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Raw materials & technologies, Production and testing

Highly accurate method measures charge and size of nano particles

Wednesday, 8 August 2012

Scientists at the University of Zurich, Switzerland, have developed a characterizing system that could increase the product quality of paints, coatings, and inks.

Scientists at the University of Zurich have developed a new measuring method for nano particles

Source: Fafoutis/Fotolia

Scientists at the University of Zurich have developed a new measuring method for nano particles

Source: Fafoutis/Fotolia

Nano particles are a millionth of a millimeter in size, making them invisible to the human eye. Unless, that is, they are under the microscope of Prof. Madhavi Krishnan, a biophysicist at the University of Zurich. Prof. Krishnan has developed a new method that measures not only the size of the particles but also their electrostatic charge. Up until now it has not been possible to determine the charge of the particles directly. The method also offers benefits for the development of paints, coatings, and inks.

Industrially relevant chemicals such as pigments, paints, inks and coatings are typically dispersions of nanometer scale particles in a fluid phase. The size and charge of these particles can fundamentally affect the properties and performance of the suspension. For example in electrostatic printing applications, the charge carried by particles in the ink, as well as their size, determine the efficiency of the printing process.

Improved product quality and efficiency

"Our method provides a tool capable of highly parallel accurate size and charge measurements on single particles that constitute such a suspension,” Prof. Madhavi Krishnan says. "This high-resolution characterization tool in the product development process will lead to enhanced product quality and consistency, reduce wastage and improve efficiency.”

In order to observe the individual particles in a solution, the Swiss scientist and her co-workers "entice” each particle into an "electrostatic trap”. It works like this: between two glass plates the size of a chip, the researchers create thousands of round energy holes. The trick is that these holes have just a weak electrostatic charge. The scientists than add a drop of the solution to the plates, whereupon each particle falls into an energy hole and remains trapped there. But the particles do not remain motionless in their trap. Instead, molecules in the solution collide with them continuously, causing the particles to move in a circular motion. "We measure these movements, and are then able to determine the charge of each individual particle”, explains Prof. Madhavi Krishnan.

Analysis in a solution, not in a vacuum

Put simply, particles with just a small charge make large circular movements in their traps, while those with a high charge move in small circles. This phenomenon can be compared to that of a light-weight ball which, when thrown, travels further than a heavy one. The US physicist Robert A. Millikan used a similar method 100 years ago in his oil drop experiment to determine the velocity of electrically charged oil drops. In 1923, he received the Nobel Prize in physics in recognition of his achievements. "But he examined the drops in a vacuum”, Prof. Krishnan explains. "We on the other hand are examining nano particles in a solution which itself influences the properties of the particles.”

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