Page 53 - Neutrons For Science - Annual Report 2016

Figure 2
Tomographic visualisation of a wax lm immersed in water caused by water penetration. The colour gradient describes the amount of water penetration.
Moisture-controlled NR measurements carried out in dehydrated, ambient and saturated humidity conditions indicated that water penetrates extensively into the wax lms measured under saturated humidity and under water, causing them to hydrate and swell signi cantly with up to 50 % of the underlying lm occupied by water. These swollen wax lms could return to their basic structural feature upon drying. NR measurements carried out in a moisture-saturated environment show that it is particularly easy for water vapour to penetrate and become trapped within the underlying wax
lms (figure 2). A steady increase in water content
with interfacial distance was observed as the water vapour readily penetrates the wax near the surface
of the underlying lm, as con rmed by spectroscopic ellipsometry. Despite moisture penetration and swelling observed under hydrated conditions, wax lms are signi cantly thinner than when immersed in water at the solid-liquid interface. Exposure to the bulk water could be said to cause more substantial swelling to the underlying wax lm, as was evident from the increase in lm thickness measured by ellipsometry and NR. Comparison of the lms measured under ambient and dehydrated environments shows little noticeable effect upon the basic wax lm structure feature and lm thickness. It can be inferred that an unhydrated cuticular wax lm will be signi cantly thinner than its hydrated counterpart.
In conclusion, this work has demonstrated that the main leaf surface could be mimicked, in terms of its main characteristics, by reconstituted lms, thus opening up
the prospect of further investigations using cuticular wax models by elaborate physical techniques. The general perception is that plant cuticular waxes form part of the hydrophobic, waterproof surface coating whose primary function is to act as a water-repellent barrier against external environmental attacks. While this is apparent on the macroscopic scale, our ndings from the behaviour of reconstituted cuticular wax lms show a signi cantly different behaviour at the molecular level. Measurements carried out in moisture-saturated solid-air and solid-
liquid environments point towards a highly porous and diffusive underlying wax lm which allows a signi cant amount of water penetration. Furthermore, to our best knowledge this is the rst study to report the swelling and dehydrating of the cuticular plant wax lms as a way
of controlling water transportation from the reconstituted wax lms. It is highly likely that the same mechanism works across real plant surfaces.
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