PIEZOELECTRIC EFFECT

1. Context

2. Background

• The effect has been known for 143 years and this time has been observed only in solids.
• The new finding challenges the theory that describes this effect as well as opens the door to previously unanticipated applications in electronic and mechanical systems.
• The effect was found in pure 1­butyl-­3­methyl imidazolium bis(trifluoromethyl­ sulfonyl)imide and 1­hexyl­-3­methyl imidazolium bis(trifluoromethyl sulfonyl)imide- both ionic liquids (liquids which are made of ions instead of molecules) at room temperature.

3. Piezoelectric effect

• In the piezoelectric effect, a body develops an electric current when it is squeezed.
• Quartz is the most famous piezoelectric crystal used in analog wristwatches and clocks.
• Such crystals are also used in other instruments where converting mechanical stress to a current is useful.
• Quartz is silicon dioxide (SiO2). The quartz crystal consists of silicon and oxygen atoms at the four vertices of a three­sided pyramid; each oxygen atom is shared by two pyramids.
• These pyramids repeat themselves to form the crystal. The effective charge of each pyramid is located slightly away from the center.
• When mechanical stress is applied, that is when the crystal is squeezed, the position of the charge is pushed further from the center, giving rise to a small voltage. This is the source of the effect.

Image Source: The Hindu

4. Why is the effect in liquids surprising?

• The piezoelectric effect has only been expected in solids thus far because the body being squeezed needs to have an organized structure, like the pyramids of quartz.
• Liquids don't have much structure as they take the shape of a container.
• Physicists explain the effect using a combination of Hooke's law that the force required to squeeze an object is linearly (i.e. nonexponentially) proportional to the amount of squeezing and the properties of dielectric materials.
• These are materials that don't conduct electricity but whose electrons are still mildly affected by an electric field. 
• Hooke's law is not clear when the body is not very compressible.
• The observation of the effect in ionic liquids appears on its face to be inconsistent with the current model.
• An implication of the findings is the existence of some manner of organization in ionic liquids that are not seen in ‘normal’ liquids.
• Normal and ionic liquids of the kind tested in the study respond very differently, at the molecular level, when an electric charge is “imposed” on them.
• Within the framework of the current understanding, the piezoelectric effect requires a ‘persistent’ order within the material.
• Normal liquids and gases have not been shown to exhibit order that persists long enough to be observed and characterized.

5. Possible New Applications

• The discovery opens the door to applications that have previously not been accessible with solid­state materials, and (room­temperature ionic liquids) are more readily recyclable and in many instances pose fewer environmental issues than many currently used piezoelectric materials.
• The liquids also displayed the inverse piezoelectric effect: they became distorted when an electric charge was applied.
• This effect could be used to control how the liquids bent light passing through them by passing different currents through them.
• That is, using this simple control mechanism, vials of these liquids could be lenses with dynamic focusing abilities.

For Prelims & Mains