Many-body dissipative particle dynamics study of the local slippage over superhydrophobic surfaces

Abstract

The gas–liquid interface (GLI) over superhydrophobic surfaces (SHSs), where the flow slips, is the key to reduce frictional drag in underwater applications. Many-body dissipative particle dynamics simulations are used to explore the slip behavior of a shear flow over a rectangular grooved SHS, and a flat GLI is obtained by tuning the contact angle of the GLI. Due to the slip, the normal profiles of the local velocity, which are perpendicular to the GLI, are curved and shifted away from the linear form near the GLI. Then, a polynomial function is proposed to fit the velocity profile to extract the local shear rate and calculate the slip length. Based on this fitting method, a hybrid slip boundary condition is derived for both longitudinal and transverse flows. That is, the shear stress and slip length are finite near the groove edge, and the stress is nearly zero and the slip length is infinite in the center region of the GLI. This new hybrid slip boundary condition not only explains the inconsistent slip conditions reported in the literature under different groove length scales, but also unifies the existing exclusive slip assumptions.

Poiseuille flow of a Bingham fluid in a channel with a superhydrophobic groovy wall

“…In this context, there are several possible scenarios for the liquid/air interface, summarized as follows: (i) the liquid/air interface is nearly flat and pinned at the edges of the groove (ideal Cassie state) (Hodes et al 2017;Arenas et al 2019;Tomlinson & Papageorgiou 2022); (ii) the interface is deformed towards the groove or towards the main flow while it is pinned at the groove edges (Crowdy 2017b;Game, Hodes & Papageorgiou 2019); (iii) the liquid partially fills the groove and the liquid/air interface is in contact with the side and bottom walls of the groove (and the interface is not anymore pinned to the groove edges) (Giacomello et al 2012;Papadopoulos et al 2013;He et al 2021). The occurrence of these situations is mostly dependent on the flow parameters and the surface properties, including the system pressure, surface tension and the fluid's rheology (Tsai et al 2009;Papadopoulos et al 2013;Annavarapu et al 2019;Rofman et al 2020;He et al 2021).…”

“…In another series of studies, the channel height has been carefully adjusted to maintain a flat liquid/air interface in molecular dynamics simulations (Bao, Priezjev & Hu 2020;Ren et al 2021). Using certain microtrench SH surfaces, the Cassie state has been stabilized for turbulent flows (Xu et al 2020(Xu et al , 2021.…”

Two local slip modes at the liquid–liquid interface over liquid-infused surfaces

“…By adjusting the channel height (H), the shape of the liquid-liquid interface (LLI) is controlled as described in literature. 10 The LLI is intentionally made to be flat in this study. The width w is varied to simulate LISs with different fluid interface fractions.…”

“…Recently, we have derived a hybrid slip boundary condition at a flat GLI over periodically grooved SHSs, based on many-body dissipative particle dynamics (MDPD) simulations. 10 It is the local shear stress and the local slip length that are finite near the groove edge, while the stress is nearly zero and the slip length appears to be infinite in the central region of the GLI. As the MDPD simulation method proved to be useful to simulate multiphase flows, [45][46][47][48][49] it is applied in this study to explore the slip behavior over LISs and to examine whether a hybrid slip boundary condition is more appropriate along the LLI.…”

“…As water is prevented from a direct contact with a solid wall, a gas-liquid interface (GLI) is formed to form an apparent slip, which makes SHSs useful for applications that require drag reduction. [8][9][10][11] Generally, the GLIs might suffer from the lack of stability. 12,13 For instance, by using confocal microscopy, Lv et al reported collapses of symmetric and asymmetric GLIs on a submerged SHS.…”