Extra force in charged resonant capacitors: a new macroscopic effect of vacuum fluctuations ?

We report on measurements of the mechanical vibrations of the fundamental mode of resonant capacitive transducers, consisting of parallel plates at different gaps (10 - 26 $μ$m). Resonant frequency data were taken vs the electric field at constant charge, at various temperatures and different configurations. All measurements exhibited unexpectedly large tuning, caused by an additional, unpredicted attractive force. Such force has a divergent behaviour characterized by a critical electrical field and start to diverge long before the critical point. Many known physical mechanisms have been considered to explain this force, but all of them had to be discarded. Thus, we turn to non-conventional physical mechanisms and suggest that vacuum fluctuations inside the gap could be a possible cause of this anomalous force.