Biomimetics of nanostructure-based passive radiative cooling properties of Silver Ants

Markus Zimmerl, Paul Kaltenböck, Karin Whitmore, Ille Gebeshuber

Atomic and Plasma Physics Group, Institute of Applied Physics, TU Wien



The increasing occurrence of hot summer days causes stress for both humans and animals, particularly in urban areas where temperatures can remain high even at night. Living nature offers potential solutions that require minimal energy and material costs. For instance, the Saharan Silver Ant (Cataglyphis bombycina) can endure the desert heat by means of passive radiative cooling induced by its triangular hairs. Shi et al (2015) (Ref. 1) experimentally demonstrated this effect.
The aim of our work is to transfer the nanostructure of the ant's body to various surfaces by using an epoxy stamp.

Assessment of shrimp shell surface modifiability and weatherability

Shrimp shells are chosen as the first target surface due to their low cost (as a waste product), biodegradability, and similarity in material to the ants' bodies (chitin).
The shells are scratched with a diamond tip in order to assess the feasibility of modifications to the surface. Some of the samples are then subjected to simulated hot and cold climates inside a climate chamber (Espec LHU-114) for three weeks.
To simulate a hot summer day, the temperature is gradually oscillating between +30°C and +50°C at low humidity levels. One cycle lasts 8 hours and is repeated 30 times. For the simulation of cold climate, the same cycle structure is used, but with temperatures ranging from -5°C and +5°C at high humidity.
Comparing the exposed to the unexposed samples provides insight into the weatherability of the shells and their scratched modifications. The measurements for this comparison are carried out with optical, confocal and electron microscopy.

Transfer of the ant hair nanostructure

A stamp of the Silver Ants hair is manufactured using the process described in the paper by Zobl et al (2016) (Ref. 2)
However, attempts to modify the shrimp shells with the stamp were unsuccessful due to the shell hardness.
As a result, a different target surface is chosen, namely a foil made from commercially available chitosan pills. The foil is produced by evenly distributing chitosan, which is dissolved in diluted acetic acid, on a flat surface. The nanostructure of the stamp is imprinted onto the foil while it is drying.
By comparing FT-IR spectroscopy measurements of the emissivity of flat and nanostructured foil, we intent to demonstrate, that it is possible to increase the IR-emissivity and therefore decrease the surface temperature purely through functionalities induced via structural modification.
This shall then be scaled up for larger surfaces, such as house facades, to reduce the need for conventional cooling.

Ref. 1: Shi, N.N., Tsai, C., Camino, F., Bernard, G.D., Yu, N., Wehner, R. (2015). Keeping cool: Enhanced optical reflection and radiative heat dissipation in Saharan silver ants. Science, Vol 349, Issue 6245, 298-301.
Ref. 2: Zobl, S., Salvenmoser, W., Schwerte, T., Gebeshuber, I., Schreiner, M. (2016). Morpho peleides butterfly wing imprints as structural colour stamp. Bioinspir. Biomim., 11 016006.

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