Ultrasonics, or what is commonly known as sonication, is an excellent example of how a scientific observation of physical properties of a material can be applied and turned into a useful tool. In the 1880's, Jacques and Pierre Curie1 made the observation that when certain asymmetric crystals such as quartz and sodium potassium tartrate (Rochelle salt) are compressed, they generate an electrical charge. This property would become known as the piezoelectric effect (piezo being the Greek word meaning "to press"). Conversely, when these same crystals are stimulated with electrical current, they contract. By rapidly alternating the electrical current, the crystal rapidly contracts and expands and thus creates a mechanical vibration. Thus, without the use of motors, electrical energy can be converted into mechanical vibration. Ultrasonics applies this physical property by creating high frequency mechanical vibrations by stimulating crystals with high frequency oscillating electrical currents.
Standard alternating electrical current has a frequency of 50 or 60 Hz (cycles per second), for Europe and the US, respectfully. Ultrasonic instruments take standard alternating current and magnify, or convert, the cycles upwards. Standard laboratory ultrasonic instruments run at 20,000 to 23,000 Hz (i.e., 20-23 kHz). An incoming electrical current is converted to a high frequency current which is then used to stimulate the piezoelectric crystals. The crystals are attached to either a probe that can be immersed into a liquid or to a pan as with ultrasonic water baths. In either case, the oscillating crystals impart vibrational energy into the liquid.
Ultrasonic probes and baths oscillate up and down at 20,000 cycles/second though the amplitude of the oscillation is very short. A typical oscillation involves a contraction when the electrical current is applied and an expansion when the current is reversed. When the probe contracts, negative pressure causes the liquid to flow up with the probe while the expansion of the crystals pushes the liquid. At a rate of 20 kHz, the liquid turns into a zone of microscopic shockwaves.
As liquids cannot flow as fast as crystals oscillate, during the contraction small vacuum cavities are formed. When the crystals expand, the cavities rapidly implode and create microscopic shock waves. This process, known as cavitation, is extremely powerful when the collective energy of all the imploding cavities is combined. The cavities are formed and collapse in microseconds which releases tremendous energy within the liquid. For example, if immersion probes are anodized (electro-plated with a colored coating as is done with heat blocks), the coating is stripped from the probe due to the force of cavitation on the surface of the probe. Any substances loosely associated with the surface are stripped, which is the basis of ultrasonic cleaning baths. Also, anyone careless enough to touch an ultrasonic probe can readily attest to the power released as it rapidly causes burns (i.e., DO NOT TOUCH A SONICATOR PROBE!).
Immersion probes are extremely useful for disrupting biological samples, mixing viscous solutions, and creating emulsions. These probes are usually made of a biologically friendly or inert metal such as titanium. The probes are machined to be "tuned" to the frequency of the oscillating electrical current so that the probe can oscillate in harmony with the crystals. Most probes attach to a base unit that houses the crystals and then taper down to a point that is from 1/16" to 1/2" in diameter. This means that the energy of a wide base if then focused in the tip of the probe giving it power as well as speed. Commercial sonicators have adjustable power (measured in Watts) with maximum ratings from 100 up to 1500 Watts.
One consequence of high power output focused in the tip of a probe is that sonication can generate substantial heat rapidly. A few second burst of a sonicator probe can cause water to boil. Consequently, when heat labile samples are processed, the samples must be kept cold and the sonication must be done in short burst interspersed with cooling periods.
In the life science laboratory, sonicators are extremely useful for disrupting cells and tissues rapidly and thoroughly. Typically samples are chilled on ice and then processed for 5 seconds. The samples are returned to ice for an additional minute before repeating the process. Many samples may only need a single treatment. By adjusting the power, many cells can be cracked without severe damage to intracellular particles. This makes ultrasonication a valuable tool for harvesting organelles.
The initial size of the particles to be disrupted is an important factor when using ultrasonics for processing. For instance, whole tissues will not disrupt but will be simply cooked. Muscle, liver, heart, kidney, spleen, and lung have all been successfully disrupted after an initial homogenization with either a glass or mechanical homogenizer. For ultrasonic processing, tissue particles should be of 100 micron or less in size. The lysate from a two step homogenization process of tissue (i.e., grinding then sonication) results in superior liberation of enzymes and analytes than grinding or sonication does alone.
1 This is the same Pierre Curie who later with his wife Marie performed the pioneering research on radiation that would earn him the Nobel Prize. Pierre Currie was only 21 at the time, along with his 24 year old brother Jacques, when they discovered the piezoelectric effect which would later be applied to microphones, sonar, and electronics.