NATURE'S MIRACLES: MECHANOLUMINESCENCE, CRYSTALLOLUMINESCENCE AND ELECTROLUMINESCENCE

 In times of a power-cut, what could be the best way to have a little light? Well, we could easily turn on a flashlight, be it the old fashioned torchlights or that little module on our smartphones. Those who are old-school would generally go after matchsticks, candles, campfire and maybe a barbeque grill. Although a knocked off drunkard might not worry about lights, he could always set his insides on fire(mind it! alcohol is flammable). And if someone is genuinely crazy, he will be creating light by vigorously shaking a solution of luminol and hydrogen peroxide. Putting some fireflies in a jar with a perforated lid could be a real page-turner. But what if he starts crushing quartz crystals, or lifesaver menthol candies, or worse, what if he starts to scrape off a small amount of sugar from a large lump?

From the previous two articles, we have learned about chemiluminescence, and its sub-types such as bioluminescence, electrochemiluminescence, lyoluminescence and candoluminescence. However, that's not the end of the story, but merely the beginning. Light emission can occur by a variety of other means, which are as follows:

Mechanoluminesence: Mechanoluminescence refers to the spectacular phenomenon of light emission resulting from mechanical action on solid substances, such as striking a piece of quartz with a hammer, whereby mechanical energy gets converted to short bursts of visible light energy. The Uncompahgre Ute tribe from Central Colorado are believed to have been the first to apply mechanoluminescence. They constructed a type of rattle from buffalo hide and filled them with quartz crystals. Upon vigorous shaking, the quartz crystals being subject to impact forces generated faint flashes of light, visible through the translucent hide. 

Quartz crystals when subject to mechanical stress can generate flashes of light
Image Credits: pxhere

Mechanoluminescence can be classified as follows: 

1. Triboluminescence: Triboluminescence is the emission of low-temperature, i.e., cold light, whenever a material is subject to the mechanical action of pulling, scraping, crushing or rubbing. The word originates from the Greek tribo, to rub, and Latin lumen, meaning light. The first recorded observations of triboluminescent phenomena date back to the writings of Sir Francis Bacon during the 16th-17th centuries. In the olden days, sugar was used only for the practice of medicine, and its production was way different from modern times. In those days, sugar was obtained as an exceptionally hard, solid lump, from which smaller portions had to be scraped off. During the scraping process, small bursts of faint light were observed. 

                                             There are other examples of triboluminescent phenomena, such as the white glow of diamonds during the cutting process. Scotch tapes, when pulled apart, display a glowing blue line at the point of separation. In a vacuum, this same mechanical action can generate X-rays strong enough to image a finger-bone. Life Saver candies contain wintergreen oil, i.e., methyl salicylate which is by itself, a fluorescent substance. When enough of them are crushed, they emit sparks of blueish light. 

Crushing a bunch of these can generate short bursts of blue light-a sure sign of triboluminescence
Image Credits: Neeraja foodie, CC BY-SA 4.0, via Wikimedia Commons

                                             Unfortunately, material scientists have not discovered the exact mechanism behind triboluminescence. However, it is believed that mechanical stress causes deformations, and separation of charges in asymmetric crystals, i.e., crystals whose lattice structure is not uniform. When opposite charges unite and recombine, they ionize the surrounding air-medium emitting a flash of light. Presence of certain impurities can also be treated as a cause of triboluminescence.  

 
2. Fractoluminescence: Fractoluminescence is synonymous with triboluminescence. The former, however, refers to the emission of light upon the fracture of a crystal. The action of rubbing is immediately followed by the fracture of crystals, and hence, fractoluminescence can be considered similar to triboluminescence. One example would be the rubbing of quartz crystals. 

3. Piezoluminescence: Some solids luminesce upon the application of pressure. Piezoluminescence comes from the Greek word piezein, meaning to press or squeeze. Piezoelectric substances such as sodium chloride, potassium chloride, potassium bromide, and even LSD can display piezoluminescent effects. 

                                          E. Newton Harvey, in his book, ''A History Of Luminescence: From The Earliest Times Until 1900'', among other things, gives a vivid description of piezoluminescent and triboluminescent properties as observed by Robert Boyle. Harvey writes that Robert Boyle had a particular diamond that used to shine in the dark, and if rubbed against clothes, could attract lighter bodies and shine like rotten wood. The same diamond, if previously heated, would shine brightly underwater and dim when moved over a hot piece of iron. These experiments and observations revealed the characteristics of thermoluminescence, electroluminescence and phosphorescence. Robert Boyle unknowingly stumbled upon piezoluminescence when he pressed a small portion of the diamond with a steel bodkin and observed a short flash of light. 

Note: Piezoelectricity and the mechanism of piezoluminescence will be explained in another article. However, at present, for the sake of simplicity, let us conclude that the production of electricity from a solid depends on their crystal lattice structure. Deformation of the crystal lattice and charge separation is immediately followed by the recombination of electrons or holes(in case of semiconductor), further accompanied by the emission of visible light. 

