Nonlinear electrokinetic phenomena in capillary systems were studied w
ith periodic variation of the pressure gradient. Output spectra of the
electrokinetic cells filled with distilled water were found to contai
n higher harmonics, while these were harmonics not found in the spectr
a of cells filled with high-grade acetone. The crucial value of shear
strain was experimentally obtained using a rheological model of viscoe
lastic liquid flow into capillaries.