unexpected magnitude of frequency shift

Hello again,
I am using the openQCM-Q to measure the viscosity of liquids. For some reason, the frequency shift is consistently about 39% larger than my calculations anticipated. I have double checked that I was calculating it correctly and my plots of frequency shift vs viscosity are good, they show a linear correlation that passes through the origin, they are just not the anticipated magnitudes by the 39%. I was wondering if anyone else has had a similar problem and might have found a solution?
any insight would be much appreciated!
thanks,
Rick

Comments

  • Hi Rick,

    it is a good result that the frequency vs dissipation experimental data shows linear correlation that passes through the origin

    I'd need more information to figure out your results and check the calculation. particularly what kind of liquid is in contact with the surface of the qcm.

    For example, if the liquid in contact with the quartz crystal surface can be modelled as a semi - infinite viscoelastic layer the following relations works:

    Frequency variation

    Dissipation variation

    where
    is the fundamental resonant frequency
    are the piezoelectric shear strength and mass density of the quartz crystal
    are the density and viscosity of the liquid

    In particular, by combining the equations above we have:

    no matter what kind of liquid is in contact with quartz crystal surface.

    if the above hypothesis is applicable to your experiment, we can check your results together on the basis of these calculations

  • Hi Marco,

    Sorry for the delay. I was hoping to figure it out, but no dice. The equation you give that should hold for any liquid in contact with the crystal doesn't appear to work for me, even for something as straightforward as DI water.

    Here is a screenshot of the GUI after DI water addition:
    https://gyazo.com/110fc6c84ee0bb7ad4ce0ae985a4388a

    Attached below is the file containing the data for this run (Jul 11th), csv isn't supported on this forum so I converted to xlsx.

    The results of a MATLAB program I made to read the QCM data suggests the delta_f should be about 2025Hz but that the measurements show a delta_f of roughly 2456Hz (over many runs, using different crystals, similar numbers occur).

    If you would like to continue this conversation outside of the forum feel free to email: rsugden@uwo.ca.
    Any help is much appreciated!

    thanks,
    Rick
  • edited July 17
    Hi Rick,

    we can continue the discussion here, it will help others to learn more about this subject.

    Indeed, in the openQCM device calibration test, we measure frequency and dissipation variation caused by the passage from air to water, when one face of a 10 MHz quartz sensors are in contact with liquid. Here below a typical openQCM calibration result

    The frequency shift caused by the passage from air to water is roughly


    and the dissipation increased by roughly:


    Our results coincide with those you have measured. Having used different sensors and openQCM devices, the results show that the measurements are reproducible and reliable, which is a fundamental feature for a device for scientific application.

    Let us come back to the equation derived by Kanazawa and Gordon Anal. Chem. 1985, 57, 1771-1772
    indeed they predict a frequency and dissipation variation for the passage from air to pure water, when one face of a 10 MHz quartz sensors is in contact with liquid, is given by:




    where the density and viscosity of pure water at room temperature is given by:



    Experimental results of frequency and dissipation variation caused the passage from air to pure water can be found in literature in Rodahl et al "Quartz crystal microbalance setup for frequency and Q‐factor measurements in gaseous and liquid environments"
    This is one of the first, pioneering and most cited work about the use of the decay method to measure simultaneously frequency and dissipation in gaseous and liquid. They use AT-cut quartz crystal with a resonant frequency of 10 MHz with only one face in contact with liquid and the other covered with a lid. The experimental results obtained by the researchers and reported in this paper are the following




    as you can verify also in this paper the experimentally measured values differ from the theoretical prediction, but despite this, the technique of decay is widely used in the field of QCM. In conclusion, the QCM setup is not straightforward and many factors must be taken into account for the prediction of experimental results observed, for example very often the same electronic interface connected to the sensor must be properly modelled.

    What is a positive value is that the openQCM device provides consistent measurements, which can be considered near to the theoretical prediction.

    Of course, discussion is still open, in order to improve together with the community help.

    marco
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