Slip theory-based numerical investigation of the fluid transport behavior on a surface with a biomimetic microstructure

Abstract

This paper presented a numerical simulation of drag reduction on a superhydrophobic surface with a groove structure. The computational fluid dynamics (CFD) method was used to analyze the effect of the groove microstructure when droplets impacted a superhydrophobic surface. The simulation results revealed three main characteristics: (1) The distance the droplet spread was larger along the direction parallel to the groove and smaller perpendicular to the groove; (2) Two protruding small spheres were formed at the edge of the droplet along the groove direction; (3) During retraction, the droplets presented a narrow, cross-shaped morphology. The effect of the groove structure in the two-dimensional microchannel on drag reduction near the wall was analyzed based on slip theory by the coupled level set/volume of fluid (CLSVOF) method. The air in the superhydrophobic pit formed a low-velocity vortex, which made the fluid roll on the air surface. The rolling on the surface produced a velocity slip at the gas-liquid interface. In addition, the superhydrophobic surface had an obvious drag reduction effect in the laminar flow state, but the drag reduction effect in the turbulent state was not ideal and even increased the flow resistance at the wall.

Rheological Properties and Drag Reduction Performance of Puffer Epidermal Mucus

“…Mucus is a gel and forms a two-phase flow with water, so the volume-of-fluid (VOF) model was chosen to study the two-phase flow. The VOF model was used to determine the position of the interface by solving the change in the volume fraction of each phase in the mixed fluid, thereby tracking the mucus–water interface. , In multiphase flow, the volume fraction of a certain fluid (α k ) is defined as: , α k = 0, there is no such fluid in the cell mesh; 0 < α k < 1, multiphase fluids coexist in the cell mesh; and α k = 1, there is only this fluid in the cell mesh. The boundary conditions are set, and the volume fraction of mucus in the set area is α k = 1, that is, the purple area in Figure a is all mucus, which is regarded as epidermal mucosa.…”

Quasi-Analytical Solution of Optimum and Maximum Depth of Transverse V-Groove for Drag Reduction at Different Reynolds Numbers

“…where 𝐾 = √ and 𝐾 = √ . Equation (25) shows that 𝜂 is related to the slip velocity and the depth of the groove. By adjusting the appropriate 𝐾 and 𝐾 values, the 𝜂 predicted by Equation ( 25) can be consistent with the numerical results.…”

“…By adjusting the appropriate K 1 and K 2 values, the η p predicted by Equation ( 25) can be consistent with the numerical results. Figure 19 shows the prediction of Equation (25) with different K 1 and K 2 . Taking the fitting degree R 2 as the objective function, K 1 and K 2 -which have the largest fitting degree-are selected as the model parameters, where K 1 = 6.93 × 10 −6 h −1.428 and K 2 = 0.00955h −0.31646 .…”

“…Step 2: Input U + s and U + ∞ into implicit Equation (18) to solve η ν , and input U + s and H + into Equation (25) to get η p .…”

“…Some studies suggested that the vortices formed within the grooves weaken the turbulence structure in the boundary layer near the wall [19,20]. The more popular perspective indicated that the vortices within the transverse grooves change the sliding friction into rolling friction at the solid-liquid interface, which is also named the "micro air-bearing phenomenon" [21][22][23][24][25]. Several studies focused on the Cassie-Baxter state to reduce the solidliquid contact area and create a superhydrophobic effect over the transverse grooves [26][27][28].…”