Microfluidic actuation by high-frequency ultrasound: acoustic streaming, pumps, and drops

Microfluidic technology is an important and active area of research. The lab-on-a-chip system has drawn together a broad range of disciplines to develop miniaturised microfluidic platforms enabling point-of-care diagnostics and rapid, low-cost scientific testing. Over the past decade the technology has moved to increasingly complex systems, which require a means to effectively actuate fluids on progressively smaller scales. The use of ultrasound to drive fluids in microfluidic applications has distinct advantages. Acoustofluidic technology such as surface acoustic wave (SAW) devices use a beam of sound to generate fast acoustic streaming within fluids, and to move drops using radiation pressure. These devices use simple structures free of moving parts, but are extremely difficult to control. The effect of sound frequency on the scale and behaviour of acoustic streaming is critically important but not well understood. In this thesis a series of experimental studies of acoustic streaming, ultrasonic pumping, and drop actuation are conducted across a frequency range between 1 MHz and 1 GHz to achieve better understanding and control of high-frequency ultrasonic fluid actuation. The jet-like acoustic streaming generated from SAW devices was measured experimentally, revealing that sound frequency, ω/2π, has a major effect on the flow. Increasing the frequency towards 1 GHz reduced the acoustic streaming to length scales on the order of 100 micrometers. A model was developed to explain the complex scaling of acoustic streaming velocity with beam power and frequency observed in experiment, finding that the peak acoustic streaming velocity, U, scales as U ~ ω^2 at low frequency, but approaches a scaling of U ~ ω^(1/2) as ω → ∞ for SAW devices, or a constant for transducers of fixed ultrasound emission area. Beam power, P, indirectly affects U by influencing the rate of jet growth, governed by a type of 'jet' Reynolds number, Re. For small Re the streaming velocity scales as U ~ P ~ Re^2 but approaches a scaling of U ~ P^(1/2) ~ Re as Re → ∞. A micropump was developed to harness the acoustic streaming generated from SAW ultrasound which, for the first time, demonstrated SAW-driven pumping between independent fluid reservoirs. The pressure-flow rate relations for SAW pumps were measured and found to be linear, revealing that SAW-based pump behaviour does not change significantly as flow resistance is altered. The efficiency of the pump was found to increase with applied power. The contact line motion of drops exposed to MHz-order vibration from piston-mode transducers was measured experimentally, revealing an ultrasonically-driven wetting phenomenon which caused lateral contact line motion. A theory was developed which attributes the ultrasonic contact line force to energy dissipation within the boundary layer, proportional to the Weber number. The force alters the contact angle and thus equilibrium state of the drop, causing contact line translation. The complex dynamic response of the drop interface is measured experimentally and characterised using timeseries and spectral analysis.