Non-locality of the contact line in dynamic wetting phenomena

Hypothesis: The notion of the contact line is fundamental to capillary science, where in a large category of wetting phenomena, it was always regarded as a one-dimensional object involving only microscopic length scales. This prevailing opinion had a strong impact and repercussions on the developing theories and methodologies used to interpret experimental data. It is hypothesised that this is not the case under certain conditions leading to non-local effects and requiring the development of a modified force balance at the contact line. Theory and simulations: Using the first principles of molecular dynamic simulations and a unique combination of steady state conditions and observables, the microscopic structure of the contact region and its connections with macroscopic quantities of capillary flows was revealed for the first time. Findings: The contact line is shown to become a non-local, macroscopic object involving rather complex interplay between microscopic distributions of density, velocity and friction force. It was established that the non-locality effects, which cannot be in principle captured by localised methodologies, kick off at a universal tipping point and lead to a modified force balance. The developed framework is applicable to a wide range of capillary flows to identify and analyse this regime in applications. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

The study challenges the traditional view of the contact line as a one-dimensional microscopic object and reveals it as a complex non-local macroscopic object involving intricate interplay between microscopic and macroscopic quantities. By developing a modified force balance approach, it identifies a universal tipping point where non-local effects kick in and lead to a modified force balance at the contact line, applicable to a wide range of capillary flows.