January 9, 2002
Method improves inkjet nozzles for printing, manufacturing
WEST LAFAYETTE, Ind. Chemical engineers at Purdue University have developed a technique to dramatically 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.
Findings are detailed in a paper in the January issue of Physics of Fluids, a journal published by the American Institute of Physics. The paper was written by Basaran and doctoral student Alvin Chen.
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.
Inkjet, or "drop-on-demand" technology, is now used in numerous applications outside of printing.
Companies use it to manufacture diagnostic strips for diabetes, photographic films, adhesive tapes and analytical devices that operate by applying DNA-laden liquids onto "gene chips." The field of combinatorial chemistry, in which thousands of experiments are carried out simultaneously to speed up the discovery of new drugs and other products, depends on equipment that rapidly deposits precisely the same quantity of liquid over and over again into separate vessels.
"In these applications, reducing drop size is really a boon because it would reduce costs," Basaran said. "The fluids in these applications are expensive."
In inkjet printers, reduced ink usage might be an unintended consumer benefit.
"Small drops enable improved printing quality, and that is the reason printer manufacturers have been seeking ways to reduce drop size," Basaran said. "Reducing the amount of ink used by commercial printers may not be a stated goal of manufacturers. But, perhaps with reduced ink usage, the cost of operating inkjet printers might become more affordable."
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 makes 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 center of the nozzle. In the final step, the nozzle expands a second time, forcing liquid in the center out of the nozzle. This forms a drop that is significantly smaller than the nozzle opening.
The Purdue researchers have applied for a patent, and they are working with companies interested in the new method.
"There has been a barrier up until now," Basaran said. "People could make drops that were either the same size as, or much bigger than, the nozzle. It was really hard to make drops that are smaller than the nozzle. This method overcame that barrier."
The Purdue engineers were not trying to find a method to decrease drop size. They made their discovery while conducting research funded by the U.S. Department of Energy aimed at better understanding how drops are formed in chemical processes, called separations, and in inkjet technology.
Finding ways to decrease the size of drops is no trivial matter, Basaran said.
"People have been trying to make progressively smaller drops since the advent of inkjet printing because smaller drops result in higher resolution," Basaran said. "But the only way they could get smaller drops was to make the nozzles smaller and smaller, and that's not easy. It's a complicated manufacturing process."
The tiniest nozzles made in labs also have been more prone to clogging and breaking.
To see exactly how drops form using the new method, the Purdue engineers used a high-speed camera provided by the 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 liter. 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 he and Chen have made 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.
Writer: Emil Venere, (765) 494-4709, firstname.lastname@example.org
Source: Osman Basaran, (765) 494-4061, email@example.com
Purdue News Service: (765) 494-2096; firstname.lastname@example.org
NOTE TO JOURNALISTS: An electronic or hard copy of the research paper described in this release is available by contacting Emil Venere, (765) 494-4709, email@example.com.
A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production
Alvin U. Chen and Osman A. Basaran, School of Chemical Engineering, Purdue University
The lack of a simple method for generating drops whose radii (Rd) are much smaller than those (R) of nozzles which produce them has heretofore been a major limitation of the drop-on-demand technique. Therefore, the only reliable way to reduce Rd to date has been to reduce R. A new method is reported which allows an order of magnitude reduction in drop volume while using the same nozzle. It is shown that the key to forming drops with Rd<R is to judiciously control the capillary, viscous and inertial time scales that govern the flow within the nozzle and the forming drop. The time scales are controlled in experiments by appropriately driving a piezoelectric sleeve surrounding a microcapillary tube, and the interplay between them is elucidated through computation.