Topography- and topology-driven spreading of non-Newtonian power-law liquids on a flat and a spherical substrate

TL;DR

This study theoretically investigates how topography and topology influence the spreading dynamics of non-Newtonian power-law liquids on flat and spherical substrates, revealing that roughness and shape accelerate droplet spreading.

Contribution

It introduces a theoretical model for droplet spreading considering topography and topology effects on non-Newtonian liquids, including new insights into relaxation behaviors and spreading exponents.

Findings

Shear-thickening liquids exhibit larger spreading exponents.

Newtonian liquids relax exponentially faster.

Non-Newtonian shear-thinning liquids relax within finite time.

Abstract

The spreading of a cap-shaped spherical droplet of non-Newtonian power-law liquids on a flat and a spherical rough and textured substrate is theoretically studied in the capillary-controlled spreading regime. A droplet whose scale is much larger than that of the roughness of substrate is considered. The equilibrium contact angle on a rough substrate is modeled by the Wenzel and the Cassie-Baxter model. Only the viscous energy dissipation within the droplet volume is considered, and that within the texture of substrate by imbibition is neglected. Then, the energy balance approach is adopted to derive the evolution equation of the contact angle. When the equilibrium contact angle vanishes, the relaxation of dynamic contact angle $\theta$ of a droplet obeys a power law decay $\theta \sim t^{-\alpha}$ except for the Newtonian and the non-Newtonian shear-thinning liquid of the Wenzel model on a spherical substrate. The spreading exponent $\alpha$ of the non-Newtonian shear-thickening liquid of the Wenzel model on a spherical substrate is larger than others. The relaxation of the Newtonian liquid of the Wenzel model on a spherical substrate is even faster showing the exponential relaxation. The relaxation of the non-Newtonian shear-thinning liquid of Wenzel model on a spherical substrate is fastest and finishes within a finite time. Thus, the topography (roughness) and the topology (flat to spherical) of substrate accelerate the spreading of droplet.