Method improves inkjet nozzles for printing and manufacturing

Chemical engineers at Purdue University have developed a technique to reduce the amount of liquid in drops emitted by nozzles such as those used in inkjet printers.

Chemical engineers at Purdue University have developed a technique to reduce the amount of liquid in drops emitted by nozzles such as those used in inkjet printers and for experiments aimed at discovering new drugs.

Because each drop is smaller, the technique might enable inkjet printers to use less ink and to produce better quality, higher resolution documents and images. For other applications, such as pharmaceutical and genomics research, the technique could decrease the cost of experiments by reducing the amount of material consumed by labs searching for new medications or studying the human genome, said Osman Basaran, a professor of chemical engineering at Purdue.

The Purdue researchers have demonstrated that their technique makes it possible to achieve at least a 10-fold reduction in the volume of liquid in each drop while using the same types of nozzles now available commercially.

The new method works by changing the ‘voltage pulses’ that command how the nozzles produce each drop of liquid.

The nozzles contain piezoceramic elements that move when electricity is applied to them. A positive electrical voltage is said to make the nozzles contract and a negative voltage makes them expand. Normally, each drop is produced by contracting the nozzle, which pushes out the liquid.

But the Purdue researchers discovered that drop size could be dramatically decreased if the following three-stage cycle were used.

First, the nozzle is made to expand, sucking liquid up into the nozzle. Then, the nozzle is contracted. However, some of the liquid that has been sucked into the nozzle from the previous expansion sticks to the sides of the nozzle, held there by friction caused by the liquid’s viscosity. When the nozzle contracts, the liquid that is closest to the sides of the nozzle does not move as quickly as the liquid in the centre of the nozzle. In the final step, the nozzle expands a second time, forcing liquid in the centre out of the nozzle. This reportedly forms a drop that is significantly smaller than the nozzle opening.

To see exactly how drops form using the new method, the Purdue engineers used a high-speed camera provided by the US Department of Energy that can take 100 million pictures per second. They also developed mathematical equations that explain precisely how the method works.

The researchers experimented with relatively large inkjet nozzles. Such nozzles were in commercial-printing use about 10 years ago and currently are widely used in scientific research labs. Those nozzles typically produced drops containing about 80 picoliters, or 80 trillionths of a litre. Commercial inkjet nozzles now on the market produce drops about one-tenth of that volume, in the range of 5-10 picoliters.

Using the new method with the old, larger nozzles enabled the researchers to produce drops in the same range as conventional nozzles, or about 8 picoliters, Basaran said.

He estimated that the computations show that reductions on a similar scale would be achieved by using the new method with modern, smaller nozzles.

For some applications, however, the larger nozzles are better than smaller ones. That’s because liquids used in certain applications, such as textile dying processes, contain particles that would clog the smallest nozzles. The method might enable such manufacturers to reduce drop sizes without using nozzles that are prone to clogging, Basaran said.

The Purdue researchers have applied for a patent, and they are working with companies interested in the new method.