Light flashes sparked by acoustic shock waves far outshine those created by manual crushing.
Mechanoluminescence, also known as triboluminescence or fractoluminescence, is light emission induced as a result of mechanical action on a solid1,2,3 — for example, Francis Bacon noted as long ago as 1605 that lumps of sugar emitted light when scraped4. Here we elicit mechanoluminescence by a new means, acoustic cavitation, and find intense luminescence and emission lines that are not generated by other mechanisms such as grinding, cleaving, rubbing, scratching, biting or thermal shock.
Acoustic cavitation is the formation, growth and implosive collapse of bubbles in a liquid irradiated with ultrasound5. As these bubbles collapse, extreme conditions of thousands of degrees6 and hundreds of atmospheres7 are generated within the bubble. Intense shock waves are launched outwards from the imploded bubble at velocities exceeding the speed of sound8. These shock waves accelerate micrometre-sized particles to high velocities and can create interparticle collisions at roughly half the speed of sound in the liquid (hundreds of metres per second)9,10. Acoustic cavitation may also facilitate interparticle collisions during bubble implosion, or by rapid expansion of a bubble (at tens of metres per second) with particles attached to its surface11.
About 36% of all inorganic and 19% of all organic compounds exhibit mechanoluminescence12, but there is much we do not know about the phenomenon. When a piezoelectric crystal (such as sucrose or resorcinol) is stressed, polarization of electrical charge is induced. At fracture, the two faces of the crack will have opposite charges; as the faces separate, the local electric field created is sufficient to cause dielectric breakdown of the solid and the intervening gases1,2,3, producing mechanoluminescence.
On fracture of sucrose in air or under nitrogen gas (N2), the emission spectra N2(C3Πu − B3Πg) and N2+(B2Σ+u − X2Σ+g) are seen. When sucrose powder is sonicated in N2-sparged dodecane, the emission is the same but its intensity is much greater (Fig. 1a). The mechanoluminescence resulting from sonication is an order of magnitude greater than that from crushing sucrose in dodecane. When sucrose is crushed by hand with a glass rod, the rod moves at tenths of metres per second; however, collisions occur at hundreds of metres per second when a shock wave is launched as a result of acoustic cavitation9,10.
We induced mechanoluminescence from resorcinol under various conditions (see Fig. 1b, which includes the solid-state photo-luminescence spectrum for comparison). When resorcinol is crushed with a glass rod in air, weak N2(C3Πu − B3Πg) and N+2(B2Σ+u − X2Σ+g) lines are seen, as well as crystal luminescence; however, when resorcinol is crushed in hexadecane under helium (He) or argon (Ar), only crystal luminescence is observed. In contrast, when a slurry of resorcinol in He- or Ar-sparged hexadecane is sonicated, intense discharge lines of He and Ar gas are created, together with a weaker crystal luminescence. When hexadecane itself, or a hexadecane slurry with hydroquinone (a non-piezoelectric, non-mechanoluminescent analogue of resorcinol), is sonicated under these conditions as controls, there is no luminescence. Other mechanoluminescent crystals that are sonicated as slurries (for example, m-aminophenol; see supplementary information) give an intense gas discharge and crystal luminescence, just like resorcinol.
The intensity of mechanoluminescence is directly proportional to the area of the new surfaces created during fracture13. The greater intensity in mechanoluminescence that we observe as a result of cavitation could be due to an increase in the frequency of mechanical action (20 kHz), which increases the rate of fracture, or to a greater intensity of collision; only the latter, however, can explain the very large increase in He or Ar line emission.
We suggest that the mechanoluminescence we observe derives from the following chain of events: acoustic cavitation induces high-velocity collisions between suspended solid particles (primarily resulting from shock waves in the liquid); this causes fracture of the crystallites and induces exceptionally high electric fields due to charge separation at the rapidly separating piezoelectric fracture faces; finally, dielectric breakdown produces an electric discharge, particularly from dissolved gases (for example, He, Ar, N2) and at the crystal surfaces.
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The authors declare no competing financial interests.
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Eddingsaas, N., Suslick, K. Light from sonication of crystal slurries. Nature 444, 163 (2006). https://doi.org/10.1038/444163a
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