An Investigation of Flow and Interface Dynamics Near a Moving Contact Line at Obtuse Contact Angles

TL;DR

This study combines experimental measurements and numerical simulations to analyze flow and interface behavior near a moving contact line at obtuse angles, validating theoretical models and addressing the contact line singularity.

Contribution

It provides detailed flow field measurements and validates theoretical models for contact line dynamics at angles greater than 90°, linking experimental and numerical approaches.

Findings

Excellent agreement with viscous theory at low Re

Accurate interface shape predictions by DRG model

Deceleration of interfacial speed near contact line

Abstract

The flow near a moving contact line is primarily governed by three key parameters: viscosity ratio, dynamic contact angle, and inertia. While the behavior of dynamic contact angles has been extensively studied in earlier experimental and theoretical works, quantitative characterization of flow configurations remains limited. The present study reports detailed measurements of flow fields, interface shapes, and interfacial speeds in the low to moderate Reynolds number ($Re$) regimes using particle image velocimetry (PIV) and high-resolution image analysis. The investigation is restricted to dynamic contact angles greater than $90^{\circ}$. In the low-$Re$ regime, excellent agreement is observed between measured streamfunction contours and the modified viscous theory of Huh \& Scriven \cite{huh1971hydrodynamic} that accounts for a curved interface. Theoretical models such as the DRG formulation, using a single fitting parameter, accurately predict interface shapes even at finite $Re$. The interfacial speed away from the contact line compares favorably with theoretical predictions, whereas a pronounced deceleration is observed close to the contact line. Complementary Volume-of-Fluid (VoF) based numerical simulations were performed using identical geometric and material parameters to validate and extend the experimental observations. The simulations successfully reproduce the interface topology, flow structure, and the deceleration of the interfacial velocity near the contact line, providing strong support to the experimental findings. We argue that this rapid reduction in speed, observed both in experiments and simulations, is critical to the resolution of the long-standing moving contact line singularity.