New concepts and technical principles are sometimes best explained using analogies that relate to something already familiar. Here, QCM-D technology is explained using a set of different musical instruments.
QCM-D is an acoustic technology, i.e. based on measurements of sound energy. Therefore, it is close at hand to use musical instrument analogies to explain the QCM-D working principle. To understand the fundamentals of the QCM-D technology, there are a few key concepts that must be understood:
In the webinar Basics of QCM-D, my colleague Fredrik, explains these three concepts using the below instruments
In brief, picturing these musical instruments, how they work and how they interact with their surroundings, Fredrik teaches us the following:
When vibrating, the quartz crystal acts like a guitar string. It would be a short and thick guitar string, only ~0.3 mm long, Fredrik explains, but it behaves in the same way. To induce a tone, the sensor is stimulated with electricity. Like a guitar string, if the string is made longer, the tone will be lower and if the string is made shorter, the tone will be higher. It is the same for the QCM-sensor, i.e. if the sensor is made thicker, by adding material, the tone will be lower, and if it is made thinner, by removing or desorbing material, the tone will be higher. The QCM sound is in the MHz regime, but it is the same phenomenon as the guitar string.
If you hit a church bell that is mounted in an undampened mounting, it will ring for a very long time, Fredrik says. If the ringing motion is dampened, however, for example by some friendly tourists giving the church bell a hug, the sound will decay faster. You can think of the hugging tourists acting like an added soft film which dampens the sound signal. In that sense, the church bell can sense its environment. If it interacts with a rigid film, or no film at all, which would be the case when its is ringing in air, the dissipation will be low, and it will ring for a long time. If it interacts with a soft film, or if a friendly tourist gives it a hug, the energy will dissipate quicker and the ringing will be a short sound.
Since we are dealing with sound waves and acoustics, we can use physical formulas for how sound waves, i.e. pressure waves, distribute through films with different material properties to establish physical models that will help us understand the relation between the sound and the material properties, Fredrik explains. For example, the viscoelastic model describes how the pressure wave moves in a layer with certain viscoelastic properties. Measuring the sound, we can then use the model to calculate what the viscoelastic properties of the layer are.
With this model in mind, we can compare QCM-D to an electric organ. The organ can produce different tones of different qualities, and so can QCM-D. We measure the different tones and listening to the different harmonics is like listening to different qualities of the sound. So, the different keys, the different tones, of the electrical organ would correspond to different thicknesses of the film on the sensor, of the mass change. Add to this the possibility of changing the quality of the sound by using the different knob settings. For example, changing from a piano sound to a trumpet or saxophone sound. This would then represent different qualities of the material and how the different harmonics behave. So, in that aspect we can say that QCM-D is like an electric organ.
Watch Fredrik’s webinar to learn more about the fundamentals of the QCM-D, the theory on how to understand the technology, what you can measure and analyze.
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Read about why it is possible to gain valuable information from a viscoelastic sample by monitoring multiple overtones in QCM measurements.
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