Tribocharging of Identical Materials
It was observed by Lowell in 1986* that identical polymers will charge one another when rubbed together asymmetrically - for example by rubbing a sphere on a plane. We illustrate here that identical polymers also charge one another when rubbed together symmetrically.
In the movie at the right, we show two common latex balloons that have been rubbed together symmetrically using method (A) described subsequently.
This experiment is easily reproduced, however electrostatics effects are notoriously variable, and this system is no exception**.    A few experimental notes may be helpful.
First, to support a balloon against its weight, it is obviously easier to use a smaller, lighter, balloon than a larger, heavier one. Up to 5" common latex balloons work well.
Second, the experiment does not work in the presence of glidants such as powders - and so using powdered latex gloves is not recommended: the powder from these gloves spreads readily. If the balloons become powdered, or are powdered during manufacture, they can be restored by washing in alcohol or in detergent and water and air drying.
Third, two methods for charging the balloons appear to work well.(A) One can press the balloons firmly together and twist them back and forth perhaps 10 or 15 times by a small angle: a squeaking sound should be produced. If squeaking is not heard, it is likely that a glidant is present from the balloon's manufacture, or contaminants may have accumulated on the balloon surface: these can be washed off as described above, or one can use the second method:
(B) One can rub the balloons together in either a circulatory or side to side motion. In this case, we have found that it may take a minute or so to build up sufficient charge to cause the balloons to stick - be patient.
Finally, balloons appear to charge best if they are maximally inflated.
For the student, we remark that the cause of this effect remains mysterious, and there is considerable room for future investigation. Important work has been performed on tribocharging of dissimilar materials - see especially literature by Harper, Moore and Lowell - however virtually nothing is known of the underlying cause of frictional or contact electrification between identical materials such as these latex balloons. The topic is clearly of importance, however, for atmospheric lightning may be influenced by such a process._________________________________________
* J. Lowell & WS Truscott, "Triboelectrification of identical insulators. I. An experimental investigation" J. Phys. D 19 (1986) 1273-80
** J. Lowell & AR Akande, "Contact electrification-why is it variable?" J. Phys. D: Appl. Phys. 21 (1988) 125-137
WR Harper, Contact and Frictional Electrification (Clarendon, Oxford, 1967)
AD Moore, & JM Crowley, Electrostatics: exploring, controlling and using static electricity 2nd Edition (Laplacian Press, Morgan Hills, CA 1997)