Focus on new faculty: Melih Eriten and the mysteries of friction
Friction occurs whenever a surface touches another. It's often the difference between the ideal of perpetual motion, and real-life mechanical systems that gradually wind down due to lost energy unless an engine or other force continues to move them.
Friction can be detrimental—for example, causing cars to waste fuel as they combat friction between the road and the tire, and between the vehicle and the air. Friction also causes wear, the loss or weakening of material.
However, friction also can be beneficial. "If friction is large enough, it can dissipate vibrations, stop a vehicle," says Melih Eriten, who recently joined the Department of Mechanical Engineering as an assistant professor. "You need a balance, depending on your application."
A native of Turkey with a strong boyhood interest in using math and physics to describe the world around him, Eriten received his undergraduate degree in mechanical engineering. He then came to the United States in 2006 to pursue his master's degree in applied mathematics at the University of Illinois. He returned to mechanical engineering for his PhD studies. After a year of postdoctoral work in aerospace engineering, studying how energy is dissipated because of friction, he joined UW-Madison in August 2012.
Eriten came to Madison already widely published, with 13 articles in conference proceedings and another 14 published in journals that include Mechanics of Materials, the International Journal of Solids and Structures, the Journal of Applied Mechanics, Tribology International, the Journal of Biomechanical Engineering, and Wear. In October 2012, Eriten received the ASME Marshall B. Peterson award for early career achievement in his field of study.
Eriten says he found the decision to move to Madison—after job offers from several other universities—"not even questionable," thanks in part to the ME department's collaborative culture. "The nature of what I do is interdisciplinary—physics, math, aerospace, civil engineering, others," he says. "We all need to work together, otherwise whatever I do cannot be applied."
For example, he's working with radiologists to study the physics of cartilage in the human knee.
The ability to predict cartilage breakdown years in advance of osteoarthritis development, for example, could lead the way to preventative treatment. Eriten envisions an algorithm that imaging software could incorporate to calculate the strength of the tissue based on characteristics of an MRI or other image: How thick is the cartilage? What material does it contain? Is it calcified or liquid?
"If you treat the tissue like it's a material, you will find specific properties of the material that will respond to the contact load," he says. "We can come up with how much load it can carry, and how much deformation it will have during certain activities like running, sitting, standing, climbing up stairs."
With that information alone, Eriten says, doctors might be able to tell patients what activities to avoid.
In another project that will require collaboration with materials scientists, Eriten wants to look more closely at one of the more puzzling mysteries of friction: What happens between two objects in the moments before they begin to slide against each other? Appropriately called "pre-sliding friction," it’s what prevents a heavy object on a shallow incline from sliding without an additional push.
Researchers can easily calculate sliding friction by performing experiments and observing the rate at which objects slide. But the same is not true of pre-sliding friction. Instruments that could measure the phenomenon on a material’s surface also would interfere with the results. "I think the material response—what the material is doing before sliding occurs—that is going to drive all those," he says.
To find the pre-sliding friction between a new iPhone screen and your finger, for example, you must figure out the deformation between those two materials: Which parts of one interact with and affect which parts of the other?
Eriten believes there could be a way to characterize those forces and predict the initiation of sliding.
At the micro- and nanoscale, even smooth surfaces contain tiny peaks and valleys that can add to friction by dragging against variations in the other surface. A precise, peak-by-peak analysis of those variations would provide a better look at the true interaction of two surfaces.
Understanding and being able to predict presliding friction has significant applications. For example, knowing that sliding is on the verge of beginning would help seismologists better predict the motion of tectonic plates, whose sliding is accompanied by the sudden bursts of energy that cause earthquakes. And pre-sliding friction has implications for the thousands of joints and contacts in assembled structures such as buildings and bridges. "They don't slide, but you still have friction," Eriten says.
Pre-sliding friction also may be a factor in premature failure of hip implants, likely in the interface where the implant stem extends into the patient's femur. "Sliding and presliding friction start to exert force on the femur," Eriten says. "If the doctor is able to see that global motion, the presliding behavior, you can start designing implants accordingly."
Friction is an old problem: Leonardo Da Vinci first described the loss of energy between moving parts more than 400 years ago. But the difficulties of measuring pre-sliding friction have, even now, left much of friction a mystery. And that challenge is part of what makes it a compelling subject for Eriten. "It's so complex that you really want to attempt to solve it,” he says. "And yet, It's still one of the unsolved problems of the world," he says.