Change of the resonance frequency of a quartz crystal microbalance in contact with an aqueous dispersion of solid particles

The change of resonance frequency of shear oscillations of an AT-cut quartz crystal with a resonance frequency f(0) of f(0) = 5 MHz operated with one face in contact with air and the other in contact with an aqueous suspension of non-porous solid particles is studied. The experiments are carried out as function of the concentration of the colloidal particles at constant temperature. The aqueous suspensions are prepared with three kinds of particles of different size (radius r approximate to 500, 200, 5 nm). It is found that the resonance frequency is independent of the concentration of the colloidal particles with r approximate to 500 and 200 nm. It has approximately the value of the resonance frequency of the fluid medium in the absence of the colloidal particles. For the colloidal particles with r approximate to 5 nm a small change of the resonance frequency with the concentration of the colloidal particles is observed. An attempt is made to rationalize this finding. The observation could be of importance to practical applications of the quartz crystal microbalance as a sensor in fluids.