Numerical and experimental investigation of droplet actuation by surface acoustic waves

Surface acoustic waves (SAWs) technology for manipulating small volumes of liquids has received much attention in recent years. SAW-based manipulation can be used for different bio-sampling functions, such as mixing, heating, pumping, jetting, separation, and atomization of droplets with volumes in the scale of microliters. Most studies in recent years have mainly focused on investigating SAW potential in different real-world microfluidics applications. However, the underlying physics of the droplet deformation by SAW still remains controversial. This thesis aims to investigate droplet deformations subjected to SAWs using both numerical and experimental methods. Different types of SAW devices with various resonant frequencies and different substrates are fabricated to carry out droplet actuation experiments. The experimental models are developed for three main reasons. First, to analyse the droplet deformation; second, to accurately define the contact angle boundary condition needed for simulations; and third, to validate the computational model. A Coupled Level Set Volume of Fluid (CLSVOF) mathematical model is developed to investigate the large deformation of sessile droplet induced by SAWs. A dynamic contact angle boundary condition is implemented to model the droplet three-phase contact line (TPCL) movement. The numerical and experimental results are quantitatively and qualitatively compared, and a remarkable agreement is achieved, which proves that the developed computational model can be used to simulate different droplet actuation scenarios. After validation of the computational model, it is used for analysing the physics of the droplet jetting and pumping. The effects of important factors such as droplet volume and SAW frequency and power on droplet pumping are investigated. Moreover, the model is used to analyse the energy budget of a droplet jetting. An investigation into the optimization of the interdigital transducers (IDT) location of the SAW devices for different microfluidic applications is also carried out using the computational model. The experimental and computational models are then employed to investigate a novel application of SAW devices to control the droplet impact. SAWs devices are used to manipulate and control the droplet dynamics. The experimental results revealed that characteristic impact parameters such as impact regime, contact time, maximum spreading and re-bouncing angle could be modified and controlled by SAWs. By changing the SAW direction and power, droplets impact behaviour can be altered. The maximum reduction of contact time up to ∼50% can be achieved, along with alterations of droplet spreading, re-bouncing angle, and movement along the inclined surfaces. On the other hand, numerical results revealed that the SAWs could be used to modify and control the internal velocity fields inside the droplet. By breaking the symmetry of the internal recirculation patterns inside the droplet during the impact on flat surfaces, the kinetic energy recovered from interfacial energy during the retraction process is increased, and the droplet can be entirely separated from the surface with a much shorter contact time. Also, numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium on both flat and inclined surface, leading to different impact behaviours. This innovative paradigm opens up new opportunities to actively program and controls the droplet impact on smooth or planar and curved surfaces, as well as rough or textured surfaces