UV patterns caused by surface architecture

[DaveO]

I was searching for UV patterns and came across this gem which pins down something that I think Birna and Andrea have long suspected:

 

Open access

 

Ultraviolet patterns of flowers revealed in polymer replica - caused by surface architecture

https://www.beilstein-journals.org/bjnano/articles/10/45

 

Conclusion

In this study we provided experimental data relating the structural

difference of the UV-reflecting and UV-absorbing areas

within flower petals. Furthermore, we provided the first quantitative

measurements of these structural differences and their influence

on the reflectivity and the scattering properties of light

on the petal surface. We showed that the structures in the

UV-absorbing area are higher in all three model plants and that

they possess smaller tip angles than in the UV-reflecting areas.

Using petal replicas, we separated this surface architecture from

the cell pigments and thereby eliminated their influence on the

UV-patterns. This procedure provides the first proof that the

UV-patterns in flowers are not just pigment-based, but also

structurally determined. Through this, differences in UV-reflectivity

between the absorbing and reflecting areas of almost 50%

have been found, which is new for the UV-regime. These

results are important from a biological point of view as they are

the first debatable measurements of the structural influence on

UV-patterns and they show strong differences even within just

the three model plants. From a technical point of view, these

results are promising and important for different types of applications

given that the optical properties of surfaces are

becoming more relevant, for example, for hierarchically structured

solar panels, biomimetic antireflective or absorbing coatings,

lenses and many more.

[Andy Perrin]

Oh, that is a nifty experiment. It looks pretty definitive. So the UV patterns (at least on those flowers) are structural coloring. That explains also why they vanish in IR — the spacing is wrong to get absorption in IR.

[colinbm]

Thanks Dave

That is some amazing research.

Cheers

Col

[DaveO]

Thanks Col,

This has been one of those niggling questions that we have come up against for some time. I think it isn't the whole answer. If we consider a typical anti-reflective coating as used on lenses, they typically use destructive interference with a layer a quarter of a wavelength thick. So for 360 nm UV we have a thickness of about 90 nm, which is about a thousand times smaller than the structures shown in the SEMs in the paper with a scale of 20 micro-metres. So the structures which lead to the UV-absorbing effect may be hidden within the details of the hairs or fibres seen on the sides of the conical cells in the SEM photos. A similar principle applies in the acoustic field where anechoic wedges work best around quarter wave heights. It would be nice to see SEMs in the nm range.

Dave (Probably wrong as usual)

[Andy Perrin]

So for 360 nm UV we have a thickness of about 90 nm, which is about a thousand times smaller than the structures shown in the SEMs in the paper with a scale of 20 micro-metres.

Yeah, they need to be looking on a different scale. Even with multilayer coatings, usually each layer is a quarter wavelength, where "wavelength" in this case is the wavelength INSIDE the material, which is different from the 360nm value in air by a factor of the refractive index. (i.e. if n=1.5, wavelength is 360nm/1.5=240nm inside the material).

[Guest]

Does anyone have any info on the transmission spectrums for the IR neutralization and UV transmission filters they used in the study (NG, and SP2 400UV,respectively)?