Why V_dc >> V_ac for electrostatic actuators ?

Consider a parallel plate capacitor with one fixed and one free electrode. If a voltage V is applied to the capacitor, the free electrode will experience an attractive electrostatic force of magnitude 

F = 1/2*e*A*V^2/g^2, where e is the dielectric constant of the material between the plates, A is the capacitor area,V is the voltage across the capacitor, and g is the electrode spacing. Because the force depends on the square of the voltage, the second harmonic of the applied voltage will be generated. For operation at the first harmonic, a dc bias Vbias is applied to the capacitor along with the RF signal such that V(t) = Vbias + Vac cos (wt +phi) . By making the bias voltage much larger than the time varying voltage, the dominant time varying force becomes F=e*A*Vbias*Vac/g^2 cos (wt + phi ).