Ultrasonic Transducer Experiments

Ultrasonic Transducers

• An ultrasonic transducer is a device that converts electrical energy to sound energy. They work by applying an alternating current to a piezoelectric material, such as quartz, sandwiched between two metal heat sinks. This causes the piezoelectric material to vibrate rapidly and generates ultrasound.

  The ultrasound used in most experiments is different from the ultrasound used in medical imaging situations such as pregnancy scans. The former is of a lower frequency, whereas the latter is of a higher frequency and safe for medical use.

Cavitation Bubbles and Sonoluminescence

• Ultrasound waves traveling through a liquid create alternating regions of high and low pressure. The low pressure causes tiny cavitation bubbles to form, which then expand and contract because of the oscillating ultrasound waves. The liquid evaporates into the bubble cavities, filling the bubble with vapor, and gases dissolved within the liquid diffuse into the bubbles.

  Eventually, the cavitation bubbles collapse and implode, creating a shockwave that subjects the contents of the bubbles to temperatures and pressures of up to 5000 Kelvin and 1000 atmospheres respectively.

  These conditions cause unique chemical reactions that would not normally occur in the laboratory, such as sonoluminescence, when collapsing bubbles emit light. The conditions inside the collapsing bubbles create very short lived and hard to identify excited molecules and ions that get rid of their excess energy in the form of light.

Ultrasonic Cleaning

• Ultrasound can be used to clean objects such as jewelry. The cavitation bubbles created by the ultrasound collapse onto the surfaces of objects submerged in the liquid. When they collapse they "jet" onto the surface and clean the surface in the process.

  You can see this effect by placing a piece of aluminum foil into an ultrasonic bath. Attach the ultrasonic transducer to the underside of a metal container, such as a tin can, and fill it with water. Turn on the transducer and hold a piece of aluminum foil in the water for around 10 seconds. Hold up the foil to a bright light to see many tiny holes punched in the foil by the collapsing and jetting cavitation bubbles.

Creating Emulsions of Immiscible Liquids

• Some liquids, such as oil and water, do not mix with one another even with vigorous stirring. These liquids are "immiscible." The cavitation bubbles produced by an ultrasonic transducer can be used to create an emulsion of two immiscible liquids in which one of the immiscible liquids is suspended within the other. The two liquids will eventually separate, but it will be slower than if you mixed them by hand.

  Add equal parts cooking oil and water to a glass jar. Mix the immiscible liquids by shaking the jar by hand, and then leave to separate. Record the time for complete separation to occur, and then insert the ultrasonic transducer into the oil and water. Turn it on to create an emulsion. Again, leave the two liquids to separate and record the time for complete separation to occur. The emulsion created by the ultrasound will take longer to separate.