Coatings and surface treatment
structure on a hydrophobic surface, you can make water become really highly repelled by this surface”. This is known as the “lotus leaf effect” because when water comes into contact with a lotus leaf, the droplets roll off, leaving the surface of the leaf dry. Although this “spectacular” effect has been observed and investigated for 20 years, it is very short-lived, Neto says: “As soon as the surface is created, it tends to become very weak if the surface, for example, is exposed to pressure.” This, she says, is because “this super-repellence comes from the surface trapping more pockets of air, and so when you press water against it, the air typically re-dissolves into the water, and you’ve lost the super hydrophobicity”.
So how can scientists retain the hydrophobic effect? The answer, says Neto, is to “use the same type of structure but replace the air pockets with another liquid, a lubricant that is immiscible with the liquid that you’re trying to repel”. Her team tackled the problem by creating a “highly wrinkled nanostructured surface that traps a silicone oil”. (The wrinkled surface was made of Teflon.) “The silicone oil spreads fully on this surface, and fills all the gaps in between the structure, and creates a smooth liquid interface on which water can then flow really easily,” she explains. In effect, the solid wall of the channel is replaced by a liquid wall, thus reducing the hydrodynamic drag.
A puzzling find
The premise of the study – which proved to be correct – was that using the silicone oil would create a slippery surface that reduced frictional drag. In fact, the effect turned out to be 50 times greater than the team had anticipated. These new slippery surfaces reduced the drag by up to 28% – something that would be expected only if the surface was infused with air rather than a viscous lubricant. It was “really puzzling,” says Neto: “The effect that we were seeing could not be justified based on the lubricant presence alone. The lubricants that we were using were more viscous than the water we were flowing on top of it, and so the existing theory did not explain it.” The challenge, says Neto, was to find out what was causing the effect. Using atomic-force microscopy to scan the surfaces underwater, they imaged the spontaneous formation of nanobubbles on the surface. (Nanobubbles are only 100nm – that is, 100 billionths of a metre – high).
It was these nanobubbles that accounted for the scale of the effect. “The surface partly converts back to a super hydrophobic state,” Neto says. It does this by capturing trapped air from the water, thus becoming a “mixed lubricant interface, which
Medical Device Developments /
www.nsmedicaldevices.com Slip and no-slip conditions
When a liquid fl ows through a channel, its velocity at the channel wall is reduced as a consequence of its interaction with the wall, an effect called frictional drag. In macroscopic fl ows, a no-slip boundary condition is usually assumed, for example, the liquid relative velocity is expected to be zero at the wall. However, in the past two decades evidence of nanoscale interfacial slip has emerged in situations when the fl ow is highly confi ned and the wettability of the solid by the liquid is poor. Slip is quantifi ed using the slip length b, the distance beyond the interface at which the liquid velocity linearly extrapolates to zero. The larger the slip length b, the larger the reduction in frictional drag.
Superhydrophobic and lubricant-infused surfaces (LIS) have been shown to reduce drag substantially, which makes them attractive to reduce the energy required to drive fl ow. Drag reduction by these surfaces is explained with the reduced contact area between the solid and the fl uid, which results in an “apparent slip” of the fl owing liquid of viscosity μw over the air (in the case of superhydrophobic surfaces) or over a lubricant of lower viscosity μo in LIS, compared to the case of a solid surface.
Source: Vega-Sánchez, C., Peppou-Chapman, S., Zhu, L. and others. ‘Nanobubbles explain the large slip observed on lubricant-infused surfaces’.
www.nature.com/articles/s41467-022-28016-1
is even more effective at resisting drag”. The research also provides a solution to the problem of fouling. In separate studies, the team investigated the attachment of bacteria and larger organisms on the same surfaces. The surfaces proved “extremely effective at preventing the attachment of both bacteria and larger algae and organisms,” she says. For microfluidic medical devices, the benefits are clear. Traditionally, if a delicate sample was placed in a
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