A piezoelectric oscillating cantilever is a structure composed of a piezoelectric material with a flexible blade attached to it. While alternating voltage is applied over the material, it starts bending according the frequency of the voltage. The blade structure has a resonance which matches the frequency. This causes the blade to oscillate with a maximum amplitude, which causes an air flow. Commercial piezoelectric oscillating fans exist and they are characterized using max air flow $[cm^3/s]$ with zero pressure difference and maximum pressure with zero air flow, which are given for the case that the fluid is air. What I ask is how do these parameters change if the fan will be put in a different gas, say in helium? For me it looks like the maximum amplitude may increase, but what happens to the pressure? On the other hand shouldn't all gases follow the ideal gas law? Does the fan behave entirely differently or similarly? Obviously the question can be expanded to a question of how any kind of fan operates in different gas mediums.
What will happen: resonating structure will feel more or less resistance, and will decrease or increase amplitude to keep the loss the same.
How to calculate that more precisely: https://en.wikipedia.org/wiki/Acoustic_impedance , the 'Characteristic specific acoustic impedance' section, there you see a formula and what is required for it. Find this values for your gas, keep the power the same, and you can calculate resulting displacement. Its crude since we dont know the shape, but at least you will able to estimate how much more or less the motion will be compared to air.
If motion is larger, structure may break. Reduce the voltage to keep displacement the same.