Capillary-driven horseshoe vortex forming around a micro-pillar

K Ozawa; H Nakamura; K Shimamura; G F Dietze; H N Yoshikawa; F Zoueshtiagh; K Kurose; L Mu; I Ueno

doi:10.1016/j.jcis.2023.03.039

Capillary-driven horseshoe vortex forming around a micro-pillar

J Colloid Interface Sci. 2023 Jul 15:642:227-234. doi: 10.1016/j.jcis.2023.03.039. Epub 2023 Mar 15.

Authors

K Ozawa¹, H Nakamura¹, K Shimamura¹, G F Dietze², H N Yoshikawa³, F Zoueshtiagh⁴, K Kurose⁵, L Mu⁶, I Ueno⁷

Affiliations

¹ Division of Mechanical Engineering, School of Science and Technology, Tokyo University of Science, 278-8510 Chiba, Japan.
² Université Paris-Saclay, CNRS, FAST, 91405 Orsay, France.
³ Université Côte d'Azur, CNRS, Institut de Physique de Nice, 06100 Nice, France.
⁴ Univ. Lille, CNRS, Centrale Lille, Univ. Polytechnique Hauts-de-France, UMR 8520 - IEMN, F-59000 Lille, France.
⁵ Department of Mechanical Engineering, Faculty of Science and Technology, Tokyo University of Science, Chiba 278-8510, Japan.
⁶ Key laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, 116024 Dalian, China.
⁷ Department of Mechanical Engineering, Faculty of Science and Technology, Tokyo University of Science, Chiba 278-8510, Japan. Electronic address: ich@rs.tus.ac.jp.

PMID: 37004257
DOI: 10.1016/j.jcis.2023.03.039

Abstract

Hypothesis: Horseshoe vortices are known to emerge around large-scale obstacles, such as bridge pillars, due to an inertia-driven adverse pressure gradient forming on the upstream-side of the obstacle. We contend that a similar flow structure can arise in thin-film Stokes flow around micro-obstacles, such as used in textured surfaces to improve wettability. This could be exploited to enhance mixing in microfluidic devices, typically limited to creeping-flow regimes.

Experiments: Numerical simulations based on the Navier-Stokes equations are carried out to elucidate the flow structure associated with the wetting dynamics of a liquid film spreading around a 50 μm diameter micro-pillar. The employed multiphase solver, which is based on the volume of fluid method, accurately reproduces the wetting dynamics observed in current and previous (Mu et al., Langmuir, 2019) experiments.

Findings: The flow structure within the liquid meniscus forming at the foot of the micro-pillar evinces a horseshoe vortex wrapping around the obstacle, notwithstanding that the Reynolds number in our system is extremely low. Here, the adverse pressure gradient driving flow reversal near the bounding wall is caused by capillarity instead of inertia. The horseshoe vortex is entangled with other vortical structures, leading to an intricate flow system with high-potential mixing capabilities.

Keywords: Capillarity; Horseshoe vortex; Meniscus; Micropillar; Wetting.