A strong and reproducible effect of an external electric field (EEF) on liquid films confined within a nanogap between a highly polished steel ball and a smooth glass disk is described. Induced by the EEF, microbubbles were observed at the edge of the contact region. This phenomenon is more obvious in polar liquid films than that in nonpolar ones. A stronger EEF causes a decrease in the film thickness in the contact region initially, and then the variation becomes much smaller. When the ball is applied with a positive EEF, the emergence of microbubbles is stable over time, while it becomes much less as time progresses. Different mechanisms of these experimental phenomena have been discussed.

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