Viscosity effects. Thermal noise spectrum of 0 – 18 kHz is shown. The natural frequencies were shifting upon increasing the viscosity of the solution surrounding the cantilevers. The first mode shifted by 4 Hz whereas the second mode shifted by 170 Hz. The shifts could only be estimated when the signals were fitted with a Lorentzian function and remained elusive. Measuring at a higher mode e.g. mode 13, (the shift for the same viscosity change by 0.257 cP) resulted in a frequency shift of 4 kHz. The frequency noise in the measurements shown was in the order of 10 Hz.