ultrasonic magic wand 12.01.2015

"Wingardium Leviosa" - such a spell was used by characters from the fairy tale series about Harry Potter to make objects fly through the air with a wave of a magic wand. But while writers and directors come up with new fantastic stories, science, step by step, brings to life things that seemed impossible before. This time it was about moving objects through the air, or levitation. Levitation can be called such a physical effect in which an object without visible support is or moves in the air. For example, if an ordinary magnet is placed over a superconducting material that was previously cooled to a low temperature, then a "miracle" will happen - the magnet will float freely in the air at a low altitude. This is due to the special behavior of the magnetic field in superconductors.

Such a phenomenon, although very curious, is of little use for practical use: the need to move magnets over a superconductor cooled to the temperature of liquid nitrogen will not arise for everyone. It is a completely different matter if a method is invented to move any objects contactlessly, regardless of their nature. Such an invention will be widely demanded in those areas where the extreme purity of materials is important. For example, in pharmaceuticals or microelectronics: there any extra contact with the material can bring unwanted impurities into it. Physicists from the University of Sao Paulo in Brazil have proposed a way to make small objects not only float in the air, but also move them in the right direction.

To overcome the force of gravity, the researchers used the pressure exerted by sound waves. You can feel the power of sound if you stand in front of a powerful speaker operating at full volume. Sound is a vibration that occurs in any medium. It can be air, water or solid materials. There are no sound waves in space, because there is no medium in which they can propagate: vacuum is a silent void. From a physical point of view, the transmission of sound in air is the movement of areas of high and low pressure. Pressure fluctuations create a force that can act on mechanical objects. This is how our ear is arranged, where the sounds of the world around us are transmitted to the brain through vibrations of the eardrum.

Now we need to remember one more feature of oscillations - the existence of standing waves. The simplest example: if you fix one end of a long rope, and move the other up and down at a constant frequency, then some points of the rope will remain motionless. The formation of such a standing wave occurs due to the superposition of two waves - the original, created by the movement of the free end of the rope, and the reflected wave. The effects that arise when sound waves are superimposed on each other formed the basis of the developed method of ultrasonic levitation. The emitter emits ultrasonic waves that are reflected from a surface located at some distance. The radiated and reflected waves add up, forming something like a corridor, in which areas of high and low pressure alternate. If an object falls into the area of ​​a standing ultrasonic wave, the energy of the waves is enough to compensate for the force of gravity, and we observe the effect of levitation.

Despite the fact that the method of ultrasonic levitation began to develop a few years ago, it was not easy to achieve a stable hold of objects in the air. For a long time it was not possible to create a stable standing ultrasonic wave of sufficient power: a slight movement of the emitter or reflector - and the effect disappeared. To solve this problem, Marco Andrade and his colleagues used the principle of standing wave formation based on multiple reflections. They made a special concave ultrasonic reflector with which they managed to achieve levitation of small plastic balls without fine-tuning the system. Even if you hold the wave reflector in your hand, the levitation effect does not disappear - until recently it seemed impossible. This technology has much more chances for real application.