Droplet behaviour under high intensity acoustic vibration

The reduction in scale of fluidic based chemical and biological processes offers significant analytical and sensitivity improvements as well as reduced reagent usage, increased automation and reduced manufacture costs. Droplets deposited on a planar surface offer a convenient way of investigating very small sample sizes. We investigate the effect of vibration of droplets in the direction normal to the surface on which they sit. When the contact line of the droplet is constrained by use of a very shallow well and suitable frequencies of vibration (order 100s Hz) are selected such that a resonant standing surface wave is established, collection of particles in predictable patterns can be achieved. When the droplet contact line is unconstrained high amplitude acoustic vibration (again order of 100s Hz) causes spreading of the droplet to occur. This effect can be so pronounced that during actuation the contact angle falls below that of the receding angle. We demonstrate the use of this effect by the merging of two droplets which are deposited a small distance away from each other. Once merged, a process which occurs due to surface energy minimisation as soon as the droplets spread such that they touch at one location, further vibration causes rapid mixing of the fluids through acoustic streaming.