Controlling the Kinetics of Contact Electrification with Patterned Surfaces
Thomas, S.W., Vella, S.J., Dickey, M.D., Kaufman, G.K., and Whitesides, G.M., Journal of American Chemical Society, 2009, 131, 8746-8747
Tribocharging, i.e. the charging of surfaces brought in contact due to the exchange of ions between the materials they enclose, is a ubiquitous problem frequently associated with spark generation when the surfaces accumulated enough charge to discharge. The authors present a potential solution to the problem which was inspired by the observation that surfaces which contain ionic functional groups tend to accumulate the same charge as that of their less mobile ion. Hence, the proposed solution is based on the creation on a surface of oppositely charged functionalized patches, so that when another surface contacts the treated surface it acquires a much smaller net charge.
The experimental system consisted of a glass surface, which acquires negative charges with friction, partially functionalized with N-trimethoxylsilylpropyl-N,N,N-trimethylammonium chloride which acquires positive charges. The second surface was that of a sphere, either a conducting one of stainless steel or an insulating one when the sphere was coated with waterproofing spray, which was free to roll on the glass. This sphere was also part of the tool for measuring the surface charges: its motion was caused by a rotating bar magnet, while an electrometer connected to the planar surface measured capacitively the charge of the surfaces in contact.
The authors first measured the rate of charging in a system containing a stainless steel sphere on a uniform glass surface, both unsilanized and fully silanized. In both cases they observed regular sparking at a rate of about once every 7 sec (Fig. 1). Then they proceeded to measure the charging rate in a system where half of the glass surface had been functionalized. In this case the surfaces never accumulated enough net charge to lead to the dielectric breakdown of air, but instead the charge stayed safely below 10% of the limiting charge (Fig.2). To study the phenomenon in a little more detail, the authors measured the surface charged in relation to the total and individual area of the treated patches on the glass and concluded that while the characteristic area of each patch did not make a difference, the total accumulated charge correlated positively with the difference in treated and untreated surface area: if only a quarter of the glass surface was silanized the net charge on the sphere was positive; if three-quarters of the glass surface were silanized the net charge on the sphere was negative; and if half of the glass surface was silanized the net charge on the sphere was very little.