Droplet entrapment on inclined substrates by enforcement of pearl formation

This study presents droplet dynamics in a rough capillary channel. Prior studies investigating the effect of roughness on fluid flow have mainly considered a continuous phase whose behavior is different from a discontinuous phase, i.e., an oil slug. To explore the dynamic behavior of droplet motion across a rough channel, a direct numerical simulation of in a three-dimensional channel is performed. Three models have been considered: model A had a rough surface only on the bottom walls, model B on both the bottom and top walls, and model C on all walls. The results show that in contrast with common observations, roughness promotes droplet mobility in comparison with smooth walls. The presence of roughness results to an additional energy required to move the droplet, and the degree of confinement increases with the roughness; thus, the difficult of mobilization increases with the increase in roughness. Different roughness parameter effects have been investigated. The results have shown that the critical pressure increases with the increase in the pillar's height and decreases with the pillars spacing. The offset leads to a decrease in flow resistance for larger contact angles. We noted also that it is more difficult to mobilize a discontinuous phase in a neutral-wet surface condition. Furthermore, discontinuous pillars in the lateral direction led to much higher resistance. Through our comprehensive numerical study, we provide valuable insights into the impact of roughness in capillary channels. These findings can be used as guidelines for designing droplet flow on complex and rough surfaces, such as microfluidic devices, and hold significant relevance in the optimization of droplet control strategies in enhanced oil recovery methods.

Effect of roughness on droplet motion in a capillary channel: A numerical study

Imani,

Zhang,

Maweja

et al. 2023

Sliding motion of highly deformed droplets on smooth and rough surfaces: Shape oscillation, chaotic breakage, corner shape, and pearling

Tsao,

Liao,

Tsao

2023

The sliding behavior of droplets on smooth and rough surfaces with various surface wettabilities is investigated by many-body dissipative particle dynamics simulations. On a smooth surface, as the driving force (Bo) increases, the droplet shape and velocity (Cac) before breakage can be classified into four distinct regimes: (I) nearly spherical cap with Cac∝Bo; (II) oval shape with negative deviation from the linear relation; (III) elongated shape without a neck, where Cac decreases with increasing Bo; and (IV) oscillation of an elongated shape with fluctuating sliding velocity. On rough surfaces, corner-shaped droplets, which are absent on a smooth surface, can be observed. A further increase in Bo leads to the formation of cusp and pearling. Different from pinching-off on rough surfaces, which produces a cascade of smaller droplets through groove-induced shedding, chaotic breakage of a droplet on a smooth surface is caused by an unsteady flow field. Finally, a universal linear relationship between the sliding velocity based on the surface velocity (Cas) and the modified driving force (Bo**) is derived to take into account the effects of surface wettability and roughness.

Successive rebounds of obliquely impinging water droplets on nanostructured superhydrophobic surfaces

Jia,

Zhang,

Wang

et al. 2024

The impact and rebound of water droplets on superhydrophobic surfaces frequently happen in nature and also in a number of industrial processes, which has thus stimulated strenuous efforts to explore the underlying hydrodynamics. Despite that massive achievements have been made over the past decades, existing works are mostly focusing on the short-time bouncing dynamics after a single impact; however, the long-term, successive droplet rebounds, which are practically more important, only received very limited attention. In this work, we perform an experimental investigation on the impact of water droplets on inclined nanostructured superhydrophobic surfaces at low Weber numbers, where massive complete rebounds arise. It was found that an obliquely impinging droplet would undergo many impacts on the superhydrophobic surface, accompanying with sliding on the surface, jumping in air, and complex shape evolutions. Based on the kinematic analyses, we demonstrate that the droplet motion on the surface can be decomposed into a perpendicular impact, which is dominated by the capillary and inertial forces, and a translational motion under the drive of gravity. By contrast, the jumping motion after droplet rebound is solely governed by the gravitational force, yet relevant droplet characteristics are affected by the energy loss during the impact on superhydrophobic surface, which sets the maximum height that the droplet rebounds to. In addition, three distinct shape evolution modes–namely, oscillation, rotation and their combination–were identified on jumping droplets, and the direction of a rotational droplet can be altered via the following impingement on the superhydrophobic surface.