Numerical and experimental study of interactions between surface acoustic waves, fluids and particles in acoustofluidic systems

Maramizonouz, Sadaf (2021) Numerical and experimental study of interactions between surface acoustic waves, fluids and particles in acoustofluidic systems. Doctoral thesis, Northumbria University.

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Abstract

Acoustofluidics refers to the multidisciplinary field of investigating the integration of acoustics with microfluidics which can be used to develop tools to manipulate and control microfluids and particles. Acoustofluidic technologies present numerous advantages including small size, simple design, low cost, reliability, efficiency, and as a result can be adapted to various applications. Moreover, for biomedical applications, acoustofluidics offer benefits such as non-invasive and contact-free manipulation with high biocompatibility, conserving cell viability and proliferation. With these advantages, acoustofluidics present a great potential to be utilised in many clinical and biomedical applications such as lab-on-chip, organ-on-chip, and controllable drug delivery platforms.

Most of the current studies on acoustofluidics are based on an experimental investigation which is essential for testing a hypothesis scientifically and to ensure that the acoustofluidic system functions properly. Experimental methods can be used for optimising acoustofluidic systems or evaluating a new design based on trial-and-error approaches. However, experimental acoustofluidcs could be costly and time-consuming. Computational modelling can provide detailed information of the underlying physics of the complex acoustofluidic systems in a more cost and time-effective manner. These details, which can be useful for adapting acoustofluidic systems in practical devices, are sometimes hard or even impossible to obtain through experimental work.

In this thesis, computational models are utilised in order to investigate the behaviour of fluids and particles in novel acoustofluidic platforms including flexible acoustofluidics and capillary bridge channels. The models are first validated using the experimental results reported in the literature and then, they are used to analyse the behaviour and to understand the underlying physics of novel acoustofluidic platforms such as flexible thin film surface acoustic wave devices and capillary bridges for the purpose of particle manipulation.

Typically, most acoustofluidic systems presented in existing literature are designed using rigid piezoelectric materials to generate acoustic fields. These rigid piezoelectric materials are generally brittle, fragile, and prone to breaking when applied with higher powers. In this thesis, flexible thin film surface acoustic wave devices with metal substrates are utilised for particle and cell manipulations, and to study their acoustofluidic behaviour in different conformations obtained through bending and to investigate the effects of bending curvatures on microparticle’s manipulation inside a microchamber. These flexible thin film devices present advantages including high wave speed and reasonable electro-mechanical properties for the flexible thin film devices.

Additionally, for continuous flow applications to enable the fluid flow, microchannels are typically fabricated with solid materials and in cleanroom environment using complicated and time-consuming processes. This thesis presents the idea to integrate flexible thin film surface acoustic wave devices with continuous flow wall-less microfluidic platforms designed using capillary bridges. Using capillary bridge channels simplifies the production of microchannels while decreasing the fabrication cost and time.

This new platform which comprises of flexible thin film surface acoustic wave devices with metal substrates and capillary bridge channels is utilised for particle and cell manipulation and is investigated in detail through both computational modelling and experimental study. These novel acoustofluidic platforms can offer potential applications in flexible microfluidics, bio-inspired and body conforming wearable devices, and wearable point-of-care applications.

The significant contributions of this thesis can be summarised as follows:

1. For the first time, flexible thin film surface acoustic wave devices with metal substrates are utilised for the purpose of particle and cell manipulation. Through both experimental study and computational modelling, the effects of various vibration modes, different bending curvatures, and twisting geometries are investigated.
It was presented that flexible surface acoustic wave devices bent in concave/convex geometries produce particle patterns converged with a slope towards/ diverged with a slope away from the centre of the curvature of the geometry.

2. Glass microtubes (with both rectangular and circular cross-sections) are integrated with flexible thin film surface acoustic wave devices for the purpose of particle and cell manipulation with and without fluid flow. The effects of different microtube cross-sections, microtube inclination angle regarding the electrodes of the surface acoustic wave device, and different fluid flow rates on particle patterning are systematically investigated.
For rectangular microtubes placed at an angle relative to the electrodes, particle pattern lines were parallel to the tube walls.
For circular microtubes, different particle patterns were observed which were dependent on their positions along the tube’s height. In the bottom/middle height of the tube, the particle pattern lines were parallel to the tube direction due to the acoustic wave propagation into the water and formation of a standing wave along the direction of the circular tube/ perpendicular to the tube direction as the standing wave propagated around the circular cross-section of the tube perpendicular to the tube direction.

3. For the first time, capillary bridge channels are integrated with flexible thin film surface acoustic wave devices with metal substrates to develop a continuous flow acoustofluidic setup for particle and cell manipulations. Through both experimental work and three-dimensional numerical modelling, the effects of different frequencies, channel geometries, particle properties, and flow rates are investigated.
It was shown that the particles were aligned on the pressure node lines of the acoustic pressure field and parallel to the air-water walls of the capillary bridge channels due to the combined effects of the acoustic wave field inside the water channel and the fluid flow.

4. The effects of acoustic streaming on fluid and microparticles in a microchannel flow are investigated through both experimental studies and three-dimensional numerical modelling. Two different modelling approaches are compared:
1st Approach: The whole acoustic field coupled to the flow field is simulated and the acoustic streaming force is calculated using the first order acoustic density and velocity which predicted the acoustofluidic system more accurately.
2nd Approach: The acoustic streaming is modelled by assuming the velocity of a one-dimensional attenuating surface acoustic wave and using the acoustic streaming force formula which is more efficient in terms of computational cost and time while still presenting results with reasonable accuracy.

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