The use of piezoelectric bimorphs to measure forces in colloidal systems

An analysis is made of the effect of errors introduced by bimorph drift and decay upon measurements of colloidal forces made with piezoelectric bimorphs. For direct measurements the errors will be small provided that the time constant of the bimorph, given by the product of its capacitance and amplifier input impedance, is much larger than the total time of measurement. With the force-feedback technique the errors will be negligible provided that, in addition, the integrator time constant is much smaller than the bimorph time constant, which condition is easily satisfied. It is important to use an amplifier with a very high input impedance to buffer bimorphs used for this type of measurement.