Long-range spontaneous droplet self-propulsion on wettability gradient surfaces

Area of Science:

• Microfluidics
• Surface Science
• Nanotechnology

Background:

• Directional and long-range droplet transport is crucial for microfluidic systems but typically requires external energy.
• Existing methods often face limitations in efficiency and control for precise liquid manipulation.

Purpose of the Study:

• To design and demonstrate a wettability gradient surface capable of driving droplet motion via structural topography.
• To elucidate the mechanism behind droplet self-propulsion on engineered surfaces.
• To enable directional and long-range droplet transportation in microfluidic applications.

Main Methods:

• Fabrication of a silicon nanopillar surface with a wettability gradient exceeding 150° (superhydrophobic to hydrophilic).
• Adjustment of hydrophilic silicon dioxide plane area to control surface properties.
• Force analysis to understand droplet self-propulsion mechanisms.
• Theoretical calculations to predict droplet displacement on various gradient surfaces.

Main Results:

• The designed surface achieved a wettability gradient from superhydrophobic to hydrophilic over 150°.
• Nanostructures were identified as critical for generating significant driving force and minimizing resistance.
• Theoretical models accurately predicted droplet self-propulsion displacement.
• Surfaces with arbitrary paths were successfully designed, enabling directional and long-range droplet transport.

Conclusions:

• A novel driving mechanism for droplet self-propulsion on wettability gradient surfaces was clarified.
• The engineered surfaces offer a new paradigm for energy-efficient, directional, and long-range droplet manipulation in microfluidics.
• This research opens avenues for advanced microfluidic device applications requiring precise liquid handling.