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 accumulate enough charge. 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 charges negatively with friction, partially functionalized with N-trimethoxylsilylpropyl-N,N,N-trimethylammonium chloride which charges positively. The second surface was that of a sphere which was free to roll on the glass - either a conducting one of stainless steel or an insulating one when coated with waterproofing spray. This sphere was also part of an apparatus for measuring surface charges: its motion was caused by a rotating bar magnet while an electrometer connected to the planar surface measured capacitively the total 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). They then 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 value (Fig.2). To study the phenomenon in a little more detail, the authors measured the surface charges in relation to the total and individual area of the treated patches on the glass (see Fig. S1 for the patterning process). They concluded that while the characteristic area of each patch did not matter, the total accumulated charge correlated positively with the difference between 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 minimal. Qualitatively similar results were obtained with stainless spheres and with acrylate-coated spheres (insulating surfaces), however stainless spheres proved better at the prevention of charge accumulation, probably due to their better conductivity which makes it more likely that charges can find a pathway to ground.
Fig.S1 Procedure for patterning glass with positively charging (red) and negatively charging (blue) regions. The patterned surfaces (schematically illustrated at the bottom) were circles (on a hexagonal grid) of positively charging, silanized glass surrounded by negatively charging, plasma-oxidized glass. (From Supplementary Information on .)
Fig 1. Representative charge vs time due to contact electrification from (a) a steel sphere rolling on clean glass or (b) an acrylate-coated steel sphere rolling on glass silanized. (c) Structure of cationic siloxane used. (From .)
Fig 2. (a,b) Rate of charging of a rolling steel sphere (a) or an acrylate-coated sphere (b) as a function of the percentage of the glass surface that was silanized. Each data point is the mean of 7-8 measurements at RH = 15-20%; the lengths of the error bars represent the standard deviations of these means. (c-h) Representative traces of contact electrification between a sphere and a glass slide silanized on 25%, 50%, or 75% of is surface area. Vertical arrows indicate electrical discharges. (From .)
This proposed approach to the prevention of sparking has several advantages and some disadvantages: it does not depend on the bulk properties of the materials in contact, however it does require that those materials be amenable to functionalization as well as to partial functionalization, and it need be optimized for each pair of surfaces; it is based on functional groups which are bonded very strongly to the surfaces of interest (covalently); and the functionalization process can be performed on large scale (should the surfaces be amenable to it). Nonetheless, in all, it seems a promising approach.