Giant drag reduction in complex fluid drops on rough hydrophobic surfaces

Area of Science:

• Fluid dynamics
• Rheology
• Surface science

Background:

• Drop impact dynamics are crucial in various industrial processes.
• Yield-stress fluids exhibit complex behavior distinct from Newtonian fluids.
• Surface properties, like roughness and hydrophobicity, significantly influence fluid interactions.

Purpose of the Study:

• To investigate the drop impact of yield-stress fluids on rough hydrophobic surfaces.
• To characterize a novel spreading regime under specific impact conditions.
• To understand the underlying mechanisms of enhanced spreading and drag reduction.

Main Methods:

• Utilized model yield-stress fluids (Carbopol microgel solutions).
• Performed drop impact experiments on surfaces with varying degrees of roughness and hydrophobicity.
• Analyzed drop spreading dynamics, focusing on maximal inertial spreading diameter.
• Employed an energy balance model and dynamic slip length analysis for interpretation.

Main Results:

• Observed a new spreading regime for yield-stress fluid drops on rough hydrophobic surfaces.
• Maximal spreading diameter was up to twice larger on rough surfaces compared to smooth surfaces at high impact velocities.
• Demonstrated apparent basal friction reductions exceeding 80% on rough surfaces.
• Correlated enhanced spreading with a dynamic slip length dependent on surface roughness and drop dynamics.

Conclusions:

• Rough hydrophobic surfaces can dramatically enhance the spreading of yield-stress fluid drops.
• Significant drag reduction is achieved through increased surface roughness, contrary to expectations for Newtonian fluids.
• The findings provide a new perspective on fluid behavior at interfaces and have implications for designing surface-fluid interactions.