Biofilm monitoring with a QCM-D: Biofilms manifest themselves by coupled dashpots rather than coupled mass

While studying biofilm formation in industrial settings with a quartz crystal microbalance with dissipation monitoring (QCM-D), it was found that the shift in half bandwidth, ΔΓ, often was much larger than the shift in frequency, Δ f . Also, ΔΓ was almost independent of overtone order. The microbial biofilms were mostly composed of prokaryotic cells, which contact the resonator surface across soft filament like structures at their cell wall called pili. A simple model is proposed, which explains the peculiar QCM response induced by these biofilms. The key element of the model is a coupled dashpot, representing small, dissipative contacts between the sample and the resonator. The dashpot replaces the coupled mass known from gravimetric sensing. The coupled-dashpot model predicts the shift in frequency to be close to zero (Δ f ≈ 0) and the shift in half bandwidth to be about the same on the different overtones (ΔΓ ≈ const ). The value of ΔΓ is proportional to the product of the number density of the contacts and their stiffness. The model – if found to agree with experiment – yields structural information on the sample and can guide data aggregation. The model was substantiated and illustrated with numerical simulations. A side aspect is a simple and economical instrument design, which brings QCM-based biofilm monitoring closer to routine practice. • New inexpensive setup for QCM measurements. • Biofilm formation mainly affects the bandwidth. • Modeling of behavior with a mechanical dashpot.