(180aj) Asymmetric Rubbing Experiments to Understand Contact Electrification of Polymeric Powders

Powders of polymeric materials are prone to static charging due to interparticle collisions (as well as particle-wall interactions) and their inherently high surface-to-volume ratios.  Curiously, theoretical and experimental results have shown that when these powders charge, there is a segregation of charge such that smaller particles charge negatively and larger particles charge positively.  In general, this can be explained by the difference in contact area between particles of different size.  However, it remains unclear what species are transferred between the contacted surfaces and how exactly this transfer takes place.

To better understand the contact charging of powders, we have recently set up asymmetric rubbing experiments¹.  Similar to a violin bow and string, two identical materials are rubbed such that one is contacted over a larger area (along its perimeter) while the other is rubbed over a smaller area (at a contact point along its surface).  Thus, we are able to control the contacting area similar to how particles of different size contact one another.  Experiments with Teflon and Nylon show that for Teflon, the larger contacted area charges positive, but for Nylon the larger contacted area charges negative.  From these results, we infer that in the case of Teflon, negatively-charged species (e.g. electrons or negatively charged ions) are transferred while in the case of Nylon positively-charged species are transferred.  Additionally, we find that the surface contacted over a larger area contains a spatial distribution of charge containing both negatively and positively charged regions, in agreement with a recent report².

1. R. Pham, R. C. Virnelson, R. M. Sankaran, and D. J. Lacks, J. Electrostatics 69, 456 (2011).

2. H. T. Bayetkin et al., Science 333, 308 (2011).