Density effects. A stop flow experiment highlighting the effects of density on frequency. Modes 9 of 4 cantilevers are shown. A stable frequency was measured initially for 5% glycerol solution before the injection of a 10 μl volume of 5% ethylene glycol (injection in the hatched blue section). The frequencies shifted by ~115 Hz in all sensors due to the decreased density of the solution and hence a lower virtual mass moved by the cantilevers (inset). The frequencies recovered to their previous values after flushing with 50 μl of 5% glycerol solution (hatched red section).