobic surfaces are influenced by gas pockets at the solid-liquid interface.
• Controlling droplet behavior is crucial for microfluidics and surface engineering.

Purpose of the Study:

• To investigate droplet actuation and mobility control on hydrophobic surfaces using backpressure.
• To understand the transition from sticky to slippery states by modulating backpressure.

Main Methods:

• Experimental measurement of sliding angles for deionized (DI) water and ethanol droplets under varying backpressure.
• 2D computational simulations using momentum conservation, continuity, and Cahn-Hilliard phase-field equations.
• Analytical calculations of contact line depinning based on force balance.

Main Results:

• Backpressure reversibly controls droplet mobility, transitioning them from sticky to slippery states.
• A 50 μL water droplet's sliding angle reduces from 45° to 0° at ~0.60 bar backpressure.
• Lower backpressure is needed for liquids with lower surface energy.

Conclusions:

• Droplet actuation is achieved by depinning the receding contact line and forward wave propagation.
• Backpressure is an effective mechanism for manipulating sessile droplet mobility on hydrophobic surfaces.
• Simulations and analytical models corroborate the experimental findings on contact line depinning.