New type of friction observed at the nanoscale

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2013

A third mechanism for friction has been discovered that holds a polymer chain in the lateral direction.

KEY CONCEPTS
• Slip and cooperative stick are two friction mechanisms already identified for a single polymer present at a solid-liquid interface.
• A new third type of friction known as desorption stick has been discovered; it is a low, two-dimensional barrier that holds a polymer chain in the lateral direction.
• There is a kind of competition among the three types of friction involving two factors (the solvent and the substrate) that determines which type of friction will occur in a specific situation.

CONTINUING RESEARCH TO DETERMINE HOW FRICTION occurs at the nanoscale is extremely important to facilitate the development of more efficient machinery. Friction remains as a constant challenge for those in the lubricant industry. In a previous TLT article, we noted that the cost of friction remains enormous and is estimated in Germany to represent about 8 percent of that country’s GDP or 200 billion Euros (1).

In that same article, research was discussed showing how friction occurs on the microscopic length scales through use of an empirical approach. The researchers developed a model for friction by passing a layer of charged, carboxylated polystyrene spheres suspended in water with a small radius of 1.95 micron through a light-created surface. The latter was created by spatially extending interference patterns of different geometries superimposed through up to five laser beams.

It was discovered in the interaction of particles from these two crystalline surfaces that compression zones and expansion zones, known as kinks and antikinks, respectively, were formed. This study indicates that kinks and antikinks are both important in gaining a better understanding of friction.

Small mechanical devices, known as microelectromechanical systems (MEMS), strongly need low friction surfaces and coatings so that they can operate effectively. Professor Thorsten Hugel, IMETUM and Physik Department at Technische Universitaet Muenchen in Munich, Germany, says, “Polymer coatings play an important role in the design of functional substrates (e.g., to reduce friction). In addition, polymers are the main components in some lubricants.”

Two known friction mechanisms have been identified for a single polymer present at a solid-liquid interface. Both of these mechanisms are based on experiments using an atomic force microscope (AFM).

The first one is slip which is characterized by a constant, small vertical force holding the polymer onto a surface until it eventually detaches. Cooperative stick is the second one and represents the formation and breaking of chemical bonds between the polymer and the surface. The term is used because the bonds are formed and broken in a cooperative manner.

Further work by Hugel and his associates with a single polymer chain has led to the discovery of a third mechanism for friction at the nanoscale.

DESORPTION STICK
Hugel and his fellow researchers have identified a new type of friction at the nanoscale, which they have named desorption stick. He says, “We discovered desorption stick because we have been able to measure the friction of a single polymer molecule at interfaces. This has not been possible before because the lateral sensitivity of an AFM is not high enough to detect single polymer events. We managed to use the much more sensitive vertical signal of the AFM to decouple the frictional force from the desorption force.”

The researchers used the AFM in an experimental approach that measured both friction and adhesion forces in one experiment while decoupling them. A specific polymer molecule on a surface is pulled up to a certain height by retracting the AFM cantilever tip more than 100 nanometers. At that point, frictional forces are measured as the AFM pulls the polymer molecule in a perpendicular fashion to the surface. Figure 1 shows a polymer chain linked to the tip of an AFM.


Figure 1. The ability to link a polymer chain to the tip of an atomic force microscope led researchers to identify a new type of friction at the nanoscale that is known as desorption stick. (Courtesy of Technische Universitaet Muenchen)

Hugel considers desorption stick to be a low, two-dimensional barrier that holds (or confines) the polymer chain in the lateral direction but does not prevent the polymer from being pulled away from the surface in the third dimension. He says, “In contrast, slip is a one-dimensional confinement in which the polymer is free to move in the plane of the surface. With cooperative stick, the polymer will remain confined in a three-dimensional fashion to the surface in its exact position until some bonds rupture.”

The researchers spent many months ensuring that desorption stick is a newly discovered frictional force. Hugel says, “We produced force-extension traces but could not explain them with previous models, even though they were reproducible.”

During this process, a series of polymers, solvents and surfaces were used to study this phenomenon. Among the polymers studied were polyglutamic acid (PGA), polystyrene (PS) and a PS-based polymer known as PSPI. Surfaces used include diamond, polytetrafluoroethylene (PTFE) and structured gallium arsenide (GaAs). Solvents evaluated were water and chloroform.

Hugel explains the choices made, “PGA is used because it is a charged, hydrophilic polymer. PS is uncharged and hydrophobic. The PS-based PSPI was added for technical reasons. PTFE and GaAs differ largely in their wettability. Diamond was used because diamond- like carbon coatings are probed for low friction interfaces, and this surface can easily be modified with various terminations.”

A change in the solvent used with a specific polymer can lead to a different friction mechanism. Hugel says, “Use of PS in an incompatible solvent such as water leads to a dense polymer conformation on the surface and results in a desorption stick mechanism occurring. If the PS is placed in the compatible solvent chloroform, the polymer takes an extended conformation in solution and friction occurs mainly by a slip mechanism.”

The prospect of three friction mechanisms taking place simultaneously for a specific system suggests that one mechanism may dominate. Hugel says, “Yes, there is a kind of competition taking place. In a compatible solvent and on homogeneously hydrophobic substrates, slip is observed without stick events. For noncompatible solvents, desorption stick occurs independently of the substrate. Intermediate cases lead to a significant and dominating amount of desorption stick.”

Future work will involve the researchers characterizing desorption stick on more complex surfaces that will primarily be polymer coatings. Hugel adds, “We are also looking to better understand how to predict macroscopic friction behavior based on our nanoscale measurements.”

Additional information on the discovery of desorption stick can be found in a recent reference (2) or by contacting Hugel at thugel@mytum.de.

REFERENCES
1. Canter, N. (2011), “Observation of Friction on Microscopic Length Scales,” TLT, 68 (4), pp. 8-9.
2. Balzer, B., Gallei, M., Hauf, M., Stallhofer, M., Wiegleb, L., Holleitner, A., Rehahn, M. and Hugel, T. (2013), “Nanoscale Friction Mechanisms at Solid-Liquid Interfaces,” Angewandte Chemie International Edition, 52 (25), pp. 6541-6544.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.