Lubricating Polymer Brushes

Since its invention in 1986, the AFM has allowed us to study the interaction between approaching surfaces at length and force scales that were previously unattainable. This has aided many fields including nanotribology, where the science of adhesion, lubrication, friction and wear can now be understood at the sub-nanometer scale. The field of nanotribology influences many applications, from the wear of micro/nanoelectromechanical systems (MEMS) devices to the design of water-based polymer brush coatings for biomedical devices. The ability to understand atomic forces that act normal to these lubricating polymer brushes (Figure 1 (a) and (b)), specifically the role of water in and around these systems will be essential in further understanding lubrication at the nanoscale.

Through FM-AFM measurements we can explored the nanotribological properties of both simple and complex systems, including the confinement of a simple non-polar liquid molecule between the AFM tip and surface. For the confinement of a simple liquid we observe oscillatory force profiles, where the spacing between the oscillations are inherent of the size of the liquid molecule and the dissipation processes show a monotonic increase.  We anticipate that the water molecules in and around these lubricating polymer layers show a similar oscillatory behavior, helping to explain the role structured water plays in the low friction behavior that has been reported in the literature.

Figure 1: (a) 150 x 150 nm and (b) 45 x 45 nm height image of polymer brush coated mica surface as imaged using the FM-AFM technique.

Figure 1: (a) 150 x 150 nm and (b) 45 x 45 nm height image of polymer brush coated mica surface as imaged using the FM-AFM technique.

References

  1. Force microscopy, Binnig, G., Ultramicroscopy, 42-44, 7-15, (1992).

  2. The nonlinear nature of friction, Urbakh, M., Klafter, J., Gourdon, D., and Israelachvili, J., Nature, 430, 525-528, (2004).

  3. Nanotribology, nanomechanics and nanomaterials characterization, Bhushan, B., Philosophical Transactions of the Royal Society of London, 366, 1351, (2008).

  4. Sweet, hairy, soft, and slippery, Lee, S., and Spencer, N., Science, 319, 575-576, (2008).

  5. Artifact-free dynamic atomic force microscopy reveals monotonic dissipation for a simple confined liquid, Kaggwa, G. B., Kilpatrick, J. I., Sader, J. E., and Jarvis, S. P., Applied Physics Letters, 93, 011909, (2008).