Flow Streaming Mediated Particle Separation inside a Droplet Induced by Surface Acoustic Waves

Abstract

[EN]In recent times, the manipulation of nanoparticles at nanoscale has become a topic of great interest among the scientific community. The advantages of using SAW represent a great advance in fields such as biomedicine or biosensors. The develop of these techniques is actually related with the advances achieved in the microfluidics science. For this reason, the knowledge of the physics that prevails in the handling of tiny size particles becomes something essential. This project wants to advance in the understanding of physics behind and discuss the effects produced by the forces involved in the interaction with nanoparticles at nanoscale. For that purpose, the experiments carried out compare the obtained results with those which were made by other groups of researchers and pretends to shed light on the gaps identified in the related literature. In this regard, an attempt is made to find an answer to how a change in, for example, surface tension, the contact angle of the drop with the substrate, the viscosity and the density of the fluid affects the movement of the particles. The use of SAW implies the development of a specific device that can channel the wave to the surface in question, this device is called interdigital transducer IDT. A SAW is the acoustic wave produced by the IDT when an electric current is applied to its electrodes. Thanks to the piezoelectric effect of the substrate where the IDT is located, the electrical signal becomes into an acoustic wave that interacts with the surface of the droplet causing an acoustic field to be created within it. With the intention of proving the predicted events that take place inside the water droplet when it is subdue to the SAW, diverse types of IDTs able to operate at different frequencies are produced. Other parameters like the input power, frequency or the particles diameter are considered also to evaluate the involved forces when handling nanoparticles inside the sessile water droplet. The combination of these different parameters directly affects the movement of the particles inside the droplet and raises the following questions. For instance, the interaction of the SAW with the droplet when it propagates along the substrate generates three known distinct types of forces inside the droplet. The acoustic streaming force ASF emerged from the propagation of the acoustic wave, the acoustic radiation force ARF and the centrifugal force Fc arisen as a consequence of the leakage of the propagated SAW over the substrate. In this sense, these forces are perceived by the particles arising a movement around the droplet in which they are immersed and forming different patterns of movement which are studied in this project. On the other hand, as it has been described in the literature, these forces are closely related with the particle diameter, the frequency and the input power applied to the IDT. In this vein, different particles sizes 7 μm and 1 μm of polystyrene nanoparticles immersed in a 2 μL droplet water volume were tested and four different IDTs with varying nominal frequencies 20,40,80 and 160 MHz are produced