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Lotus Effect and Friction biomimetics Review Lotus Effect and Friction: Does Nonsticky Mean Slippery? Md Syam Hasan and Michael Nosonovsky * Mechanical Engineering Department, University of Wisconsin-Milwaukee, 3200 N Cramer St, Milwaukee, WI 53211, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-414-229-2816 Received: 3 March 2020; Accepted: 11 June 2020; Published: 12 June 2020 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: wetting; friction; adhesion; biomimetic surfaces 1. Introduction Biomimetics is mimicking living nature for engineering applications. The term “biomimetics” was suggested by Otto Schmitt in the 1950s. A similar concept was developed also by Jack E. Steele in 1958 under the name “bionics.” Both concepts were popularized during the next decades; however, today the word “bionics” is common mostly in popular culture, while “biomimetics” is used in scientific and engineering literature. More rigorous definitions (such as the International Organization for Standardization Standard 18458 of 2015) distinguish between biomimetics (“an interdisciplinary cooperation of biology and technology or other fields of innovation with the goal of solving practical problems through the function analysis of biological systems, their abstraction into models, and the transfer into and application of these models to the solution”), bionics (“a technical discipline that seeks to replicate, increase, or replace biological functions by their electronic and/or mechanical equivalents”), biomimicry or biomimetism (“philosophy and interdisciplinary design approaches taking nature as a model to meet the challenges of sustainable development”), and bioinspiration (“a creative approach based on the observation of biological systems”) [1]. Biomimetic approaches are currently quite common in a wide range of fields including materials science, artificial intelligence, robotics, biomedical applications, and so on. Particularly important applications are found in materials science, and they include self-healing, self-lubricating, and self-cleaning materials and surfaces [2]. The most common example of a successful use of biomimetics in surface science is the lotus effect for water-repellency and self-cleaning. Among natural leaves, the lotus (Nelumbo nucifera) leaf exhibits extreme water repellence and hydrophobic behavior. When a water droplet is placed on a lotus leaf Biomimetics 2020, 5, 28; doi:10.3390/biomimetics5020028 www.mdpi.com/journal/biomimetics Biomimetics 2020, 5, x FOR PEER REVIEW 2 of 20 Biomimetics 2020, 5, 28 2 of 20 Biomimetics 2020, 5, x FOR PEER REVIEW 2 of 20 Biomimetics 20 202020, ,5 5, ,x x FOR FOR PEER PEER REVIEW REVIEW 2 of 220 of lotus20 leaf surface, the adhesion between the water and dust particles is greater than the adhesion lotus leaf surface, the adhesion between the water and dust particles is greater than the adhesion surface, the adhesion between the water and dust particles is greaterbetween than the the adhesion dust and between the leaf the surface. As a result, the water droplet picks up the dust particles and lotuslotus leafleaf surface,surface, the the adhesion adhesion between between the the water water and and dust dust particles particles is greateris greater than than the theadhesion adhesion between the dust and the leaf surface. As a result, the water droplet picks up the dust particles and rolls off the leaf surface immediately. The lotus ( (Puṇḍarīka) or प (Padma) in Sanskrit, between thethedust dustdustand and and the thethe leaf leaf surface. surface. surface. As As a result,a As result, a result,the the water water the droplet droplet water picks picks droplet up upthe thedust picks dust particles upparticles the and dust and particlesrolls off andthe leaf rolls surface off the immediately leaf . The lotus (पुडरकप (ुडरकPuṇḍarīka) or प (Padma) in Sanskrit, rolls off the leaf surface immediately. The