Drop Retention and Departure in Adiabatic Shear Flow on Structured Superhydrophobic Surfaces

Area of Science:

• Surface science
• Fluid dynamics
• Materials science

Background:

• Droplet retention on surfaces is governed by surface tension, drop geometry, and contact angle.
• External forces exceeding retention force initiate droplet movement.
• Understanding droplet departure is crucial for applications involving fluid manipulation on surfaces.

Purpose of the Study:

• To investigate droplet departure conditions on structured superhydrophobic surfaces under applied shear flow.
• To analyze the influence of surface microstructure and solid fraction on droplet mobility.
• To determine the relationship between surface properties and the critical shear velocity for droplet removal.

Main Methods:

• Experiments were conducted on five microstructured superhydrophobic surfaces, one nanostructured carbon nanotube surface, and one smooth hydrophobic surface.
• Droplet volumes (5-50 μL) were subjected to increasing air velocities until departure.
• High-speed imaging tracked droplet dimensions and contact angles; air velocity at departure was measured.

Main Results:

• Droplet departure velocity is strongly dependent on surface solid fraction, decreasing as solid fraction decreases.
• Contact angle hysteresis increases prior to departure and serves as an indicator of droplet mobility.
• A calculated coefficient of drag for departing droplets decreases with increasing Reynolds number.

Conclusions:

• Structured superhydrophobic surfaces can significantly reduce the shear flow required for droplet departure.
• Lower surface solid fractions enhance droplet mobility, making them more susceptible to removal by shear flow.
• Contact angle hysteresis and surface solid fraction are key parameters for predicting droplet behavior on superhydrophobic surfaces.