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31 March 2023: Scientists spot piezoelectric effect in liquids for the first time

What’s in news?

Scientists have discovered the piezoelectric effect in liquids for the first time. The impact has been known for 143 years and has only been noticed in solids during that period. The new discovery calls into question the theory that explains this effect and opens the door to previously unknown uses in electronic and mechanical systems.

Key findings:

  • In an experiment, a container is filled with each liquid of varying thickness and squeezed with a piston. A cable inside the piston that is linked to an externally circuit and has an indicator that indicates when a current flows through it.
    • At room temperature, the effect was observed in purified 1-butyl-3-methyl imidazolium bis(trifluoromethyl-sulfonyl)imide and 1-hexyl-3-methyl imidazolium bis(trifluoromethyl-sulfonyl)imide, both ionic liquids (liquids composed of ions rather than molecules).
  • According to the article, the magnitude of the piezoelectric effect in the first liquid was 16 millivolt per newton (mV/N) and 17 mV/N in the second, both within a 1 mV/N margin. 
  • Using these figures, they determined that the piezoelectric constant – the strength of the effect in these materials – is a factor of ten lower than that of quartz, a comparatively minor variation.

Piezoelectric effect:

  • When a body is compressed, it generates an electric charge, which is known as the piezoelectric effect. 
  • Quartz is the most well-known piezoelectric crystal, and it is used in analogue wristwatches and timepieces. Cigarette lighters, electric guitars, TV remote controls, audio transducers, and other devices that transform mechanical tension to electricity use such crystals.
    • Quartz is silicon dioxide (SiO2). The quartz crystal is made up of silicon and oxygen atoms that are arranged at the four corners of a three-sided pyramid; each oxygen atom is shared by two pyramids. These pyramids are repeated to create the crystal.
  • Each pyramid’s functional charge is situated slightly far away from the middle. When mechanical tension is applied, i.e. when the crystal is squeezed, the charge location is moved further from the center, resulting in a small voltage. This is where the impact comes from.

Piezoelectric effect in solids and liquids:

  • The piezoelectric effect has only been predicted in solids thus far because the substance being compressed must have an organised structure, such as quartz pyramids. Liquids lack this framework and instead take on the form of their container.

Previous and current knowledge of piezoelectric effect:

  • The effect is explained by physicists using a mix of Hooke’s law, which states that the force needed to compress an object is linearly (i.e. non-exponentially) proportionate to the quantity of pressing, and the characteristics of dielectric materials. 
    • These are materials that do not transmit electricity but have electrons that are slightly influenced by an electric field.
    • Hooke’s law is not clear when the body isn’t very compressible.
  • The piezoelectric effect, according to current knowledge, needs ‘persistent’ order within the substance. Normal liquids and gases have not been demonstrated to have order that lasts long enough to be witnessed and described.
    • The results imply the presence of some type of organisation in ionic liquids that is not seen in ‘normal’ liquids.”
    • This is because, according to the paper, when an electric charge is “imposed” on ‘normal’ and ionic liquids of the type examined in the research, they react very differently at the molecular level.

Potential new applications:

  • The finding opens up uses previously unavailable with solid-state materials, and [room-temperature ionic liquids] are more easily recyclable and, in many cases, cause fewer environmental problems than many presently used piezoelectric materials.
  • When an electric charge was given to the liquids, they twisted due to the inverse piezoelectric effect. By passing various currents through them, this fact could be used to influence how the liquids bent light travelling through them. That is, using this basic control device, vials of these liquids could be transformed into lenses capable of dynamic focusing.