Surfing droplets on nanoscopic films driven by surface acoustic waves

Area of Science:

• Microfluidics
• Surface Science
• Acoustics

Background:

• Forming micron-sized droplets on open surfaces is difficult, especially for highly spreading liquids.
• Existing microfluidic techniques face challenges with precise droplet generation from thin films.

Purpose of the Study:

• To investigate the formation of microdroplets from thin liquid films using nanoscale acoustic wave excitation.
• To understand the mechanism behind droplet
• surfing
• behavior and predict droplet characteristics.

Main Methods:

• Utilizing high-frequency nanoscale acoustic wave excitation on nanoscopically thick films of low surface tension and low-viscosity liquids.
• Employing theoretical scaling and numerical simulations to analyze droplet formation and dynamics.
• Comparing theoretical predictions with experimental observations.

Main Results:

• Successfully formed microdroplets from thin liquid films via acoustic wave excitation.
• Observed droplets "surfing" on an air layer atop the film, driven by acoustic radiation pressure.
• Demonstrated droplet generation through shear-driven pinch-off from acoustically induced fluid protrusions.
• Validated theoretical predictions for droplet size (d_d) and velocity (U_d) against experimental data.

Conclusions:

• Acoustic wave excitation provides a novel method for generating microdroplets from thin liquid films.
• The observed "surfing" phenomenon is attributed to acoustic radiation pressure acting on the droplets.
• The study offers a theoretical framework for predicting droplet size and velocity in this microfluidic system.