In a physics first, Tadmor demonstrates new fact of friction
11/20/2009
Lamar University associate professor Rafael Tadmor and a team of students recently found a way to bring what had only been a physics theory to the realm of experimental fact. Their effort, and the device they built to test the theories will be discussed in a number of leading scientific journals.
For decades, students have learned in introductory physics classes that the friction force — which must be overcome to, say, slide a block down an inclined plane — is the product of a friction coefficient and the force normal to the surface. That relationship has been superseded over the past 50 years by the recognition that the lateral friction is, in fact, proportional to the true contact area. The approach taught in introductory physics courses is a special but common case in which the contact area scales linearly with the normal force.
Using a special device that allows complete decoupling of normal and lateral retention forces, a group of student researchers led by Rafael Tadmor, associate professor in the Dan F. Smith Department of Chemical Engineering at Lamar University, found a system in which the lateral force decreases – rather than increases – with the normal force in spite of the fact that the contact area increases. This discovery will be published in one of the most prestigious journals in science.
The article, “Measurement of lateral adhesion forces at the interface between a liquid drop and a substrate,” was accepted for publication and will appear in a future issue of Physical Review Letters, published by the American Physical Society. The findings will also be presented on the society’s public website (physics.aps.org) and two publications, Physics Today and Physical Review Focus, written for a more general audience.
Tadmor holds a bachelor’s and master’s degree from Technical Israel Institute of Technology and the Ph.D. from Weizmann Institute of Science, Israel, and has been on faculty at Lamar since 2003. He was joined in the research by Ph.D. candidates Prashant Bahadur and Aisha Leh, master’s students Lan Dang and Hartmann N’guessan, and Rajiv Jaini, an undergraduate chemical engineering major.
By measuring liquid drops on surfaces, Tadmor and the students observed a lowered lateral force despite a larger normal force and an increased contact area. Key to their observations was the ability to decouple the normal and lateral forces while monitoring the drop. David Day, a technician from the Lamar’s mechanical engineering department, was particularly helpful in building the novel instrument that allowed measurements of the lateral adhesion forces at the point where the solid and liquid surfaces interface in a way that is decoupled from the normal forces, Tadmor said.
To achieve that separation of forces, the researchers mounted the sample at an adjustable angle in a horizontal centrifuge arm that could be rotated about the vertical axis at a variable speed. A camera transmitted video to a nearby computer allowing the researchers to compare the situation in which the drop was on top of a horizontal substrate to that in which the drop was hanging below a horizontal substrate. Tadmor found that the hanging drop had the larger lateral retention force, despite a smaller contact area and a smaller normal force. That seemingly counterintuitive result agrees with theories that incorporate the effects of surface deformation and molecular reorientation, Tadmor said.
“It is always a significant accomplishment for a research work to be accepted for publication in a premium scientific journal,” said T.C. Ho, chair of the Department of Chemical Engineering. “Publication promotes the visibility of Lamar's research capability and creates a positive image of Lamar's research quality to the scientific research community. That is essential for the department, the college and the university to be positively recognized and will, in turn, lead to more successful awards in grant applications.”
Physical Review Letters is the world's foremost physics letters journal, providing rapid publication of short reports of significant fundamental research in all fields of physics. International in scope, the journal provides its diverse readership with weekly coverage of major advances in physics and cross-disciplinary developments. The publication’s topical sections are devoted to general physics; gravitation and astrophysics; elementary particles and fields; nuclear physics; atomic, molecular, and optical physics; nonlinear dynamics, fluid dynamics, classical optics; plasma and beam physics; condensed matter; and soft-matter, biological, and interdisciplinary physics.
This research was also recognized by the American Institute of Chemical Engineers when a presentation of the research made by Rajiv Jaini, an undergraduate student who works with Tadmor, was awarded first prize at its annual meeting this month.
For more information on Lamar University or the College of Engineering, visit lamar.edu.
