Displacement of Droplets on a Surface Using Ultrasonic Vibration

Liquids handling is an important issue in biomedical analysis. We propose two different devices for ultrasonic droplets displacement. Each one involve a different physical phenomenon. The first device proposed is a low frequency standing waves using flexural plate waves (FPW). In this case, acoustic radiation pressure is involved for the displacement. The second one uses high frequency travelling waves: Surface Acoustic Waves (SAW). The droplet is moved thanks to acoustic streaming. Some theoretical and experimental results are presented . Introduction The study of the interaction between a vibrating surface and a liquid led us towards the design of a new method of handling of small quantities of liquid. We propose two different devices to move small droplets that emphasize the interaction between liquid and on the one hand FPW and on the other hand SAW. The first device is a metallic beam with two stacks of multi-layers piezoelectric actuators on each end. When a standing wave is generated with sufficiently high vibration amplitude, it has been observed that a droplet moves towards the nearest anti-node. A theoretical explanation of the phenomenon has been performed . For each flexural mode, the position of the antinodes is fixed by the boundary conditions. So if different flexural modes are excited we obtain different equilibrium positions. Consequently, we achieve to move the droplet by switching the excitation frequency of the beam. With the second device Rayleigh waves are generated by inter-digital transducers (IDT) of aluminium deposited on a LiNbO3 layer. The Rayleigh wavelength is about 100μm at 80 MHz. The surface acoustic waves are propagating on the surface and absorbed by the droplet. The motion effect is caused by the acoustic streaming . The fluid is intended to be pushed away from IDT. We can move the droplet in two dimensions by laying out several IDTs all around the desired surface of displacement. Effect of acoustic radiation pressure He very first interest of effect of acoustic radiation pressure has came from experimental experimentations. When one spreads a liquid layer on a plate, the free surface of the liquid is plane. If FPW of the plate is excited, a permanent deformation of the liquid surface can be observed as one can see on figure 1. Flexural plate wave (FPW) Permanent deformation of the liquid layer