Measuring Van der Waals forces more effectively

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2011

The new technique uses a beam of atoms incident on a nanograting.

KEY CONCEPTS
• Van der Waals forces between two interacting neutral particles are very weak and very difficult to measure.
• A new technique measures Van der Waals forces with improved precision using a beam of atoms incident on a nanograting.
• For the alkali metals tested, empirical results could not be explained by models just accounting for valence electrons. A more sophisticated theory, including core electrons and valence electrons is needed.

Van der Waals (VdW) forces are very weak but can become a very important factor, particularly in studying phenomena at the nanoscale. For example, VdW forces affect how friction behaves at the nanoscale; therefore, better measurements of VdW forces will lead to a better understanding of this important tribological property.

In a previous TLT article, work was discussed that friction is dependent upon the thickness of atomic layers encountered at the nanoscale (1). As the number of atomic layers increases from one to around four, friction decreases but then levels off when additional layers are added. This work was done through the use of atomic force microscopy (AFM). The researchers noted that the reason for higher friction with a single atomic layer is due to an attractive VdW force between the AFM tip and a single sheet of atoms causing the sheet to bend, leading to more contact area and higher friction.

Associate professor Alex Cronin in the department of physics at the University of Arizona in Tucson, explains the origin of VdW forces. He says, “A weak attractive force can form between two neutral particles that interact with each other. This occurs because quantum mechanical fluctuations can allow a neutral atom to develop a fluctuating dipole moment so that it can become attracted to another neutral atom. The same mechanism leads to an attractive VdW force between a single atom and a surface (made up of many atoms) as well as between two surfaces.”

VdW forces are difficult to measure because the interaction is weak compared to electrostatic interactions and covalent bonds. Cronin says, “Most molecular bonds are stronger than VdW forces.”

Development of more effective techniques to measure VdW forces will enable researchers to gain a better understanding of interactions at the nanoscale.

CONTRIBUTION OF CORE ELECTRONS
Dr. Vincent Lonij, who recently obtained his doctorate in physics from the University of Arizona, worked with Cronin and other co-workers to develop a new technique to measure VdW forces with improved precision. The improved precision allowed them to detect, for the first time, the impact of atomic core electrons (not just valence electrons) on VdW forces.

The researchers focused on determination of the C₃ VdW potential coefficient. This potential coefficient measures the strength of the interaction between an atom and a surface.

Lonij says, “The VdW potential is inversely proportional to the cube of the distance between the atom and the surface. This is why the coefficient is referred to as C₃.” The inverse cube dependence of the magnitude of the VdW force explains why VdW forces are only apparent when two objects are in very close proximity to each other.

The researchers studied the VdW forces that occur when a beam of atoms incident on a nanograting. Lonij says, “Atoms of an element such as sodium are evaporated in an oven and allowed to escape through a 50-micron diameter aperture to produce a beam of atoms. This beam is then incident on a nanograting made from silicon nitride.”

The nanograting consists of a series of regularly spaced slits. “It looks like a miniature picket fence,” says Lonij, “where the distance between openings is 100 nanometers and each slit opening is 50 nanometers wide.” The experiment is sensitive to VdW forces that attract the atoms to the silicon nitride surface of the nanograting bars.

This approach relies on quantum mechanics that indicates atoms can behave both as particles and as waves. For this reason, a beam of atoms, just like a beam of light, can diffract from a set of periodically spaced openings.

Lonij says, “The propagation of this atom-wave through the grating slits is affected by the VdW interaction just like a beam of light is affected by the index of refraction of a medium. We are able to measure C₃ by studying how the interaction between the atoms and the grating bars affects the paths of diffracted atoms.”

Figure 1 shows the experimental setup used by the research team. In doing this work, the researchers determined ratios of C₃ values for pairs of the following four alkali metals: lithium, sodium, potassium and rubidium.


Figure 1. A new technique more precisely measures VdW forces using the experimental setup shown. (Courtesy of the University of Arizona)

Cronin says, “We found that it was easier to determine the C₃ ratios for the alkali metals tested. This is the first time that this ratio was empirically measured with a high level of accuracy.”

Alkali atoms each have one valence electron, which is why these atoms are often described using a single electron model. However, the empirical results generated by the researchers were inconsistent with this simple description. Cronin says, “We found that one electron models just accounting for the valence electron are far off the mark from our results.” Instead, a more sophisticated theory, including core electrons and valence electrons, was needed to explain the results.

Future work will involve applying this technique to other atoms such as strontium and ytterbium. Cronin adds, “We also want to look at specific molecules such as benzene that are nonpolar yet still interact with surfaces because of the VdW force. Benzene is particularly interesting because the VdW force will depend on whether the ring is oriented parallel or perpendicular to the surface.”

One future potential area of research is to determine if (non-contact) friction between atoms has an effect on VdW forces. Cronin says, “In all of our experiments, we did not observe any energy dissipation due to friction. We are not sure if VdW friction exists, but we may look into whether it has an impact in slowing down the velocity of an atom as it approaches a surface.”

Further information on this research can be found in a recent paper (2) or by contacting Cronin at cronin@physics.arizona.edu.

REFERENCES
1. Canter, N. (2010), “Size Does Matter for Nanoscale Friction,” TLT, 66(8), pp. 10–11.
2. Lonij, V.P., Klauss, C.E., Holmgren, W.F. and Cronin, A.D. (2010), “Atom Diffraction Reveals the Impact of Atomic Core Electrons on Atom-Surface Potentials,” Physical Review Letters, 105(23), pp. 233202.


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.