We learn in introductory physics classes that the friction force is the product of a friction coefficient and the force normal to the interface. That relationship, has been superseded over the past 50 years by the recognition that the lateral friction or retention force is, in fact, proportional to the true contact area. In new measurements of liquid drops on surfaces, Rafael Tadmor and colleagues at Lamar University in Beaumont, Texas, observe the opposite behavior: a lowered lateral force despite a larger normal force and an increased contact area. Key to the observations was the ability to decouple the normal and lateral forces while monitor- ing the drop. To achieve that, the researchers established the instrument (see figure on the right) with the help of this NSF award. These discoveries allow better understanding of wet friction and friction minimization for construction of energy efficient bearings. measuring chamber motor Control box counter balance Figure: The device developed at Lamar University: Centrifugal Adhesion Balance (CAB). It consists of a chamber (1) with a camera, light source, and sample holder. The chamber can rotate vertically, and is fixed to an arm that rotates horizontally, thereby allowing complete decoupling of normal and lateral forces. The two drop pictures were taken while the CAB was stationary (top) and during rotation (bottom). Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate Rafael Tadmor, Lamar University Beaumont, DMR Reference: R. Tadmor, P. Bahadur, A. Leh, H.E. N’guessan, R. Jaini, and L. Dang “Measurement of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate”, Phys. Rev. Lett. 103, , (2009).

Broader Impact: The newly developed CAB is gaining wide interest in the surface science community and we are collaborating with scientists at several universities (incl. Rice University, University of Georgia at Athens, University of Bridgeport) to use this new technology, which can lead to significant enhancement in understanding of surface phenomena. Education: The participants including undergraduate and graduate students are educated with the invented instrumentation (CAB) for surface characterization. Undergraduate student Rajiv Jaini who works on the project, is a co-author on the PRL paper (referenced in the other slide), and presented his study in three conferences, in each of which he picked an award: 3 rd place in AIChE Regional Conference at University of Texas-Austin; (2009); 1 st Place (2009) AIChE National Conference in Nashville, TN; 1 st place in AIChE Regional Conference TX (2010). PIC middle left: Tadmor and three of his graduate students assemble the CAB. Outreach: PI Tadmor is the 2010 incumbent of the Honorary University Lecturer award of Lamar University and will give a lecture to the general audience about the instrument on 10/11/10. In addition, the instrument was recommended by the Phys. Rev. Lett. editorial for reading across disciplines. Yet in addition, stories about the instrument were made by leading journals around the world, including: Physical Review Focus; Physics Today; Physics World; Tribology and Lubrication Technology; C&EN; and a few other journals in different languages. PIC top left: The PI, Dr. Tadmor, explains about the CAB to school children at the Harmony Academy Middle School, in Beaumont, Texas. Right: Tadmor works with PhD student P. Yadav in the lab. Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate Rafael Tadmor, Lamar University Beaumont, DMR