Dynamics of piezo-actuated vibrating structures: Results of simulations and interferometric measurements

The research activity carried out within the Laboratory of Electrooptics of the Department of electrical, computer and biomedical engineering at the University of Pavia has demonstrated the application of laser diode feedback interferometry, also known as self-mixing interferometry (SMI), for the non-contact characterization of piezoelectrically actuated planar structures and fluid chambers, and the exploitation of their vibrational modes for particle manipulation. In this work, a near-infrared laser diode is focused by a lens on the vibrating structure under test. A fraction of the backscattered light re-enters the laser cavity and modulates the optical power, which is detected by the monitor photodiode integrated in the laser package and converted into a voltage signal by a transimpedance amplifier. In the frequency domain, the piezoelectric actuator is driven by white noise and the spectrum is acquired by a dynamic signal analyzer. In the time domain, the actuator is driven by sinusoidal signals and the interferometric fringes, each corresponding to a displacement of half the laser wavelength, are acquired by an oscilloscope. The data are then loaded and elaborated in MATLAB environment to reconstruct displacement, displacement sensitivity and phase delay. The vibrational behavior has been evaluated at different spatial scales. Indeed, in the first part of the work a piezo-MEMS speaker and a piezo-actuated Plexiglas plate were characterized in terms of resonance frequencies and mode shapes. These results were then used to validate the experimental and numerical procedures on well-defined structures. In the second part, the same tools have been applied to a Petri dish and to a closed polymeric microchannel, with the piezoelectric actuator bonded to the structure. Direct Chladni patterns in dry conditions, with particles collected along the nodal lines, and inverse Chladni patterns in water, with particles driven towards the antinodes, have been studied. Thus, density-based separation of particles has been demonstrated, and iron particles have been trapped in the water-filled microchannel at its fundamental resonance. Preliminary tests with yeast cells and polystyrene microspheres have been carried out to explore the application to biological samples. A 3D numerical model has been implemented in COMSOL Multiphysics, combining structural mechanics, electrostatics, pressure acoustics and particle tracing. The model has allowed to evaluate resonance frequencies, mode shapes, acoustic pressure fields and particle trajectories. Experimental results have been found in good agreement with numerical results. The potentialities of the platform have been explored through parameters as resonance frequency, displacement sensitivity and phase delay, and the limits of the present driving hardware for the manipulation of cell-scale particles have been identified.