  Even though contact electrification has been studied for a long time, the
mechanism of charge transfer still remains elusive. Most of previous reports
only focus on the driving force of charge transfer. However, to better
understand the mechanism of charge transfer, we believe contact itself for
supplying transferring path that charges taking also need to be understood.
Here, we focus on the in-plane symmetry of contact in contact electrification
by utilizing a uniaxial strain to change the material's isotropic nature to
anisotropic. A clear anisotropic charge transfer is observed by contacting
axially stretched rubber films at different rotational angles, which could
arise from fluctuation of contacting area in microscale. A universal ellipse
model is also proposed for qualitatively describing the anisotropy of contact
regardless of the specific driving force of contact electrification.