4. Sonoluminescence: Sonoluminescence, or, the conversion of sound energy into visible light energy, was discovered around 1934 by two German physicists. When an intense sound wave(between 20-60,000 Hz and above)passes through a liquid or gas, it creates bubbles by a process known as cavitation. Sonoluminescence materializes the concentration of these sound waves into tiny bubbles that implode in effect of the sound wave. Recent research suggests that the temperature inside these tiny bubbles can be higher than 10,000 kelvins and may even exceed a million Kelvins. 

A single sonoluminescent bubble trapped inside an acoustic chamber
Image Credits:CC BY-SA 4.0, via Wikimedia Commons

                                 In the above photograph, a small luminous air bubble is visible, suspended in an acoustic chamber filled with water with ultrasound frequencies being applied. The exact mechanism behind this phenomenon is not known. However, a simple explanation is as follows: Sound waves, i.e., acoustic vibrations are nothing but the compression and relaxation of the material medium through which energy travels as a series of high and low pressure gradients, known as peaks and troughs. When the pressure of the sound waves exceed the static and the vapour pressure of the fluid, it tears apart the liquid layers and forms a cavity which is an air bubble. As the pressure inside the bubble is much much lower than the pressure outside, the bubble collapses and rebounds, sometimes violently, thereby reaching a temperature around thousands of degrees kelvin and emits visible light. This phenomenon is also known as creating a star-in-a-jar as the temperature inside the bubbles typically exceed that of a star.  

Crystalloluminescence: Crystalloluminescence is the emission of light during the formation of crystals. It is the opposite phenomenon of triboluminescence, piezoluminescence and fractouminescence, which results from the breakage of crystals. During the late 18th century, it was observed that rapid crystallisation of potassium sulfate in an aqueous medium was followed by the emission of bright streaks of greenish light. J.J. Berzelius and Friedrich Wohler, apart from noticing the glow of potassium sulfate during crystallisation, also noticed that the newly formed crystals, if stroked, would keep on glowing for a sufficiently long time. The effect disappeared if the substance was dissolved and re-crystallised. Other crystalloluminescent substances include cobalt sulfate, potash, platinum compounds, benzoic acid, manganese dioxide-potassium chlorate mixture, arsenious acid and even during the phase transition of liquid water to ice. Although the true mechanism of crystalloluminescence is not fully understood, light emission might be related to the formation of molecules from charged, dissociated ions present in the aqueous solution. 

Electroluminescence: Electroluminescence can be defined to be the emission of light in response to an electrical passage or an electric field through a material substance. In a broader perspective, it can be regarded as the emission of light resulting from an electrical discharge, or the bombardment of electrons through a solid, liquid and gaseous matter. The spectacular northern lights, i.e., the aurora borealis or the streaks of luminosity observed during meteor showers, can be treated as a classic examples of naturally occurring electroluminescent phenomena which has fascinated man since prehistoric times. 

 

                                           Artificial electroluminescence was first observed in 1675 by the French astronomer Jean Pickard, who noticed that the air above the mercury in his barometer tube started to glow during the descent of the mercury column. These observations were later independently confirmed by Johann Bernoulli. Francis Hauksbee, being famous for his outstanding work on electrostatics, was intrigued by the mysterious phenomenon of the so-called barometric light. Through extensive research, he observed that a small amount of mercury, if inserted into a partially evacuated glass sphere, and upon charging the whole setup with static electricity, the air inside started to glow. Sometimes not even a vacuum was necessary as the glow was also evident during the violent shaking of a little amount of mercury in a glass tube. 

                                    

Electroluminescent backlight in a Casio digital watch
Image Credits: Multicherry, CC BY-SA 4.0, via Wikimedia Commons

                                        At present, electroluminescence is used in devices such as glow in the dark watch dials, LCDs, and LED lights. Phosphor based substances are typically used as a luminescent material in watches, as shown in  the above image. Whenever electrical energy passes through a phosphor substance, the electrons in the phosphor are excited to a high energy state. Light emission occurs following the transition of these excited electrons to their ground state. In semiconductor diodes, light emission follows directly from the recombination of electrons and holes(to be explained in a future article). Some common electroluminescent substances include powdered Zinc Sulfide doped with copper(green light), silver(blue light), manganese(orange-red light)

                                          

                                      Many other instances of electroluminescent phenomena have been observed for the past three hundred years and counting, not to mention the discovery of cathode rays(cathodoluminescence) and the characteristic green glow inside the Crookes ray tubes, which ultimately took mankind on a new journey.   

References: 

1. https://www.bbc.co.uk/bang/handson/sugar_glow.shtml
2. https://pubs.acs.org/doi/10.1021/accountsmr.1c00041
3. https://news.illinois.edu/view/6367/207368
4. Harvey, Edmund Newton. A History Of Luminescence; From The Earliest Times Until 1900. The American Philosophical Society, Philadelphia.
5. https://www.britannica.com/science/luminescence/Luminescence-excitation#ref582024

Video Resources: 

1. https://youtu.be/0TVZd8dZysY for sonoluminescene
   
2. https://youtu.be/FFukQowqGPg  for triboluminescence in quartz
   
3. https://youtu.be/tW8q_JfmcbU  for triboluminescence in menthol candies