When water does not reach its boiling point
We usually take it for granted that pure water will always boil at 100 degrees Celsius (°C). Except that it doesn’t. The temperature at which a liquid transforms into a gas is known as the boiling point. When a liquid’s vapor pressure equals the gas’s atmospheric pressure outside of it, boiling takes place. As a result, the boiling point of the liquid likewise changes as the outside pressure does. Water has a normal boiling point of 100°C when the atmospheric pressure is exactly one atmosphere (1 atm). The boiling point of a liquid will decrease if the air pressure is less than 1 atm. At the top of Mount Everest, 8,849 meters above sea level, the atmospheric pressure is so low that water only boils at 71°C, making it incredibly challenging to cook even an egg. Contrarily, in a hermetically sealed pressure cooker, the heating of the air it holds increases the pressure, preventing the water from boiling until a temperature of 120°C, at which point food cooks significantly more quickly. The underlying idea behind these events is typically referred to as vapor pressure.
The phenomenon of cavitation and Bernoulli’s principle
The development of a vapor bubble in a liquid media at extremely low pressures, followed by a fast collapse of the same bubble, constitutes the unique hydrodynamic phenomenon known as cavitation. In hydraulic machinery, the liquid’s velocity during fluid transit frequently produces the pressure gradients required to cause cavitation.
Cavitation can also happen in a liquid when a solid body with sharp edges (like a disk) accelerates quickly and suddenly in still water. There may be bubbles close to these edges. Cavitation becomes important when a centrifugal pump, boat propellers, or control valve malfunctions as a result of accidental cavitation-related damage, primarily the shattering of metal.
Since Bernoulli’s principle is the root of most of the issues with unwanted cavitation creation, understanding it will help us identify the cause of these unintended effects. According to Bernoulli’s principle, a fluid’s (whether liquid or gas) pressure decreases as its velocity rises and vice versa. Bernoulli’s principle is the primary reason that airplanes fly. In this case, the airplane’s wing shape is important. The reason for this is that the upper portion is more curved than the lower portion, which is more straight. As a result, the air above the wing has a larger surface area and can move more quickly (at a lower pressure) than the air below. Airplanes fly because there is a difference in air pressure above and below the wings. In this case, the increased pressure from the bottom of the wing exerts a force that pushes the air upwards.
In terms of cavitation, the change in pressure caused by changes in fluid velocity is critical. If the pressure decreases below the liquid’s vapor pressure, the liquid changes phases and becomes a vapor generating a bubble. If the pressure rises above the vapor pressure, the bubble will collapse. In reality, the bubble implodes with great force. As a result, severe and costly metal damage will occur if the violent implosion occurs repeatedly against the side of a piece of metal over time. Thus, preventing cavitation is a major problem for those who design hydraulic machinery.
But a liquid that is static or almost static can also experience cavitation. When an oscillating pressure field is applied to the free surface of a liquid in a reservoir, cavitation bubbles may form within the liquid bulk if the amplitude of the oscillation is large enough. This type of cavitation is known as acoustic cavitation, a concept that we will return to later.
Naturally, the benefits of a sonic weapon like this were first recognized by nature. The mantis shrimp, or Odontodactylus scyllarus (Fig. 1), are well known for their novel way of shattering their shells with quick, strong blows from their appendages. These underwater attacks occur at rates of up to 23 meters per second, which causes cavitation between their appendages and hard-shelled prey. Consequently, the prey experiences two impacts: the first from the appendages and the second from the cavitation bubbles. The shock wave produced by cavitation can kill or stun creatures even if the initial blow misses.
Figure1. The mantis shrimp strikes hard objects with a pair of large predatory appendages at such high speeds that cavitation bubbles form between the appendage and the impact surface. Credits: Dorothea OLDANI on Unsplash.
And, as expected, humans have discovered a way to exploit the properties of this phenomenon in order to improve the efficiency of their own weapons; this is the case of supercavitation. By keeping a single bubble of vaporized water around torpedoes, supercavitation can significantly reduce drag and provide increased velocity ranges. The Russian VA-111 Shkval torpedo and the Iranian Hoot can travel at a speed of 370 km/h, while the German Barracuda can travel at 800 km/h. Since a propeller would not function properly inside a vapor bubble, these torpedoes use rocket propulsion instead.
But when appropriately handled, this virtually singular capacity for energy concentration may also be used in remarkably positive ways. As we will see later, there is another type of cavitation in which the bubbles are produced by sound waves rather than the movement of an object. This is how ultrasonic cleaning devices work: an ultrasound source generates millions of tiny cavitation bubbles in a fluid to remove dirt from delicate objects like jewelry, optical and surgical instruments, watches, and electronic components.
Cavitation bubbles are now also employed in a wide variety of surgical and medical procedures, for example, the removal of kidney and gall stones (lithotripsy). In the lithotripsy techniques, the stone’s substance is crushed by the cavitation created at the focal point until it is reduced to powder. Another example is the emulsification of tissue during cataract surgery. In this technique, an ultrasonically vibrating probe is positioned very close to the tissue or solid substance using a method termed high intensity focused ultrasound. The cavitation created at the tip of this probe produces the desired cleansing effect by creating the desired damaging effect on the target tissue.
