Forcing the details of contact charging.

Forcing the details of contact charging

When two different surfaces are brought into contact and then separated from each other, they often end up oppositely charged. Such electrostatic charging occurs when balloons rub against sweaters, shoes shuffle across carpets and toner particles in photocopiers bump into carrier beads. Now, with the aid of a novel instrument for pinpointing the location of small amounts of excess charge on an insulating surface, researchers are getting their best look yet at what happens during contact electrification.

Despite its everyday occurrence and technological importance, contact electrification has long mystified scientists. The main questions concern whether two materials in contact exchange electrons, ions or charged bits of material and precisely where those mobile charges end up when the materials are separated.

"If it were possible to identify such sites with near-atomic resolution, then a deeper understanding of the [contact] electrification process might result," Bruce D. Terris and his colleagues at the IBM Almaden Research Center in San Jose, Calif., write in the Dec. 11 PHYSICAL REVIEW LETTERS.

To locate charges deposited by a single contact between a metal and an insulator, Terris and his group use a specially modified force microscope. This scanning microscope's key element is an L-shaped piece of nickel wire mounted so that its sharp tip hangs over a sample. The wire vibrates with a characteristic frequency, and any forces acting on it change this frequency. A special modification of the basic instrument allows researchers to locate and distinguish between positive and negative charges in a single scan across a surface.

"It's a new way to look at [contact electrification]," Terris says.

Initial experiments involved contact between the microscope's nickel tip and a thin sheet of a polymer known as polymethyl methacrylate. Although results varied somewhat from trial to trial, the researchers found that charged areas were often substantially larger than the expected contact area and contained both positive and negative charges.

"This bipolar charging is a surprising result and, to our knowledge, has not been observed in previous [contact charging] experiments," they report.

At present, the force microscope can detect clusters of charge equivalent to three electrons with a spatial resolution of 2,000 angstroms. "Our hope is to develop more sensitive techniques so you can really look at specific sites on a surface to see where and what the charge is," Terris says.