Observation of friction on microscopic length scales

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2012

Researchers develop an empirical approach that shows how friction occurs at the atomic scale.

KEY CONCEPTS
• A newly developed empirical approach shows how friction takes place on the microscopic length scales.
• Friction was modeled through passing a layer of charged polystyrene spheres with a small radius of 1.95 microns across a light created surface.
• Some particles move closer together to form compression zones knowns as kinks, while other particles move further apart to form expansion zones known as antikinks.

FRICTION IS STILL A CHALLENGING ISSUE TO OVERCOME IN LUBRICATION SYSTEMS. The cost of friction remains enormous—in Germany it is estimated to be 8% of German GDP or 200 billion Euros.

Continuing work on determining how friction occurs on the atomic scale shows that it differs from the friction seen on the macroscale. This means that the friction generated as a macroscopic object slides across a surface differs from how one atomic layer slides across a second atomic layer.

Dr. Clemens Bechinger, professor at the Physikalisches Institut of the Universitat of Stuttgart and the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, says, “Typically, one measures friction between macroscopic objects by instruments such as the pin-on-disk tribometers. In this case, a disk of the material rotates, and a pin of another material is pressed on it from above. By a spring mechanism, the lateral force (i.e., friction) can be measured.”

On the atomic scale, measurement of friction has been conducted through the atomic force microscope (AFM). In a previous TLT article, researchers used the AFM in a technique known as friction force microscopy by sliding the tip of this instrument across atomically thin layers of materials such as hexagonal boron nitride (1).

That work showed that friction is a function of the number of layers of atoms encountered by the AFM tip. Friction decreased as the number of layers rose from one to four and then leveled off as more layers were added.

Actually witnessing how friction occurs at the atomic scale will provide further assistance for development of micro- and nanomachines. This goal has now been achieved.

KINKS AND ANTIKINKS
Bechinger and his fellow researchers developed an empirical approach to show how friction takes place on microscopic length scales. For that purpose, they used a monolayer of highly charged carboxylated polystyrene spheres with a radius of 1.95 microns, which are suspended in water. These spheres are all negatively charged and repel each other electronically to form a two-dimensional triangular crystal upon application of gravitational and optical forces. This serves as a model system of an atomic surface.

The second surface is also crystalline but is created entirely with light. Bechinger says, “We use the optical tweezing of light, i.e., the fact that a particle with a high index of refraction experiences a force toward regions of high intensity of a light field. We create spatially extended interference patterns of different geometries by superimposing up to five laser beams. They can be regarded as an optical potential landscape where the particles prefer to sit in regions with high laser intensity.”

The researchers pulled the layer of charged polystyrene spheres across the light-created surface. Bechinger says, “We consider this process to be analogous to pulling one egg carton over a second egg carton.”

As the two layers interacted with each other, the researchers expected that the particles in both layers will move simultaneously in a jerky fashion and then move back into their original positions after the layers separate. Instead, the two atomic layers behaved differently.

As shown in Figure 1, some particles moved while others did not. The blue spheres in Figure 1 slide together to form a compression zone, while the green particles remained in place on the surface. Areas where the particles moved closer together are known as kinks.


Figure 1. As two microscopic surfaces interact with each other, some particles move while others remain in place. The particles (shown in blue) slide together to form a compression zone known as a kink, while the particles (shown in green) remain in place on the surface. (Courtesy of the Universitat of Stuttgart and the Max Planck Institute for Intelligent Systems)

In other cases, particles moved farther apart as one layer passed over the other. This is a situation where an expansion zone was formed, which is known as an antikink.

Bechinger explains, “Kinks are regions where the particles are closer than on average (compression zone), while the opposite is true for antikinks. Such compression and expansion zones can move much more easily across a surface than if all the particles would keep the same distance while sliding across the surface.”

The formation of kinks and antikinks is due to energetic reasons. Bechinger adds, “It turns out that the energy required for formation of compression/ expansion zones is smaller than the energy gain when sliding the system with these local deformations across the surface.”

The use of the light crystalline surface provided the researchers with two important advantages in studying friction at the atomic scale. Bechinger says, “First, we can image this interface, which is normally hidden (e.g., in a pin-on-disk device or an AFM tip). Second, we can continuously vary the length scales and the symmetry of the substrate potential just by slightly changing the number and angle of incidence of the laser beams forming the interference pattern.”

Besides studying crystalline layers, the researchers also prepared an optical quasicrystal surface. Bechinger says, “Quasicrystals are perfectly ordered materials that lack periodicity. It is known from macroscopic experiments (e.g., pin-on-disk studies) that quasicrystals have a friction coefficient about one order of magnitude smaller than comparable crystals.”

The lack of periodicity in quasicrystals affects the formation and movement of kinks and antikinks across the surface. Bechinger says, “In the case of crystalline surfaces, kinks and antikinks run quite undisturbed across the entire surface. This is not true for quasicrystalline surfaces because they can become stuck at quasicrystalline surfaces.”

The observation of kinks and antikinks confirms theoretical predictions, which have previously claimed their existence on the atomic level. Bechinger says, “In addition, our experiments demonstrate how important they are for the understanding of friction at small-length scales.”

Future work will involve learning more about how friction depends on the size of the contact and how the added presence of vibrations affects the process. Bechinger says, “Due to the complex mechanism responsible for friction, we expect a nontrivial dependence of the friction force on the contact size. We intend to study vibrations because they are typically present in macroscopic systems once you start sliding them.”

Additional information can be found in a recent article (2) or by contacting Bechinger at c.bechinger@physik.uni-stuttgart.de.

REFERENCES
1. Canter, N. (2010), “Size Does Matter for Nanoscale Friction,” TLT, 66 (8), pp. 10-11.
2. Bohlein, T., Mikhael, J. and Bechinger, C. (2012), “Observation of Kinks and Antikinks in Colloidal Monolayers,” Nature Materials, 11(2), pp. 126-130.


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.