Abstract
It has become well-known that bees, marine fishes and birds, perceived geomagnetic
field by utilizing biological magnetic compass. Also, a recent paper reported the evidence that
cows and deer have a tendency to align along north-south direction of Earth provably
detecting the geomagnetic fields. We focused on the magnetic field effects on structural
colours or light reflection changes in biogenic crystals in fish scales. The magnetic control of
colour has been investigated in several fields. Liquid oxygen with a blue colour became
transparent under magnetic fields of Tesla order due to the spin state conversion. Biological
samples, such as chromatophore cells of fish scales, showed a structural colour change by
means of light scattering and interference under strong magnetic fields. The changes of light
reflection and colours in fish scales should have a biological advantage in competition and
communications. It is an interesting question whether the magnetic fields, including the
geomagnetic fields, affect the light refraction and reflection in fish scales. In the present study,
in the latter case; light scattering control by the magnetic orientation of organic and
diamagnetic molecules was applied for developing a functional diamagnetic fluid, controlling
its turbidity and transparency. The light scattering fluid was prepared by suspending the
guanine crystal particles, which were collected from goldfish scales, in an aqueous solution.
The typical size of a guanine particle was 5μm x 20μm x O. lfm, and its shape was of a thin
plate. The suspension of guanine with a density of 1.0 x 10 particles/ml was used to fill a
quartz cell The guanine particles floating in the suspension strongly scattered the light
incidence of 168k LUX and blinded the background where the light incidence came from the
top. Simultaneously with the magnetic field-on, the blinded liquid became transparent and its
background was clearly exhibited. The magnetically induced transparency of the guanine
suspension occurred reversibly, and its change was detectable at 0.1 T order, but was very
distinct at 6 T. The primal mechanism of the transparency was the magnetic orientation of the
guanine particles which resulted in suppressing the light scattering. Additionally, we
demonstrated the threshold of the obtained phenomena was several hundreds mT. The
developed method can be utilized for the visibility control of light reflective objects by using
the magnetic fields of milli Tesla order.
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