Schematic diagrams of (a) the setup, (b) the smooth rectangular microchannel, and (c) the microstructured channel, showing the trapped air inside the grooves when water flows through the channel.
Representative SEM images of the used microchannels: (a) a smooth, flat PDMS microchannel of height and of width ; (b) a structured PDMS microchannel possessing longitudinal grooves of height and of width ; (c) a rough, porous PVDF rectangular microchannel of in height, in width, and in length; (d) a patterned PVDF microchannel with grooves of , , and , showing its porosity and additional surface roughness.
Average velocity profile over a smooth rectangular PDMS microchannel [Fig. 2(a)]. The solid line is a quadratic fit, showing a parabolic profile in the laminar flow. The inset shows the detailed velocity profile close to the hydrophobic PDMS surface; the solid line is the best linear fit.
Detailed streamwise velocity profiles at different channel heights when water flows over a micropatterned PDMS surface [as shown in Fig. 2(b)].
measurements of the detailed velocity profile over a microstructured porous PVDF substrate, as shown in Fig. 2(d).
Slip length vs aspect ratio over various hydrophobic microstructured surfaces. Data are obtained by velocity profiles averaged over different areas: (1) the effective slip length over the combination of the air-liquid and liquid-solid interfaces of PDMS (◼) and PVDF (◆) and (2) locally average slip length over the air-liquid interfaces of PDMS (●) and PVDF (▲), with the line showing the best linear fit of for PDMS (●).
(a) Dimensionless slip length vs aspect ratio over various microstructured channels of PDMS [the same data (●) and (◼), as shown in Fig. 6]. The solid line is the theoretical prediction which is described in Eq. (3) (Ref. 13). Here, the upper axis shows the corresponding confinement parameter , suggesting a small influence of the confinement. (b) A sketch of a bending air-liquid meniscus complying with the wetting angle in a microstructured channel section.
A confocal microscope image showing bending liquid-gas menisci for water passing through a micropatterned channel with a combination of the liquid-solid interfaces of the width of and the liquid-gas interfaces of the width of .
measurements of the velocity profile inside PVDF microgrooves below the base line , showing curved menisci toward trapped air. In (a), different symbols indicate the velocity profile at different scanned planes: (∗) at the solid-liquid edge of the microridges, (×), (▲), and (◼). (b) The zoomed-in velocity profile obtained from focused microspheres even deeper inside the same microgrooves as in (a), revealing the longitudinal velocity in the planes of (●) and (◆), and the transverse velocity in the same plane of (▼).
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