Analysis of mechanical dynamics in acoustofluidics: assessing the impact of frequency sweeping on PZT performance and acoustic phenomena

In this work, we combine acoustic streaming measurements, direct vibration measurements and the theory of wave dispersion to interpret changes in acoustic streaming. The vibration dynamics of the PZT are studied under frequency sweeping actuation, varying the sweep period between 1 ms and 1000 ms, while the acoustofluidic chip is characterised and compared with theoretical expectations from wave dispersion theory. This analysis enables acoustofluidic device operators to obtain relevant information on actuation such as the amplitude loss factor, ageing, mechanical vibration dynamics and the influence on acoustofluidic phenomena. First, the PZT is shown to operate in a linear regime and then the electromechanical coupling factor was extracted (226.7pmV). As the stresses applied to the PZT influence the mechanical vibration amplitude, the stress is evaluated by applying the theory of linear piezoelectricity (σ = 0.23mNmm².). Then, the amplitude damping factor (ζ(%) = 67 %) is introduced to quantify mechanical losses due to the acoustofluidic assembly and it was found to be independent of the actuation mode. The streaming velocity is then evaluated while varying the sweep period and in continuous mode. The results show that streaming did not change significantly with the sweep period, and the amplitude of streaming velocity was reduced to approximately 60 % of its value compared to continuous mode. Connecting the theory of wave dissipation and energy spectral density, it was possible to establish that the energy spectral density remains approximately constant on average, explaining the streaming results. Overall, frequency sweeping offers a broader-band and a more robust operation, but at the cost of reduced acoustofluidic forces.