A focused laser beam is another method for producing cavitation in a specific region. Photodisruption occurs when focused laser light creates cavitation bubbles that cut through tissue, resulting in precise microscopic incisions that have a high potential for use in many surgical procedures. The vapor bubble created by the laser causes the tissue to rupture; the temperature needed to achieve this action is between 100 and 305 °C. Laser pulses, also known as light scalpels, have become a well-established tool in non-invasive intraocular surgery.
The use of cavitation, on the other hand, has enormous potential in a number of crucial applications in additive manufacturing. However, how the cavitation phenomenon, which is well recognized to be a power of destruction, can be a force in the creation of new structures?
A quick advance in 3D printing technology
Additive manufacturing (AM), also known as 3D printing, is rapidly changing the way the manufacturing industry and medical devices must be designed as well as what can be produced and prototyped. A computer-aided design model can be directly transformed into a 3D object, avoiding the lengthy processes of traditional fabrication methods. This technology first appeared in the 1980s and quickly gained popularity since it allowed for the creation of tools and devices that could be customized for any user.
Different kinds of 3D printers use various technologies that handle various materials in various ways. However, to precisely manipulate polymers layer by layer, the majority of existing 3D printing techniques rely either on photo (UV light)- or thermo (heat) activated polymerization reactions. An alternative may be provided by the development of a new platform technology called direct sound printing (DSP), which creates new things using ultrasonic waves.
A study that was published in Nature Communications details the procedure. In direct sound printing, ultra-high frequency sound waves are concentrated for only picoseconds on a spot of liquid resin. The authors demonstrate the use of focused ultrasound waves to induce sonochemical reactions in microscopic cavitation areas or tiny bubbles. In that bubble extremes of the temperature of about 15000 degrees Celsius, and pressure that is about 1,000 times the air pressure can solidify the liquid resin and produce complicated shapes that are pre-designed and impossible to create with current methods.
The researchers use an ultrasonic field to selectively solidify liquid resin polydimethylsiloxane (PDMS), a polymer used in additive manufacturing, and deposit it on a plate form or other previously solidified object. A computer controls 3D printers by reading digital model data from computer-aided design software or a computed tomography scan. The computer moves the ultrasonic along a predetermined path, pixel by pixel, creating the desired object. Concordia University researchers have already tested this new technique on real pig tissues. In this experiment, focused ultrasound penetrates 15 mm tissue (skin, fat, and muscle) and 18 mm liquid polymer (total impression depth is 33mm) to successfully imprint a device into the tissue.
Direct sound printing offers fundamental advantages for aerospace applications, such as significant cost and lead-time savings, novel materials and inventive design solutions, mass reduction of components through highly effective and lightweight designs, and the abolition of conventional joining techniques allowing metal shell repair and maintenance teams to service parts deep within an aircraft’s fuselage that are difficult to reach by other means. However, direct sound printing has begun to gain the attention of the medical device industry.
Examples of today’s 3D printing technology include artificial stents, tissue engineering, surgical models, and porous trabecular prosthetic bones for the hips and knees, tissue engineering, tissue and organ models, prosthetics and replica fabrication, implants, and many other applications are available (Fig. 2). However, direct sound printing has the potential to open up previously unimagined possibilities in 3D bioprinting. Because sound can penetrate solid bodies or devices, it is possible to create medical implants and devices for almost every body part directly inside the body in the range of a few dozens of millimeters rather than surgically inserting implants created outside the body.
Figure 2. Recent advancements in direct sound printing have prompted scientists to experiment with and evaluate the technology’s applicability in the medical field. This includes rapidly evolving areas like tissue engineering, tissue and organ models, prosthetics and replica fabrication, implants, and many others. Credits: Nature Communications
Prospects for 3D printing
Similar to how computers and smartphones have changed our reality, 3D printing will significantly affect it. Customized and made-to-order consumer goods will be available, and 3D printing will allow producers to make completely new versions of commonplace items. In 2019, the global 3D printed market was valued at $11.58 billion USD, and it is expected to exceed $33 billion USD by 2027, (at a 14 percent growth rate per year).
In the field of health care, this innovative technology enables a wide range of devices. As research in this field expands and advances, more treatments for patients will become available, improving their quality of life in ways that would not be possible without the development and application of 3D direct sound printing. In the near future and has the potential to change many people’s lives if it is implemented on a larger scale and other issues, such as cost, accuracy, and speed are eventually overcome.
- Jean-Pierre, F. Fundamentals of Cavitation. vol. 76 (2004).
- Habibi, M., Foroughi, S., Karamzadeh, V. & Packirisamy, M. Direct sound printing. Nat. Commun. 13, 1800 (2022).
- Al-Dulimi, Z., Wallis, M., Tan, D. K., Maniruzzaman, M. & Nokhodchi, A. 3D printing technology as innovative solutions for biomedical applications. Drug Discov. Today 26, 360– 383 (2021).
- Brennen, C. E. Cavitation in medicine. Interface Focus 5, 20150022 (2015).
- Patek, S. N. & Caldwell, R. L. Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarus. J. Exp. Biol. 208, 3655–3664 (2005).