Droplet Impact on Surfaces with Asymmetric Microscopic Features

Area of Science:

• Fluid dynamics
• Surface science
• Materials science

Background:

• Liquid drop impact is crucial for industrial processes like cleaning, cooling, and coating.
• The influence of surface topography (pores, roughness, texture) on droplet impact dynamics remains poorly understood.

Purpose of the Study:

• To experimentally investigate the initial stages of droplet impact on surfaces with asymmetric (tilted) ridges.
• To determine how surface texture influences droplet spreading asymmetry at low impact velocities.

Main Methods:

• Experimental study of low-velocity droplet impacts on surfaces with tilted ridges.
• Analysis of the role of impact velocity, surface tension, and line friction.
• Definition and application of the line-friction capillary number (Ca_f) to quantify texture influence.

Main Results:

• Spreading asymmetry is driven by the interplay between impact velocity and capillary speed, which depends on line friction.
• At low line-friction capillary numbers (Ca_f ≪ 1), impact is asymmetric: friction governs spreading against ridge inclination, while corners pin the contact line with inclination.
• At high line-friction capillary numbers (Ca_f ≫ 1), surface geometric details have minimal impact on dynamics.

Conclusions:

• The line-friction capillary number (Ca_f) effectively characterizes the influence of surface topology on droplet impact.
• Surface texture significantly impacts droplet spreading asymmetry, particularly under conditions of low line friction.
• Understanding these dynamics is key for optimizing surface designs in various applications.