Jennie Ringberg Dec 15, ’15 ~ 3 min

# What does QCM-D have in common with a church bell?

## Surface Science Blog

How does QCM-D work, really? Perhaps you know that a QCM-D sensor is made out of quartz, which is a piezoelectric material. But what does that really mean? And how can that property of the quartz give you information about how much mass you have put on your sensor, or how soft that material on the sensor is?

Let’s think of it this way: visualize a church bell, it could look like the one imaged above. What happens if you hit that church bell? You will get a tone back, a tone with a certain frequency, right? Let us imagine that two people, for some reason, are hugging that church bell. What would happen with the tone if you hit the church bell now? The tone, the frequency, will be lower since mass is now added to the bell.

The same goes for the QCM-D sensor. When you apply a voltage to it, it will start oscillating at its resonance frequency. If mass is added to the sensor, the frequency of the oscillation will decrease. From the frequency decrease, you can get information about the mass that has been added. That is one part of the QCM-D fundamentals.

But how to understand the dissipation and how that can give me information about how soft the material is? Let us go back to the church bell. When the two persons are hugging it, it will not only change tone but also, the tone will fade out faster now. The two “soft persons” let energy get lost in the system so the ringing will stop faster. Let us pretend that we would have glued two stones onto the church bell instead, then the ringing would have continued for a longer time since the stones are rigid and will not dampen the motion like the “soft persons” do.

This is how dissipation can tell us about the softness of the material on the surface. We actually measure the time it takes for the sensor to come to a stop after oscillating it and the faster it stops, the softer material it is.

## Explore the blog

You have only scratched the surface.