Friction, lubrication, and wear between two moving surfaces in contact with one another are important in applications ranging from car engines to non-stick coatings. Typically, when two surfaces are pressed together with increasing force, the lateral force required to slide the two surfaces apart increases. Recent work has discovered cases where this is not true, i.e., as the two surfaces are pressed together with increasing force it becomes easier to slide the surfaces apart. This unexpected behavior has been observed in the case for some liquids sliding in contact with select solid surfaces. Since liquid lubrication on solids is a critical component of many systems (i.e., oil lubricating a car engine), understanding this effect could be of great benefit technologically and to society. The goal of this project is to better understand the behavior of liquid drops on surfaces and to develop a better method for predicting the forces exerted on a liquid-solid surface when it is in motion. A new property for describing the interaction of has been proposed, namely interfacial modulus. The work will be used as the basis for a number of educational outreach activities to high school students and to provide research experiences for undergraduate students including a focus on students from under-represented groups.
Tribological science teaches that lateral frictional force increases as the contact area between two surfaces increases and that frictional force is proportional to normal force. However, recent work by this research group has uncovered a system that violates this basic principle. It has been demonstrated that for some liquids on a solid surface, lateral force decreases with normal force despite the fact that the contact area between the two surfaces increased. The discovery was enabled by a new instrument, the Centrifugal Adhesion Balance (CAB), that allows for independent control and measurement of the normal and lateral forces for two surfaces in contact. A systematic study will be conducted whereby the relationships between surface chemical and physical properties to the interfacial modulus and to surface damage will be established for a set of model fluid/solid systems. Surface roughness is expected to be a critical parameter and a range of nanostructured surfaces are included in the study.