Improved Fluid Handling and Manipulation using a Floating Electrode Unidirectional Transducer

Recently, surface acoustic waves (SAW) have found their way into various microfluidic applications to provide useful functionality and efficiency. Traditionally, the standard interdigital transducer (IDT) design is used in acoustofluidics to generate propagating SAW later used to manipulate fluids or particle suspensions. In this configuration, the acoustic energy generated in the fluid is a narrow beam, much narrower than the wave's propagation length in the fluid. This gives rise to issues like reflections and caustics, making it difficult to control and use. To overcome this, we employ a floating-electrode unidirectional transducer (FEUDT) that produces a unidirectional surface acoustic wave, replacing the beam-like attenuation (in case of an IDT) with a continuous pressure SAW over a longer length scale in a desired direction. We establish the differences between the acoustic wave field in the absence and presence of a fluid medium for both these transducers, illustrating vortex and shear flows uniquely generated by the FEUDT system. Additionally, we show the efficiency and unidirectionality of the FEUDT in pumping fluids in a semi-infinite volume of fluid. We have observed fluid velocities up to 18 mm/s, compared to 2.5 mm/s for a standard IDT using the same experimental settings.