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.
Keywords: Contact line; Dynamic wetting; Meso-scale; Molecular dynamics simulations; Nano-scale.
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