Rose petal effect: A subtle combination of nano‐scale roughness and chemical variability

Lisa Almonte,Victoria Fernández,Carlos Pimentel,Enrique Rodríguez‐Cañas,José Abad,Jaime Colchero

doi:10.1002/nano.202100193

Abstract

AbstractRose petals may involve high water contact angles together with drop adhesion which are antagonistic wetting properties. Petal surfaces have a cuticle which is generally considered a continuous, hydrophobic lipid coating. The peculiar properties of rose petals are not fully understood and have been associated with high surface roughness at different scales. Here, the chemical and structural features of natural upper and lower petal surfaces are analyzed by atomic force microscopy (AFM). Both rose petal surfaces are statistically equivalent and have very high roughness at all scales from 5 nm to 10 μm. At the nanoscale, surfaces are fractal‐like with an extreme fractal dimension close to df = 2.5. A major nanoscale variability is also observed which leads to large (nanoscale) wettability changes. To model the effect of roughness and chemical variability on wetting properties, a single wetting parameter is introduced. This approach enables to explain the Rose petal effect using a conceptually simple scheme. The described fundamental mechanisms leading to high contact angles together with drop adhesion can be applied to any natural and synthetic surface. Apart from introducing a new approach for characterizing a biological surface, these results can trigger new developments on nanoscale wetting and bio‐inspired functional surfaces.

Highlights

Characterizing the multifunctional properties of biological surfaces such as plant leaves, is interesting under a Life sciences perspective, and for the development of bio-inspired materials and Biomimetics.[1]
The structures observed in these atomic force microscopy (AFM) and scanning electron microscopy (SEM) images are in good agreement with the images of petal cross-sections (Figure 1), in particular, the papillae of the upper surface and the cuticular folds of the lower surface of rose petals are clearly resolved
Extreme roughness and chemical variability are the basis for the Rose petal effect

Summary

Introduction

Characterizing the multifunctional properties of biological surfaces such as plant leaves, is interesting under a Life sciences perspective, and for the development of bio-inspired materials and Biomimetics.[1] Being the interface between aerial plant parts and the surrounding environment, plant surfaces play a major role for plant growth and survival.[2] The morphology and chemical composition of plant surfaces determine a wealth of fundamental processes; in particular, wettability, adhesion or repulsion of water drops and transport phenomena across the cuticle.[3] The outer surface of most aerial plant organs is protected by epidermal cells of different shape, function and composition.[4] The external cell wall of epidermal cells has a lipid-rich area named cuticle,[5,6] which is a composite material made of rather hydrophobic (lipids) and hydrophilic (polysaccharides) chemical constituents.[7] The cuticle has traditionally been considered a continuous lipid layer covering aerial plant parts, as stated in the first half of the XIX Century.[8,9] While the chemical heterogeneity of the cuticle has been highlighted in recent studies,[7,10,11,12,13] it is still generally regarded as a continuous, hydrophobic coating made of cutin and waxes embedded (intra-cuticular) or deposited (epicuticular) onto the surface.[5,10]

Results

Discussion

Conclusion