The Restitution Coefficient of Impacting Low-Viscosity Nanodroplets on Solid Surfaces

Area of Science:

• Fluid dynamics
• Nanotechnology
• Surface science

Background:

• Droplet bouncing relies on energy conversion during impact, spreading, and retraction.
• Nanoscale energy conversion during droplet retraction is poorly understood, limiting nanodroplet dynamics insights.

Purpose of the Study:

• Investigate nanodroplet retraction dynamics and energy conversion on superhydrophobic surfaces.
• Understand scale effects influencing nanodroplet bouncing.

Main Methods:

• Molecular dynamics simulations of low-viscosity nanodroplets impacting superhydrophobic surfaces.
• Analysis of energy conversion mechanisms (surface, kinetic, viscous dissipation) during retraction.
• Extraction of velocity contours to identify dissipation processes (bulk and rim).

Main Results:

• Nanodroplet retraction exhibits an additional crossover stage beyond macroscale inertial and capillary stages.
• In the crossover stage, kinetic and surface energies decrease simultaneously due to scale effects and viscous dissipation.
• Deformation-induced bulk dissipation is continuous, while rim dissipation occurs only in inertial and crossover stages.

Conclusions:

• A theoretical model for the restitution coefficient was developed based on revealed energy conversion mechanisms.
• The model is validated against simulation and existing data across various Weber numbers and liquids.
• Findings offer physical insights into nanodroplet bouncing and a basis for cross-scale modeling.