From dessert to defense, agriculture to airplanes

Author: Linda Thomas Terhune
Early in his undergraduate career at Michigan State University, Paul Sojka decided his chosen discipline, physics, was not going to be a big money maker. So, after completing a bachelor’s degree in physics, Sojka moved on to Plan B — mechanical engineering.

The decision to change focus has resulted in a career-long pursuit of better ways to deliver sprays. His findings have a wide range of applications — from controlling the spread of airborne pesticides to creating more effective pharmaceutical tablets and more efficient systems that use fuels.

Sojka says that as he considered areas within mechanical engineering as a focus for graduate study he was most interested in thermal mechanics. “For me, power-producing things were the most interesting, things that used chemistry to produce power,” he says. Sojka developed his current focus after joining the Purdue faculty in 1983. His faculty mentor, Arthur LeFebvre, was renowned for his work in the application of fuel sprays in gas turbine combustor systems and led the School of Mechanical Engineering from 1976 to 1980.

It was during his master’s and doctoral work at Michigan State that Sojka made the commitment to mechanical engineering, a process that was completed when he joined the Purdue mechanical engineering faculty in 1983. His interest in sprays came through work with Arthur LeFebvre, who was renowned for his work in the application of fuel sprays in gas turbine combustor systems.

He has studied super-critical jets and sprays for the last six years, during which time his research group has developed the experimental protocol necessary to make spray mass flow rate, patternation, drop size and velocity measurements consistent. His work is a mix of analytical, theoretical, and experimental, with contributions to both fundamental and applied problems.

Once the domain of the aviation industry and the military, related to gas turbine engines, spray formation now has applications across the board.

Sojka’s projects range from developing uniform coatings on medicine tablets for consistent drug delivery to a partnership with food science professor Carlos Corvalan developing pesticide sprays that won’t be carried from farm fields on a light wind. The interest he shares with Corvalan has translational value. The mechanical properties that make for more consistent tablet coatings — viscosity and elasticity — also make for effective gelled airplane and rocket fuels.

Sojka and Corvalan are collaborating on gelled fuels research with Steve Heister, professor of aeronautics and astronautics and director of a $6.4 million U.S. Army Multidisciplinary University Research Initiative (MURI) to develop unique high-energy fuels.

Gelled fuels are attractive because they resist leaking and allow for better control of rockets than is possible with the solid fuels now used. Motors running on gelled fuels could be throttled up and down and controlled more precisely than conventional rockets that use solid propellants.

Sojka’s part of the research involves building an experiment to take high-speed videos of the gelatinous fuel, which forms during the fuel-injection process. “The fluid mechanics of gels are quite challenging,” Sojka says. “The viscous properties of the gel change depending on how fast it’s flowing, which is not true of common liquids such as water or gasoline.

“The basic science we’re developing will lead to improved pharmaceutical manufacturing and production efficiencies, better crop production, improved food process engineering, spray drying of foods, and medicinal coatings — from dessert to defense and agriculture to airplanes.”