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

Abstract

Rose 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.

Cuticle structure and chemical composition of waxes in Phaeoceros laevis (L.) Prosk (Notothyladaceae, Anthocerotophyta)

“…Unlike the sporophyte, no cuticle was observed on the gametophyte by LM proceedings (Figure 1D), but a very thin osmiophilic layer external to the gametophyte epidermis was observed by TEM (Figure 1H). As well in the present study, Cook & Graham (1998) (Almonte et al, 2021), have demonstrated the presence of cell wall constituents among the cuticle layers, contradicting the traditional idea that the cuticle is free of polysaccharides and "continuous" (Brongniart, 1830;Jeffree et al, 2006). This cuticle chemical heterogeneity suggests a variation of the hydrophobic property in different regions of the same plant surface, facilitating the bidirectional permeability of water and solutes, the wettability of plant surfaces, altering mechanical resistance and, therefore, changing the interaction with microorganisms and contaminants deposited on these surfaces (Guzmań-Delgado et al, 2014;Almonte et al, 2021).…”

“…As well in the present study, Cook & Graham (1998) (Almonte et al, 2021), have demonstrated the presence of cell wall constituents among the cuticle layers, contradicting the traditional idea that the cuticle is free of polysaccharides and "continuous" (Brongniart, 1830;Jeffree et al, 2006). This cuticle chemical heterogeneity suggests a variation of the hydrophobic property in different regions of the same plant surface, facilitating the bidirectional permeability of water and solutes, the wettability of plant surfaces, altering mechanical resistance and, therefore, changing the interaction with microorganisms and contaminants deposited on these surfaces (Guzmań-Delgado et al, 2014;Almonte et al, 2021). The simpler cuticle detected by TEM analysis in the P. laevis gametophyte (Figures 1F, H) may be related to a lower lipidization, perhaps due to a lower wax content (Figure 3A), observed in the present study, and a greater presence of wall polysaccharides associated with the cuticular layer.…”

Droplet Impact Dynamics on Biomimetic Replica of Yellow Rose Petals: Rebound to Micropinning Transition

“…This same rationale also applies for the negative replicas, which have the same degree of homogenization as that of the positive replicas themselves and thus explains the higher h c for YRN than on RRN. So, in previous studies, the influence of the complex hierarchical structures that are omnipresent on these biological surfaces over the static wetting states have been studied, 13,14,32 but the dynamic wetting states over such short times have been missing. This is shown here for the first time that over a much shorter time such as during droplet impact, the same hierarchical structures that show sticky superhydrophobicity reveal a myriad of other wetting states including the completely opposite superhydrophobicity as well.…”

“…The “lotus effect” is a hallmark of such structural superhydrophobicity, which has been exploited exhaustively by the research community for its adaptation in the fabrication of self-cleaning and low-maintenance surfaces. − On the other hand, sticky hydrophobicity of the rose petals has recently gained much attention from researchers as they exhibit contrasting properties of high equilibrium contact angle (i.e., low foothold) along with their strong adhesive nature. − The origin of such sticky hydrophobicity of the petals is believed to be the Cassie-impregnated wetting state, where the liquid gets into the microbumps present on the surface of the petal but is unable to impregnate the nanowrinkles interspersed on the microbumps. , Owing to this unique hierarchical patterns, the liquid drop remains totally stuck on the surface, even when the conformation of the drop is changed from the sessile to the pendant. A more recent study revealed that this petal effect may also find its origin in the chemical heterogeneity at the nanoscale . Another molecular dynamics simulation study showed that the higher wettability of the nanowrinkles is the reason for such high adhesion .…”

“…A more recent study revealed that this petal effect may also find its origin in the chemical heterogeneity at the nanoscale. 13 Another molecular dynamics simulation study showed that the higher wettability of the nanowrinkles is the reason for such high adhesion. 14 However, it is envisaged that such a surface can be employed to hold a drop of liquid against gravity and to suppress any spill to the neighboring areas after depositing a droplet, 9,10,12,15,16 including possible fabrication of stable slippery surfaces by anchoring the slip enhancing the oil layer against the gravity-mediated drainage 17 and also for maneuvering droplets over them.…”