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Review
. 2020 Jun 12;5(2):28.
doi: 10.3390/biomimetics5020028.

Lotus Effect and Friction: Does Nonsticky Mean Slippery?

Md Syam Hasan  1 , Michael Nosonovsky  1
Affiliations
Review

Lotus Effect and Friction: Does Nonsticky Mean Slippery?

Md Syam Hasan et al. Biomimetics (Basel). .

Abstract

Lotus-effect-based superhydrophobicity is one of the most celebrated applications of biomimetics in materials science. Due to a combination of controlled surface roughness (surface patterns) and low-surface energy coatings, superhydrophobic surfaces repel water and, to some extent, other liquids. However, many applications require surfaces which are water-repellent but provide high friction. An example would be highway or runway pavements, which should support high wheel-pavement traction. Despite a common perception that making a surface non-wet also makes it slippery, the correlation between non-wetting and low friction is not always direct. This is because friction and wetting involve many mechanisms and because adhesion cannot be characterized by a single factor. We review relevant adhesion mechanisms and parameters (the interfacial energy, contact angle, contact angle hysteresis, and specific fracture energy) and discuss the complex interrelation between friction and wetting, which is crucial for the design of biomimetic functional surfaces.

Keywords: adhesion; biomimetic surfaces; friction; wetting.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Parameters interrelating friction and wetting.
Figure 2
Figure 2
(a) Adhesion between interlocked asperities and (b) deformation of asperities in the shear direction.
Figure 3
Figure 3
Coefficient of friction (COF) vs. country: Data on the COF of identical steel/aluminum-oxide sample pairs measured in different laboratories in different countries of the world. Each flag represents an experimental point from a laboratory.
Figure 4
Figure 4
“Mode II” crack propagation for in-plane shear loading, red arrows showing applied forces.
Figure 5
Figure 5
(a,b) Schematics of the sessile droplet method to measure contact angle hysteresis (CAH) and (c) a schematic of advancing and receding contact angles on the tilting plate.
Figure 6
Figure 6
The Stribeck curve for fluid-lubricated contacts.
Figure 7
Figure 7
(a) Contact angle (CA) vs. surface roughness for hydrophobic tiles and (b) COF vs. roughness for hydrophilic and hydrophobic samples (reproduced from Reference [71] with permission).
Figure 8
Figure 8
COF vs. CA for hydrophobic tile samples.
See this image and copyright information in PMC

References

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    1. Nosonovsky M., Rohatgi P.K. Biomimetics in Materials Science: Self-Healing, Self-Lubricating, and Self-Cleaning Materials. Springer; New York, NY, USA: 2012.
    1. Nosonovsky M. Slippery when wetted. Nature. 2011;477:412–413. doi: 10.1038/477412a. - DOI - PubMed
    1. Nosonovsky M. After Which Plant the Lotus-Effect Is Called? [(accessed on 9 May 2020)]; Available online: https://sites.uwm.edu/nosonovs/2020/04/23/lotus-in-sanskrit/
    1. Bhushan B., Nosonovsky M., Jung Y. Nanotribology and Nanomechanics. Springer; Berlin/Heidelberg, Germany: 2008. Lotus Effect: Roughness-Induced Superhydrophobic Surfaces.

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