"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Tribology teaches that the lateral force required to slide two surfaces against each other (friction force) is in fact proportional to the contact area. The Amonton law, therefore, is a special case in which the contact area of a rough surface happens to increase linearly with the load. The PI recently presented a system in which the lateral force decreases with the normal force in spite of the fact that the contact area increases. This happens for drops on surfaces, and the discovery was enabled using an instrument that allowed for the first time some limited decoupling between normal and lateral retention forces, namely the Centrifugal Adhesion Balance (CAB). In this proposal the PI will develop the CAB such that it will enable for the first time complete independent manipulations of the normal and lateral retention forces, which is essential for the study of the dependence of lateral retention forces on the normal forces. Enabling the measurement will change the way we consider the mutual influence of orthogonal retention forces. This will create a new paradigm in surface science (aiming to find relation between normal and lateral retention forces) leading to transformative research.
Another feature that this will add to the CAB is the possibility to decrease the normal force under zero lateral force. In this configuration the drop can only move normal to the surface, and will eventually fly off the surface at some negative normal force. Such an experiment will be the first experimental direct test for the Dupre equation - a central issue in surface science - thereby bearing the potential to radically change our understanding of the Young-Dupre equation thereby leading again to a transformative research. This instrument will be used to study drops on surfaces and also any other case in which two orthogonal retention forces may influence each other.
Intellectual Merit A new instrument, Centrifugal Adhesion Balance (CAB), was established and broke ground in surface science. The CAB, allows measurement for properties that were only assess theoretically before. For example, it enables: 1. Measurements of the adhesion force at the solid liquid interface (normal adhesion). 2. Measurements of the lateral (shear) force at any normal force (for example, one can induce shear at zero gravity or at another arbitrary gravity). 3. Relate the contact angles (advancing, receding, and as placed contact angles) to the lateral needed to slide a drop on a surface at a given normal forces. This measurements can allow the calculation of the interfacial modulus, an important interfacial property. None of these could have been done before. All there were done with the help of this NSF grant. Broader Impact A company, Wet Scientific (see Wetscientific.com) commercialized the modified CAB and sales began already at the end of 2013. The CAB is used for other commercial applications in diverse fields ranging from contact lenses to mist eliminator wires (a wire mesh common in the chemical engineering industry). Technical Aspects The ability of the CAB to perform such measurements and obtain such data is by decoupling the normal and lateral forces. This is achieved by two synchronized motors. One centrifugal (parallel to the earth) and the other at the end of the centrifugal arm, which tilts the surface with respect to gravity in the radial direction. The two motors are synchronized through a prescribed function so that they move at the same time to provide the desired conditions (such as constant normal force at changing lateral force or vice versa). References: 1. R. Tadmor, "Misconceptions in Wetting phenomena", Langmuir, 29, 15474, (2013). 2. Patent number 8,447,531 "Device for measuring retention force" (accepted on May 21st 2013) R. Tadmor sole inventor. 3. A. Leh, H.E. N'guessan, J.-G. Fan, P. Bahadur, R. Tadmor, Y.-P. Zhao, "On the Role of the Three-Phase Contact Line in Surface Deformation", Langmuir, 28, 5795, (2012). 4. H.E. N'guessan, A. Leh, P. Cox, P. Bahadur, R. Tadmor, P. Parta, R. Vajtai, P.M. Ajayan, P. Wasnik "Water tribology on graphene", Nature Communications, 3, 1242, (2012). 5. R. Tadmor, "Approaches in wetting phenomena", Soft Matter, 7, 1577, (2011).
