Interdigitated transducers (IDTs) were originally designed as delay lines for radars. Half a century later, they have found new life as actuators for microfluidic systems. By generating strong acoustic fields, they trigger nonlinear effects that enable pumping and mixing of fluids, and moving particles without contact. However, the transition from signal processing to actuators comes with a range of challenges concerning power density and spatial resolution that have spurred exciting developments in solid-state acoustics and especially in IDT design. Assuming some familiarity with acoustofluidics, this paper aims to provide a tutorial for IDT design and characterization for the purpose of acoustofluidic actuation. It is targeted at a diverse audience of researchers in various fields, including fluid mechanics, acoustics, and microelectronics.
While gold is a stable metal in water, it is not uncommon for microfluidic experimenters using biologically-relevant fluids such as phosphate-buffered saline (PBS) to witness their precious gold electrodes quickly vanish from the microchannel once the voltage exceeds a few volts. This stability issue concerns multiple fields where high voltage provides superior actuator or sensor performance, such as resistive pulse sensing (RPS), electroosmosis, electrowetting and so on. One solution to protect metallic electrodes is using alternative voltages (AV) as opposed to continuous voltages. After recalling that gold dissolution is enabled by the chloride ions present in most biologically-relevant solutions, we explore the stability conditions of the electrodes for voltages from 1 to 20 Vpp (Peak to Peak voltage amplitude), actuation frequencies between 0 and 5 kHz, and for various pH and electrolytes (NaCl, Na2SO4, HCl). We find that the dissolution threshold voltage depends on the ratio of reaction to diffusion rate given by the Damkhöler number Da. In mass-transfer limited regime, the dissolution threshold is independent of the frequency, whereas the dissolution voltage is observed to grow as Da−1/2 in the reaction limited regime. These findings provide guidelines to design more reliable electrowetting, electroosmosis, dielectrophoresis and resistive pulse sensing devices.
Acoustic
mixing of droplets is a promising way to implement
biosensors
that combine high speed and minimal reagent consumption. To date,
this type of droplet mixing is driven by a volume force resulting
from the absorption of high-frequency acoustic waves in the bulk of
the fluid. Here, we show that the speed of these sensors is limited
by the slow advection of analyte to the sensor surface due to the
formation of a hydrodynamic boundary layer. We eliminate this hydrodynamic
boundary layer by using much lower ultrasonic frequencies to excite
the droplet, which drives a Rayleigh streaming that behaves essentially
like a slip velocity. At equal average flow velocity in the droplet,
both experiment and three-dimensional simulations show that this provides
a three-fold speedup compared to Eckart streaming. Experimentally,
we further shorten a SARS-CoV-2 antibody immunoassay from 20 min to
40 s taking advantage of Rayleigh acoustic streaming.
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