Computer program reveals optimum structure for new composites

Techniques could help bring efficiency of biology to man-made materials

By Steven Schultz

Princeton NJ -- A Princeton chemist has developed a general mathematical system for designing materials that perform two functions at once, even when the desired properties sometimes conflict with each other.

Salvatore Torquato
 
  
Salvatore Torquato and colleagues used computers to calculate the optimum structure for any material that is a composite of two substances with differing properties. The achievement is the first simple example of a mathematically rigorous method for optimizing the design of multifunctional composites, which are an increasingly common kind of material.

The approach could help bring to man-made materials the efficiency of design that characterizes so many biological materials. "Biological materials are inherently multifunctional," said Torquato. "They have evolved over millions of years to cope with a wide range of situations, so they perform a variety of functions well."

  

Princeton chemist Salvatore Torquato and colleagues created a computer program that finds the optimum microstructure for composite materials that excel at two functions at once. These examples show how two hypothetical materials could form a composite that is as good as possible at conducting both heat and electricity. The structure on the left is the same as the one above but with the surrounding material removed. The units could be virtually any size, from hundreds of nanometers (billionths of meters) up to centimeters or meters.
 

A tree, for example, has to support its weight and resist winds while transporting liquids up and down its length, said Torquato, who is a professor in the Princeton Materials Institute as well as the Department of Chemistry. "Until our work, however, there has been no clear and simple example that rigorously demonstrates the effect of competing property demands on composite microstructures."

In addition to its possible applications in materials science, the method may help biologists study natural materials, such as the walls of a cell, to understand why they are built as they are. "Using rigorous optimization techniques, we are now in a position to test some of the basic tenets of biology," Torquato said. "Are there elements of biology -- perhaps subsystems within an organism or cell -- that are optimized in any sense?"

Torquato and co-authors Sangil Hyun, a postdoctoral fellow, and Aleksandar Donev, a graduate student, described their findings in a paper published in the Dec. 23, 2002, edition of Physical Review Letters.

In their paper, the scientists demonstrated their approach by finding the ideal structure for a composite that is good at conducting both electricity and heat. Many materials already are good at both those tasks, but Torquato chose ones that are good at only one or the other. Running the scientists' program, the computer arrived at surprisingly complex shapes as the optimum way in which the two materials should mix with each other at a microscopic scale.

The technique is general and could be used to optimize many properties, Torquato said. The technology already exists to make materials assemble themselves into finely tuned micro-scale patterns like the ones the scientists generated in their demonstration, Torquato said.

"I think it's phenomenal work and it's something that is very needed and timely," said Jeff Brinker, a senior scientist at Sandia National Laboratory and professor of chemical and nuclear engineering at the University of New Mexico. Brinker is preparing to collaborate with Torquato to test the idea in actual materials.

As fabrication techniques improve, materials scientists increasingly need such theoretical work to guide them, Brinker said. "How should we direct the self assembly? Sometimes it's not very intuitive what the optimum structure should be."

The shapes produced by the computer are interesting in themselves, said Torquato. The best structure for simultaneous heat and electricity flow turned out to be a complex shape called a "bicontinuous triply periodic minimal surface," which Torquato recognized from other situations. A minimal surface is one that takes up the least amount of area for a given volume. A soap bubble is a common example of a minimal surface. Usually, this shape arises from a need to minimize surface tension. The researchers were surprised to see a minimal surface in their ideal conductor because neither of their stipulated properties has anything to do with surface tension.

Studying these non-intuitive shapes may offer insights into the relation between structure and function in both biological and man-made materials, Torquato said. "These results and the shapes we found suggest to me that there are incredibly rich opportunities that have not even been tapped into," he said.

 
top
 

 


February 10, 2003
Vol. 92, No. 15
archives   previous   next

Contents

Page one
Abraham: U.S. participation in international fusion effort builds on success at PPPL
Astrophysicist reaches for the stars and more
Computer program reveals optimum structure for new composites

Inside
Figuring out how the universe works
New P-Rides bus service launched
Science on Saturday lectures offered

People
Nugent named president of Kenyon
Hargadon chosen to deliver baccalaureate address
New faculty members appointed

Sections
Nassau Notes
By the numbers: Campus buildings: additions and subtractions
Calendar of events


The Bulletin is published weekly during the academic year, except during University breaks and exam weeks, by the Office of Communications. Second class postage paid at Princeton. Postmaster: Send address changes to Princeton Weekly Bulletin, Office of Communications, Princeton University, 22 Chambers St., Suite 201, Princeton, NJ 08542. Permission is given to adapt, reprint or excerpt material from the Bulletin for use in other media.


Subscriptions. The Bulletin is distributed free to faculty, staff and students. Others may subscribe to the Bulletin for $28 for the academic year (half price for current Princeton parents and people over 65). Send a check to Office of Communications, Princeton University, 22 Chambers St., Suite 201, Princeton, NJ 08542.


Deadline. In general, the copy deadline for each issue is the Friday 10 days in advance of the Monday cover date. The deadline for the Bulletin that covers Feb. 24-Mar. 2 is Friday, Feb. 14. A complete publication schedule is available at deadlines or by calling (609) 258-3601.


Editor: Ruth Stevens
Calendar editor: Carolyn Geller
Staff writers: Jennifer Greenstein Altmann, Steven Schultz
Contributing writers: Karin Dienst, Eric Quinones, Evelyn Tu
Photographer: Denise Applewhite
Design: Mahlon Lovett, Laurel Masten Cantor, Margaret Westergaard
Web edition: Mahlon
Lovett