Viscosity effects. A stop flow experiment highlighting the effects of viscosity on frequency. A stable frequency was measured initially for 5% glycerol solution before the injection of a 10 μl volume of 12% ethylene glycol (injection took place in the hatched blue section). The frequency shifted towards lower frequencies for all modes (mode 9 by ~385 Hz, mode 10 by ~575 Hz and mode 11 by ~810 Hz) due to the increased viscosity. The frequency recovered to its previous values after flushing with 100 μl of 5% glycerol solution (hatched red section).