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 George Whitesides, 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.
Figure 1 above demonstrates control conditions. To put perspective on the study, take the mixing fuel tank: the sphere is fuel and the surface is the container of the tank or the pipes that deliver the fluid to the tank. To reduce this charge build up, the authors propose a patterned "mosaic" of positively and negatively charged regions on the surface of the plane to suppress the build up of charge on the rolling sphere. Essentially, and certainly not to take away from the work... just to describe the experiment in a simple way, the authors propose that patterning the glass surface leaving some bare (negatively charged) and coating some with 1 (positively charged) will reduce build up of charge on the sphere. The logic, I suppose as I think of it, is like having a floor with patterned hot and cold regions so your feet neither become too cold nor too hot. As the authors describe the process, "Because the sphere acquired both positive and negative charges, it accumulated net charge more slowly than it did on either homogeneous surface."
1 was patterned on glass in a hexagonal array of circular posts on a PDMS stamp which protected the glass (for the bare, negatively charged part of the surface). Using air plasma oxidization on the exposed areas of the glass, the silanol groups of the glass were regenerated. This procedure, which is more clearly detailed by the authors, was used to produce wafers that were 25%, 50%, and 75% silanized with 1; of course, the more silanized the surface was with 1, the more positive charge was one the surface. The full results of the experimentation are shown in Figure 2 below. The interesting data comes from the 50% silanized surface where charge accumulation leveled off at 100-200 pC which, as the authors report is 4-5 times less than the minimal charged required to cause discharge (a spark!). See Figure 2:
Image not very clear, find paper at: http://pubs.acs.org/doi/pdf/10.1021/ja902862b for clearer images.
The first column (on left) is the stainless steel bare ball rolled on the different silanized wafers. As mentioned before, at the 50% silanization level, minimal net charge is experienced on the ball whereas at 25% and 75% silanization, there is a net positive and net negative charge build up on the ball approaching the dangerous discharge threshold, or at least substantially closer to discharge than when the the surface is 50% silanized. I put a red box around the 50% silanized experiment to emphasize that it appears the hypothesis of the authors is supported and especially so for this percentage of silanization; this makes sense intuitively: half the surface is positively charged and half negatively charged, thus the charges are 'canceled' or the ion-transfer is net of (approximately zero). Similar results are observed for the coated ball but where however exaggerated; the authors propose that this is due to the decreased conductivity of the coated balls. Still though, suppression of charge build up was achieved with the coated ball and it, like with the bare ball, "correlated linearly with the percentage of glass silanized"."
The authors note 4 unique characteristics of their method for suppressing charge build up: "(i) it does not require any of the materials to be conductive; (ii) it relies on functional groups that are covalently bound to one of the contacting materials and should therefore be less resistant to wear than topical antistatic coatings; (iii) it relies only on surface chemistry; the bulk properties of the contacting materials remain unchanged; (iv) the process to pattern the surface is simple and easy to perform on large surface areas." The third and fourth points are crucial for practical use of such a surface treatment method. Just about all materials used in the design of devices big or small are very well studied and understood. There is no way that a "new material" would be deemed acceptable for mechanical use within a reasonable time period and hence surface modifications that do not impact the bulk properties of the material are much more acceptable and apt to be used. Also, the ease of this method is similarly crucial, and the covalent nature of the treatment suggests the longevity of surface modification would surpass current methods of charge suppression. However, as the experimentation suggests, and the authors responsibly note, the charge suppression is varied among materials (bare steel versus coated steel) so particular modification to the surface of a material would need to be considered given what is rubbing (or flowing... or rolling) against it.
Connection to Soft Matter
In lecture we discussed the wet contact versus the dry contact and resulting build up of contact charge (or tribocharge). It was noted that dry contacts have no voltage where as wet contacts have a change in voltage, therefore "the electrolytic currents can do work." Essentially, the dry contact will result in no electric 'shock' or discharge; however, in reality we live in a very wet-contact world. Humidity in the experiments in this paper was maintained at 15-20%. This wasn't arbitrary but done to maintain a relatively constant medium for discharge. Zero humidity would have resulted in perpetual charge build up (like in space for example) which is unrealistic for our more terrestrial purposes. Humidity is crucial for contact charge build up and discharge, its why there are huge lightning storms in the summer when a front of humid air comes through. Similarly, all the systems I described in the beginning of the wiki entry involve humidity and consequently, wet contacts. Take the fuel mixing tank as an example, why is there a potential created between the container surface and the moving liquid or vapors? Because their contact is wet and this resulting potential can result in discharge or spark (...sparks and fuel vapor don't mix well... they blow up). The authors here present a means of reducing this potential in what seems to be a dry contact situation but, as we can see because of humidity, we're really talking about a wet contact problem (and just about everything is a wet contact problem!).
More Thoughts: (call me crazy but...) I wonder a little about the body when we start to think about wet contacts and the resulting potential. Ok, so there aren't too many flammable vapors in our body but there certainly are some (think end of digestive track). We know there are numerous potentials generated in our body as well (think heart, neural, etc) created by the flow of ions. But are these the only potentials generated? Like I mentioned, we are full of wet contacts and therefore maybe there are contact voltages and charges created all the time. Since we are mostly water, its possible any spark and fire created in our body is put out by this water but I don't think this is happening (is heart burn really a fire burn!?). Of course this isn't the case but since there is a possibility that we are creating contact charges, are the surfaces in our body then equipped to reduce charge build up... sort of a natural 'surface modification' like the authors of this paper propose? Lets take the intestines, and face the facts that they are full of flammable moving gas. As this gas moves, does it acquire a tribocharge? If it does, is it then possible that the surface of the intestines it comes in contact with is patterned in a "mosaic" of nature positive and negative regions which minimizes the charge build up? If not, we can imagine how painful it would be if a the charged gas discharged within us and caught fire. Since this doesn't happen, its possible that the surface modifications done in this experiment have already been 'done' in our body to reduce charge build up... our bodies are way ahead of us in terms of what we know...