Controlling the Kinetics of 'Contact Electrification' with Patterned Surfaces
Original entry by Andrew Capulli, AP225 Fall 2011
"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
All it takes is a single spark... There are numerous examples of tragedy from the discharge of contact charged (tribocharged) surfaces. Be it the explosion of fuel transfer systems or helicopters landing, these sometimes 'playful' static charges we're all familiar with by rubbing our feet on carpet and shocking each other, can turn into costly and very deadly 'shock' discharges. NASA follows a strict "triboelectrification rule" which grounds any mission if the clouds a shuttle is predicted to fly through may result in a potentially unsafe surface charge on the vehicle (see triboelectric effect: http://en.wikipedia.org/wiki/Triboelectric_effect). There must then be a means of controlling the triboelectric effect... minimize its effect on contacting materials so as to reduce charge build up which would then reduce the deadly discharge. This is the goal of the authors as they provide a means of surface modification based on the ion-transfer mechanism. The ion-transfer mechanism can be briefly summarized in Figure 1 below. Figure 1 is taken from another paper by Professor Whitesides entitled: "Ionic Electrets: Electrostatic Charging of Surfaces by Transferring Mobile Ions upon Contact" and can be found at: http://gmwgroup.harvard.edu/pubs/pdf/988.pdf. Essentially, a surface with covalently bound ions is neutralized by the 'mobile counterion'. If another surface comes in contact with the given surface, the mobile counterion is 'transferred' to this other surface and consequently a net charge is left on each surface (see Figure 1 below):
Suppression of Net Charge
The authors aim to "suppress the net contact electrification of surfaces and electrical discharges between them" via the use of patterns of oppositely charged functional groups on the surface of a material. Using the "rolling sphere tool" (RST) and maintaining the humidity (15-20%), the Whitesides et al manipulate surface chemistry to reduce accumulating charge. The RST is described briefly in the paper: it is essentially an electrode that measures the the charge on a planar surface and the sphere as the sphere rolls (and accumulates charge via the ion-transfer mechanism). Positively charged self assembled monolayers of ammonium-terminated siloxanes (N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (which the authors refer to as "1"... and I will as well) and the negatively charging glass surface were used as the as the "oppositely charged" functional groups aimed to suppress the net charge build up on the rolling sphere. Two types of rolling spheres were used: bare steel as well as insulator coated steel (acrylic waterproofing spray).
Figure 1 below shows the charge vs time data of the bare steel ball and the coated steel ball. As evident by the graphs, the bare steel ball rolling on the glass surface accumulated charge which was then discharged to the ambient air approximately every 7s. The coated ball (b) rolled on a surface 100% silanized with 1 and had less of a dramatic profile although charge build up was observed.