Friction is a common phenomenon, frequently experienced in daily life. The detrimental effects of friction are easy to point out: the squeaky door caused by unnecessary rubbing of the metal hinge parts, the heavy clothes dresser that is difficult to move across the floor, tire tread and brake pad wear caused by friction during abrupt car braking. Despite numerous negative examples of friction, there are also many devices and tasks dependent on friction. Maintaining traction on an icy sidewalk, warming one’s hands by rubbing them together, and playing a violin are just a few examples of the positive effects of friction.
Tribology is the study of friction, wear, and lubrication, and like many other scientific fields, historical tribology has grown out of the complex relationship between scientific curiosity and technological need. Mediating friction for technological improvement motivates better scientific understanding, which in turn brings about novel applications that make use of the newly-discovered fundamental concepts.
The goal of my research is to seek out a better understanding of the fundamental mechanisms of friction. I use the atomic force microscope (AFM) to determine nanoscale constitutive laws for frictional contacts. In my graduate work I studied the frictional mechanisms of self-assembled monolayers, such as octadecyltricholorsilane (OTS).
In this research, we work to uncover a more fundamental understanding of friction using the Atomic Force Microscope (AFM).
The schematic drawing above shows an AFM tip, typically with a 10-100 nm radius of curvature, attached to a compliant cantilever spring. This tip is brought into contact with nominally flat surface. At low applied loads, the tip can form a nanometer-scale single contact point (an “asperity”) with the sample surfaces, thus providing, in principle, a well-defined interface. In practice, careful measurement of the tip shape is needed since substantial variations in tip size and shape are common for commercial AFM cantilevers. The cantilever’s deflections are recorded using, most commonly, a reflected optical beam. These deflections are converted to forces by using Hooke’s Law. The normal and lateral forces can be measured with sub-nanoNewton precision, provided that appropriate calibration methods are used. The tip is rastered over the surface with sub-Ångstrom displacement precision in x, y, and z using piezoelectric scanning tubes.
A brand new state-of-the-art AFM was installed in the fall of 2007 in the Flater Nanotribology research lab. This system is designed and manufactured by Veeco Instruments in Santa Barbara, CA. This AFM, as pictured below, is a MultiModeV NanoscopeV AFM.
AFM tip deconvolution methods: "Pursuing improved atomic force microscope tip shape quantification methods at Luther College"
Undergraduate researchers: Isaac White (2007 - 2009), Braulio Dumba (2008 - 2010), George Zacharackis-Jutz (2010-2011)
An important aspect of friction research is characterization of both sides of the tribological interface. For nanotribology studies usingthe AFM, it is crucial to determine the physical and chemical properties of both the surface and the AFM tip. Often the tip can be received from the manufacturer in unacceptable condition, with a tip apex that is broken or blunt. Techniques to determine tip geometry include direct imaging with a Scanning Electron Microscope or Transmission Electron Microscope (TEM), and indirect methods.
Tip deconvolution is an indirect method to determine tip geometry. Imaging sharp features on a sample surface with an AFM tip results in an inverse image of the tip itself. If the surface feature is not infinitely sharp (which no real surface can be), mathematical methods must be used to extract an upper bound to the tip geometry.
The goal of this project is to compare TEM tip imaging with the indirect tip deconvolution method to quantify both techniques, as well as to determine the pros and cons of both methods. This information is important for future nanotribology research at Luther, as well as for the AFM community as a whole.
This work is published in Ultramicroscopy:
E.E.Flater, G.E.Zacharakis-Jutz*, B.G.Dumba*, I.A.White*, C.A.Clifford, "Toward Easy and Reliable AFM Tip Shape Determination using Blind Tip Reconstruction", Ultramicroscopy 146 (2014) p.130-143.
If you are interested is using our MATLAB-based blind tip reconstruction algorithms, they are available for free at nanoHUB.org.
Microscale investigation of bare and coated aluminum oxide surfaces
Undergraduate researchers: Luwa Matthews (2010-2012), Sarice Barkley (2010), Erik Linn-Molin (2012-2013), Steve Sorenson (2013-2015), Jesse Hitz Graff (Fall 2014-)
Research/senior project student advising