# Presentation: 056147

## Correlation of Topography with Friction in Alkane FilmsH. Taub, University of Missouri-Columbia, DMR-0411748 

-      Alkane molecules are the principal constituents of commercial lubricants such as engine oils.  Our current research deals with what makes alkane films good lubricants.  How do microscopic properties such as the alkane molecule’s shape, orientation, and flexibility affect  the film structure?   And how is its structure correlated with the frictional force exerted on a nanoscale object sliding over the film?

-        We have used Atomic Force Microscopy to map the topographic landscape of a solid alkane film coating a silicon wafer used in electronic device fabrication.  Height steps  as small as one nanometer (1 nm = 109 m) can be resolved.   The image in (a) is complex consisting of a bulk particle (white) and molecular layers in which the rod-shaped molecules are oriented with their long axis perpendicular (orange and yellow) or parallel to the surface (brown).  A close-up of a bulk particle is shown in (b).

- J. Chem. Phys., 1 Oct. 2005

-       Despite its complex topography, the solid alkane film has only two principal levels of friction:  a lower level atop features in which the molecules are oriented perpendicular to the surface and a higher level on layers in which the molecules are oriented parallel to the surface.  AFM studies of the wetting behavior of fluid alkane films are now in progress.

- (a)

- (b)

**Notes:**
	Alkanes (chemical formula CnH2n+2) are flexible chain-like molecules that are the principal constituents of commercial lubricants such as engine oils.  A microscopic understanding of the structure and dynamics of very thin alkane films could potentially lead to significant advances in developing more effective and durable lubricants.

	Our current research is focused on trying to understand at the atomic level what makes a molecule a good lubricant.  How do microscopic properties such as the alkane molecule’s shape, orientation, and flexibility affect the film structure?  How do these microscopic properties determine the structure of the solid and liquid film?  And how is the film structure correlated with the frictional force exerted on an object sliding over the film?.  These questions are addressed more easily in a solid alkane film that we have studied initially.  

	With Atomic Force Microscopy (AFM), we have measured the topographic landscape of a solid alkane film coating a silicon wafer used in electronic device fabrication.  The molecule that we have selected, dotriacontane, is an intermediate-length alkane with carbon number n = 32.  In the AFM images, one can resolve height features as small a one nanometer (1 nm =109 m).   The image in (a) is complex and contains bulk particles (white) and molecular layers in which the rod-shaped molecules are oriented with their long axis perpendicular to the surface (orange and yellow).  The lowest level (brown) is the top of a layer of molecules oriented parallel to surface.  The image in (b) shows a closer look at a mesa-like bulk particle in a solid dotriacontane film, revealing a terraced structure on top.  The depth of a terrace is about equal to the length of the fully extended  dotriacontane molecule, 4 nm.

	Despite its complex topography, the solid dotriacontane film has only two principal levels of friction:  a lower level atop features in which the rod-like molecules are oriented perpendicular to the surface [white,orange, yellow in (a)] and a  higher level on layers in which the molecules are oriented parallel to the surface (brown).  We conclude that the molecular orientation is the dominant factor in determining the magnitude of the lateral frictional force acting on a nanoscale object sliding over the surface of the solid film.  AFM studies of the wetting behavior of fluid alkane films are now in progress. 

	This work will be published in the October 1, 2005 issue of the Journal of Chemical Physics. 

## Correlation of Topography and Friction in Alkane FilmsH. Taub, University of Missouri-Columbia, DMR-0411748

-      Shown in the photo are undergraduate student (Matthew Simpson) and  postdoc (Sven Trogisch) who conducted the AFM measurements.  Simpson is the recipient of a Goldwater Scholarship and is planning to pursue a combined physics Ph.D./M.D. program as a graduate student.  Dr. Trogisch has accepted an industrial research position in Dresden, Germany.  Currently, postdoc Mengjun Bai is working with Simpson on AFM studies of the wetting behavior of fluid alkane films.  Armand Diama is a graduate student performing quasielastic neutron scattering experiments on alkane films.  His measurements investigate the conformational changes of the alkane molecules which may affect the film’s lubricating properties.

**Correlation of Topography and Friction in Alkane Films**H. Taub, University of Missouri-Columbia, DMR-0411748

**Societal Impact****:**
-      Alkane molecules are the principal constituents of commercial lubricants.  A microscopic understanding of the structure and dynamics of very thin alkane films could lead to the development of better and more durable lubricants in applications ranging from computer hard drives to automobile engines.