Modeling QCM-D Response to Deposition and Attachment of Microparticles and Living Cells

Quartz crystal microbalance with dissipation monitoring (QCM-D) is a powerful tool for studying adhesion, yet its use for analyzing the deposition of microparticles and living cells on surfaces has been hampered by difficulties in interpretation. Here we report a new quantitative model of QCM-D response, presented as an equivalent acoustic impedance circuit. As an essential feature, the particle interaction with surrounding fluid is modeled by relations for a freely oscillating rotating and translating sphere in an unbounded fluid, which is a valid approximation for microparticles. This helps deduce from the measured reponse the parameters pertinent to the contact mechanics. We use the model to analyze deposition of different microparticles as well as Pseudomonas fluorescens bacteria on several substrates using QCM-D combined with real-time microscopy. The parameter space is increased by varying particle type and size, substrate surface chemistry and rigidity, and ionic strength of the solution, which allows observation of diverse responses and transition from inertial to elastic loading, including rarely observed resonant regimes. Ultimately, we find that the model describes reasonably well the observed response for different microparticles and substrates, as well as for bacteria, and enables extraction of the contact characteristics in elastic and mixed loading regimes. It also reveals discrepancies between measured and anticipated parameters for large particles. The new model can be a useful tool for interpreting and quantifying QCM-D data on the adhesion of particles and living cells to surfaces, including time-dependent adhesion phenomena.