lotus (पडरक (Puṇḍarīka) or प (Padma) in Sanskrit, পুরীকপুরীক in Bengali, and 蓮華 (Liánhuá) in Chinese) became a symbol of purity in many Asian rolls off thesurface leaf surface immediately. immediately The. The lotus lotus ( ु(पुडरक(Pu (Pun. dṇḍ. arar¯ıkaīka) oror प ((Padma) )in in Sanskrit, Sanskrit, in Bengali,in Bengali, and and 蓮華 (Liánhuá) in Chinese) became a symbol of purity in many Asian পুরীক in Bengali, and 蓮華 (Liánhuá) in Chinese) became a symbol of purity in many Asian পুরীক in(Li Bengali,ánhu áand) in 蓮華 Chinese) (Liánhuá became) in Chinese a symbol) became of a puritysymbol inof manypurity in Asian many cultures,Asiancultures, cultures, as reflectedas reflectedas reflected in Hindu inin Hindu and and and Buddhist Buddhist sacred sacred texts such text ass Bhagavatsuch as Gita Bhagavat 5:10 (“Having Gita 5:10 (“Having cultures, asas reflectedreflected inin Hindu Hindu and and Buddhist Buddhist sacred sacred text texts suchs such as asBhagavat Bhagavat Gita Gita 5:10 5:10 (“Having (“Having abandoned attachment he acts unaffected by Evil, as Lotus leaf is not wetted”) or the Lotus Sutra 14:46 abandonedBuddhist attachment he sacred acts unaffected texts such by Evil, as asBhagavat Lotus leaf is Gita not wetted”)5:10 (“Having or the Lotus abandoned Sutra 14:46 abandoned attachment attachment he acts una heff ectedacts unaffected by Evil, as Lotus leaf is not wetted”) or the Lotus Sutra 14:46 abandoned attachment he acts unaffected by Evil, as Lotus leaf is not wetted”) or the Lotus Sutra 14:46 (“They have learned the bodhisattva way and are untainted by worldly things, just as the lotus flower (“They have learned the bodhisattva way and are untainted by worldly things, just as the lotus flower is also me wayntioned and as aare symbol untainted of purity by in worldlythe Quran things,(Al-Waqi ‘justah 56:28) as the lotus flower (ﺳﺪﺮ)They haveby learned Evil, the as bodhisattva Lotus leaf way is and not are wetted”) untainted orby worldly the Lotus things, Sutra just as14:46 the lotus (“They flower(“They havein the learnedhave water learned”) the; the bodhisattval otusthe bodhisattva“) (is also mentioned as a symbol of purity in the Quran (Al-Waqi‘ah 56:28 (ﺳﺪﺮ) in the water”); the lotus (of) Purification is is also me 1:189)ntioned [1,3 ,as4]. Therea symbol are more of purity than 70 in words the Quranfor lotus ( Alin -Waqi‘ah 56:28 ﺳﺪﺮ ) untainted) is also mentioned by worldly as a symbol things, of purity just in asthethe Quran lotus (Al-Waqi flower‘ah 56:28) inin the theand water”);water the ”Hadith); thethe (The lotuslotus Book ﺳﺪﺮ) in the waterway”); the and lotus are and the Hadith (The Book of Purification 1:189) [1,3,4]. There are more than 70 words for lotus in and the Hadith (The Book of Purification 1:189) [1,3,4]. There are more than 70 words for lotus in Sanskrit [4]. Sanskrit [4]also. mentioned as a symbol of purity in the Quran (Al-Waqi‘ah 56:28)and and the the HadithHadith (The(The Book Book of ofPurification 1:189) [1,3,4]. There are more than 70 words for lotus in Sanskrit [4]. The superhydrophobic and self-cleaning mechanism of the lotus leaf is due to a special The superhydrophobic and self-cleaning mechanism of the lotus leaf is due to a special Sanskrit [4]. The superhydrophobicPurification 1:189) and [self1,3-,cleaning4]. There mechanism are more of thanthe lotus 70 words leaf is fordue lotusto a special in Sanskrit hierarchical [4]. roughness profile of its surface combined with wax coating, and it is called the “lotus hierarchical roughness profile of its surface combined with wax coating, and it is called the “lotus The superhydrophobic and self-cleaning mechanism of the lotus leaf is dueTheffect”e tosuperhydrophobic [5 a]. special Quantitively, hierarchical superhydrophobic and self-cleaning surfaces are mechanism characterized byof highthe value lotuss of leafthe apparent is due to a special effect”hierarchical [5]. 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