Spreading of aqueous droplets with common and superspreading surfactants. A molecular dynamics study

TL;DR

This molecular dynamics study investigates how surfactant properties influence droplet spreading, revealing that interface adsorption ability is crucial for superspreading, with implications for controlling wetting in various applications.

Contribution

The paper introduces a coarse-grained molecular dynamics model to analyze the mechanisms behind superspreading and compares common and superspreading surfactants, highlighting the role of adsorption at interfaces.

Findings

Adsorption at the contact line is key to superspreading.

Surfactants forming aggregates hinder fast interface replenishment.

Surfactant properties like diffusion are secondary in spreading behavior.

Abstract

The surfactant-driven spreading of droplets is an essential process in many applications ranging from coating flow technology to enhanced oil recovery. Despite the significant advancement in describing spreading processes in surfactant-laden droplets, including the exciting phenomena of superspreading, many features of the underlying mechanisms require further understanding. Here, we have carried out molecular dynamics simulations of a coarse-grained model with a force-field obtained from the statistical associating fluid theory to study droplets laden with common and superspreading surfactants. We have confirmed the important elements of the superspreading mechanism, i.e. the adsorption of surfactant at the contact line and the fast replenishment of surfactant from the bulk. Through a detailed comparison of a range of droplets with different surfactants, our analysis has indicated that the ability of surfactant to adsorb at the interfaces is the key feature of the superspreading mechanism. To this end, surfactants that tend to form aggregates and have a strong hydrophobic attraction in the aggregated cores prevent the fast replenishment of the interfaces, resulting in reduced spreading ability. We also show that various surfactant properties, such as diffusion and architecture, play a secondary role in the spreading process. Moreover, we discuss various drop properties such as the height, contact angle, and surfactant surface concentration, highlighting differences between superspreading and common surfactants. We anticipate that our study will provide further insight for applications requiring the control of wetting.