June 16, 1999 Chemists from Utrecht University and Philips Research have been able to regulate the flow of light through a liquid by means of an electrical field. As part of a project funded by the NWO’s Foundation for Fundamental Research on Matter (FOM), they developed a liquid containing metal "nano-rods". A liquid of this type can be used in electrical sun screens, for example in a car window, with the light transmission being adjusted by means of a knob.
The ability of the liquid to screen out the sun is based on the tiny metal rods it contains, which move at random in suspension. The rods have a diameter which can be altered from 12 to 22 nanometres and an adjustable length of between 40 and 730 nanometres. They absorb light of a certain colour, and the electrons resonate with the light over the length of the rod. The colour of the light absorbed depends on the length of the rods. If all the rods are oriented in the same direction, the liquid hardly absorbs any light at all.
The orientation of the rods can be adjusted by means of an electrical field which causes the transport of a charge in the rods. This means that the mid-point of the positive charge no longer coincides with that of the negative charge, so that the rods are subject to a force which causes them to orient themselves in the direction of the electrical field. The chemists found that the strength of the field required to orient the rods in an alternating field is dependent on their length.
The phenomenon of light absorption by nano-rods was already known in the 1930s. However, there had up to now been hardly any systematic research, because it proved to be a difficult matter to produce a liquid in which all the rods are of equal size and do not coagulate. To produce such a liquid, the Dutch chemists filled a finely perforated mould with gold. They then dissolved the membrane of the mould so that the rods remained behind on the base. These were then transferred to water by means of ultrasonic vibration. A special anticoagulant ensured that the gold rods did not form aggregates.
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