Drag reductions and the air-water interface stability of superhydrophobic surfaces in rectangular channel flow

Flow in a rectangular channel with superhydrophobic (SH) top and bottom walls was investigated experimentally. Different SH surfaces, including hierarchical structured surfaces and surfaces with different micropost sizes (width and spacing) but the same solid fraction, were fabricated and measured. Pressure loss and flow rate in the channel with SH top and bottom walls were measured, with Reynolds number changing from 700 to 4700, and the corresponding friction factor for the SH surface was calculated. The statuses of the air plastron on different SH surfaces were observed during the experiment. In our experiment, compared with the experiment for the smooth surface, drag reductions were observed for all SH surfaces, with the largest drag reduction of 42.2%. It was found that the hierarchy of the microstructure can increase the drag reduction by decreasing the solid fraction and enhancing the stability of the air-water interface. With a fixed solid fraction, the drag reduction decreases as the post size (width and spacing) increases, due to the increasing curvature and instability effects of the air-water interface. A correlation parameter between the contact angle hysteresis, the air-water interface stability, and the drag reduction of the SH surfaces was found.

Figure 1

Scanning electron microscopy (SEM) and atomic force microcopy (AFM) images of the SH surface. (a) SEM image of the primary micropost textures. (b) SEM image of the surface with hierarchical textures (microposts with dispersed nanoparticles). (c,d) are AFM topographic images of the nanoparticles deposited on a flat silicon wafer, with test area of $9\mu {\mathrm{m}}^2$. In (c,d), the surface roughness $R_t$ (maximum peak-to-valley height) is 359 nm.

Figure 2

Solid and air region of the nanoparticles’ surface in the center plane.

Figure 3

A schematic diagram of the channel used in the experiment. The top and bottom walls in the middle part of the channel (shown in red) are designed to be replaceable for different patterned surfaces.

Figure 4

Schematic of the experiment for flow in the channel (measuring zone with the SH surface is marked by a red dash-dotted line box).

Figure 5

The statuses of the air plastron on SH surfaces during the measurement. The air plastron area is detected and calculated by a matlab algorithm, and the corresponding air plastron area proportions of different SH surfaces are shown on the right.

Figure 6

Friction factor $f$ for smooth and SH surfaces.

Figure 7

Schematic geometry of the air-water interface in the channel. The top and bottom of the channel are SH surfaces with regular post patterns (with width $w$, space $s$, and height $h$). The air-water interface is a curve, penetrating into the space of the microtexture, with a maximum value of ${\delta }^{*}$.

Figure 8

Drag reduction vs the contact angle hysteresis (a) and the contact angle (b) of different SH surfaces.

Figure 9

Drag reduction ($D_R$) vs the underwater surface property (USP), which is related to the contact angle, contact angle hysteresis, and the air-water interface stability. The black solid squares represent $D_R$ in laminar flow, while the red circles represent $D_R$ in turbulent flow.