Context: Recently, for the first time, scientists have reported evidence of the piezoelectric effect in liquids which earlier believed to be found in solids only.

What is the History behind the Piezoelectric Effect?

• In 1880, Jacques and Pierre Curie discovered that pressure generates electrical charges in certain types of crystals such as quartz and tourmaline.

• They called this phenomenon the "piezoelectric effect". The word "piezo" is derived from the Greek word piezein, which means to squeeze or press.

• Later they noticed that electric fields can deform piezoelectric materials. This effect is called the "inverse piezoelectric effect".

• After these first demonstrations, more work was undertaken to explore the great potential of the piezoelectric effect.

About Piezoelectric effect 

• It is a phenomenon where specific types of material (e.g., quartz, topaz, etc.) produce an electric charge proportional to the mechanical stress applied to them.

• It occurs when there is a conversion of kinetic or mechanical energy due to crystal deformation, into electrical energy. 

• Piezoelectric materials are materials that can produce electricity due to mechanical stress. 

• When a piezoelectric material is placed under mechanical stress, there is a shift of the positive and negative charge centers in the material, which then results in an external electric field. 

• Going the other direction, with an inverse piezoelectric effect, an external electric field causes a physical deformation in a piezoelectric material.

About Piezoelectric Materials

• A crystal is any solid with atoms or molecules that are arranged in a very orderly way based on repetitions of the same basic atomic building block (the unit cell).

• In most crystals (such as in metals), the unit cell is symmetrical; in piezoelectric crystals, it is not.

• Normally, piezoelectric crystals are electrically neutral.

• The atom arrangement may not be symmetrical, but the electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby.

• However, stretching or squeezing a piezoelectric crystal deforms the structure, pushing some of the atoms closer together or further apart.

• This upsets the balance of positive and negative, and causes net electrical charges to appear.

Examples of Piezoelectric Materials

• There are a variety of piezoelectric materials, both man-made and natural, that can produce electricity.

• Natural:

• The most well-known, and the first commercially available piezoelectric material used in electronic devices is the quartz crystal.

• Other naturally-occurring piezoelectric materials include topaz, tourmaline, and even bone.

• Man-made:

• A breakthrough came when scientists developed a new class of man-made materials, called ferroelectrics, which exhibited piezoelectric characteristics many times more powerful than piezoelectric crystals.

• Scientists kept searching for higher performance materials and this resulted in the development of barium titanate and lead zirconate titanate.

Applications of Piezoelectric Effect

• Developed in the 1900s to detect icebergs, the sonar device is the first practical application for piezoelectric devices.

• Sensors, high voltage generators, electronic frequency generators, microbalances, inkjet printers, and ultra-fine focusing and alignment of optical assemblies among others all exploit piezoelectric technology.

• It is also the basis of a number of scientific instrumental techniques with atomic resolution, such as scanning tunnelling microscopes.

Discovery of Piezoelectric Effect in Liquids

• The researchers at Michigan State University were studying properties of ionic liquids.

• Ionic liquids are made from salts with unsymmetrical, flexible organic cations and symmetrical weakly coordinating anions.

• The liquid piezoelectric material was discovered as the researchers applied pressure with a piston to a sample of an ionic liquid in a cylinder.

• To their surprise, they found that this led to the release of electricity.

• They also found that the amount of electricity released was proportional to the amount of pressure applied.

What is the Significance of this Discovery?

• The reason the piezoelectric effect has only been expected in solids thus far is that the body being squeezed needs to have an organised structure, like the pyramids of quartz.

• Liquids do not have such structure; instead, they take the shape of their container.

• The researchers suggest that liquid piezoelectric materials could prove to be useful, especially ones made using ionic liquids, because they would be more environmentally friendly than solid materials.

• They also note that liquid piezoelectric materials could allow more variety in device shape, opening up wider design opportunities.