Drop friction on liquid-infused materials

Abstract

We discuss in this paper the nature of the friction generated as a drop glides on a textured material infused by another liquid. Different regimes are found, depending on the viscosities of both liquids. While a viscous drop simply obeys a Stokes-type friction, the force opposing a drop moving on a viscous substrate becomes non-linear in velocity. A liquid on an infused material is surrounded by a meniscus, and this specific feature is proposed to be responsible for the special frictions observed on both adhesive and non-adhesive substrates.

Hydration lubrication of polyzwitterionic brushes leads to nearly friction- and adhesion-free droplet motion

“…For example, for polyzwitterionic brush surfaces F fric, adh varies nonlinearly with U (∝ U 2/3 and U 0.6 , respectively), whereas for lotus-leaf effect surfaces F fric, adh is relatively insensitive to U ∼ mm/s or less [20,22,34,39]. Table I summarizes the functional forms of F fric, adh for three surface classes, as reported here and elsewhere in the literature [17,20,23,33,34]. The values of |F fric, adh | are for droplets of size 2R = 1-4 mm and speeds U = 0.01-2 mm/s, and as discussed previously, |F fric, adh | is much lower for polyzwitterionic brushes as compared to lotus-leaf effect and lubricated surfaces.…”

“…In summary, we have clarified the origin of underwater oil-repellency for polyzwitterionic brush surfaces, in particular the role played by hydration lubrication. We were able to measure the thickness of the water film beneath the oil droplet with nanometric accuracy, as well [17,20,23,33,34]. φ is the solid surface fraction.…”

Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces

“…Finally, a suitable lubricant oil can be added to the surface, which can be structured as is the case for slippery liquid-infused porous surfaces [9,10] or flat as is the case of lubricant-infused organogels [11,12] [ Fig. 1(c)]; any liquid can then easily be removed, as long as there is a stable intercalated lubricant layer [13][14][15][16].…”

Guided droplet transport on synthetic slippery surfaces inspired by a pitcher plant

“…Lubricated topographies, however, would be able to overcome such shortcomings thanks to the pinning introduced by the feature and the droplet's tendency to adhere to the lubricating liquid (whilst being completely mobile). Furthermore, directed motion on lubricated topographies could be created with a vanishingly small tilt of the surface [5,6,[11][12][13][14]22].…”

“…Both SLIPS and LIS are modelled on the peristome surface of the Nepenthes trap in that they are preferentially wetted by a film of lubricating liquid that repels foreign liquids (immiscible to the lubricating liquid) [5,6]. Since their conception, these surfaces have often out-performed their conventional superhydrophobic counterparts in terms of their liquid-shedding abilities [5,6,[11][12][13]. However, the lack of drop-solid interaction on these surfaces also means that controlling the motion of liquid droplets is inherently difficult [11,14].…”