Piezoelectric MEMS Flexural-Plate-Wave Transducer for Alignment of Microparticles in a Drying Droplet

The possibility of obtaining aligned clusters of microparticles in a drying water droplet by employing standing flexural plate waves (FPWs) generated by a piezoelectric MEMS transducer has been explored. The MEMS device has a squared cavity etched out in a silicon (Si) substrate forming a 6\times6 mm diaphragm composed of a stack of doped Si and aluminum nitride (AlN) layers. Metal interdigital transducers (IDTs) placed at the edges of the diaphragm allow to electrically drive the AlN layer to excite FPWs in the diaphragm at its first antisymmetric Lamb mode (A0). The working principle has been validated through finite-element analysis and experimentally verified. For experimental testing, a droplet of tap water with an approximate radius of 850 \mu \text{m} was placed on the diaphragm and let dry at room temperature while applying voltage excitations to two opposed IDTs. After droplet evaporation, dry clusters of microparticles, originally dispersed therein, have been successfully patterned with a regular spacing of half wavelength, as expected, and remained adhered to the diaphragm without the need for additional substances for clot creation. The innovative exploitation of a noncontact and noninvasive flow-field-based approach combined with the evaporation process in a piezoelectric MEMS transducer can enhance the assembly control of microparticles or biological cells in lab-on-chip applications.