Dynamic Wetting by Concentrated Granular Suspensions

TL;DR

This study investigates dynamic wetting of dense granular suspensions by comparing two silica–dispersion systems that exhibit markedly different interparticle interactions. By combining DropSlider-based moving-contact experiments with astigmatism particle tracking velocimetry and comprehensive rheology, the authors map how local flow near the advancing contact line departs from Newtonian predictions as non-Newtonian effects grow. The weakly interacting (TDE) suspension largely follows hydrodynamic expectations at moderate concentrations but deviates at high loading due to frictionally connected microstructures, while the strongly interacting (NaSCN) suspension forms a yield-stress, plug-like flow with a thin yielded boundary layer and a promoted transition to nonflowing bulk. The findings highlight the breakdown of classical Cox–Voinov/Moffatt-type descriptions for dense suspensions and emphasize the need to incorporate particle-scale friction and yield phenomena when modeling dynamic wetting in coatings and printing applications.

Abstract

Many functional materials, such as paints and inks used in applications like coating and 3D printing, are concentrated granular suspensions. In such systems, the contact line dynamics and the internal structure of the suspension interact through shear rate dependent viscosity and microstructural rearrangements. The local shear rate increases sharply near moving contact lines, leading to the non-Newtonian rheology of dense suspensions in this region. While hydrodynamic solutions can describe dilute suspensions, their applicability near advancing contact lines in dense suspensions remains unclear. This study quantifies the deviation from the Newtonian solution by systematically varying interparticle interactions through the choice of dispersion medium. We use silica particles suspended in two refractive index-matched fluids: (i) aqueous 2,2'-thiodiethanol (weak interactions) and (ii) aqueous sodium thiocyanate solution (strong interactions). These systems exhibit substantially different rheological responses, shear-thickening and yield-stress behaviour, respectively. Using astigmatism particle tracking velocimetry (APTV), we resolve the three-dimensional trajectories of tracer particles within a drop driven over a substrate, in an arrangement enabling tracking the internal flows over a long travel distance of the drop. We observe distinct flow behaviours depending on the particle interactions and the resulting suspension rheology. The more the particle interactions play a role, i.e., the more pronounced the non-Newtonian effects are, the stronger the measured flow profiles differ from the Newtonian solution to the hydrodynamic equations. In the shear-thickening suspension, a notable deviation from Newtonian behaviour is observed. Conversely, the yield-stress suspension exhibits plug flow over the substrate, with Newtonian-like behaviour restricted to the yielded region near the substrate.

Figures (25)

• Figure 1: Schematic representation of a droplet's advancing contact line and the lubrication flow near it. The Stokes equation is solved within the lubrication approximation, following the individual works of Cox cox1986dynamics and Voinov voinov1976hydrodynamics.
• Figure 2: Scanning electron microscopy image of spherical silica particles with a diameter of 5µm, used in the preparation of the suspensions.
• Figure 3: Schematic representation of the DropSlider configuration mounted on an inverted microscope for APTV measurements. Two geometries are used: (a) disk geometry for TDE-based suspensions, and (b) prism geometry for NaSCN-based suspensions.
• Figure 4: Schematic representation of APTV setup utilized for three-dimensional tracking of fluorescently labelled tracer particles by integrating a cylindrical lens in front of the camera sensor. The framed region highlights the deformation of particle shapes based on their relative distance to the focal planes.
• Figure 5: Shear rate dependent viscosity of dense granular suspensions composed of 5µm silica particles dispersed in TDE and NaSCN solutions, measured at 31 wt.% and 28 wt.% respectively.