Научный совет РАН по акустике

Статья в газете China Daily об акустической левитации
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China Daily, Dec, 15, 2006, No 8320

Fish can fly, Xi'an scientists show in ultrasound levitation

By Wu Chong

No strings attached, Chinese scientists have levitated small live animals, including tadpoles, fish and spiders using ultrasound - for the first time ever. Researchers at the Northwestern Polytechnical University in Xi'an conducted the levitation experiment by employing an ultrasound emitter and reflector to generate a sound pressure field. The emitter produced roughly 20-millimetre-wavelength sounds which balanced the relevant gravity and therefore could levitate objects half that wavelength or less in theory, Xie Wenjun, a material physicist at the university who led the research, told China Daily. Xie and his colleagues reported their findings online on November 20 in the US journal Applied Physics Letters.

Their work could help in the manufacture of medicines or materials that requires a non-container environment. The scientists started the experiment three years ago after their previous success in levitating globs of the heaviest solid and liquid - iridium and mercury. "Our aim is to find out how to process some metals without the aid of containers," Xie said. At times, compounds are too corrosive for containers to hold, or they react with containers in other undesirable ways. "When we wanted to further examine how to achieve better stability of the levitated object, the wild idea of using live animals came to us," the researcher said.

After the scientists had the ultrasound field in place, they used tweezers to carefully place animals between the emitter and reflector. It turned out that they could float ants, beetles, spiders, ladybugs, bees, tadpoles and fish up to 1 centimetre in midair. When they levitated the fish, the researchers added water through a syringe to the ultrasound field every minute to keep them alive. "The water also levitated, spreading over the fish like a thin film," Xie said.

The research required each levitation to last more than half an hour. During the experiment, the animals looked agitated and tried to escape from the field - the ant tried crawling in the air and struggled to break away by rapidly flexing its legs; and the ladybug tried flying away. "We had to control the levitation force carefully, because they tried to fly away," Xie was quoted as saying in Live Science. "An interesting moment was when my colleagues and I had to catch escaping ladybugs."

The scientists measured the pressure on the levitated animal and found it "much smaller than the gravity force it was under," said Xie. "So we think it didn't do any harm to the object." The technology of acoustic levitation can be traced back to 1866, when German physicist August Adolph Eduard Eberhard Kundt invented the "Kundt's tube," a glass tube in which dust is shown to collect at the nodes of standing waves, enabling the measurement of sound velocity in gases and solids.

Статью комментирует сотрудник бывшего отдела ультразвука Акустического института Виктор Петрович Юшин:

Исследования в области акустической левитации мы проводили в 70 -е годы. Эта технология перспективна с точки зрения подвешивания в газовой среде различных (в том числе неметаллических) тел без механического контакта их с опорами и без воздействия на них газовых струй.

Изложенный опыт повторяет классический эксперимент в стоячей волне, в данном случае на частоте 16 кГц. Вес взвешиваемых объектов около 1 г, такой же, какой и был достигнут в наших экспериментах.

Несомненно, новым достижением можно отнести подвешивание рыбок в пузыре воды, которая покрывает их пленкой. Мы с водой тоже экспериментировали, но без рыб. Водяные шарики ведут себя неустойчиво, распадаются на все более мелкие капли и дрейфуют к краям области облучения, хотя другие тела, например, перфокарты, втягиваются в поле. Живые организмы в поле особенно не страдают, т.к. отношение импеданса тела к импедансу воздуха очень велико и звук практически не попадает внутрь водяной капли.

Также интересно заявление, что наши коллеги из Сианьского университета поддерживали левитацию в течение получаса. Мы экспериментировали по 10-15 минут, задавалось это временем нагрева излучателя до недопустимых величин.