Good vibrations

Droplet vibration is an important phenomenon which has important applications in processes such as inkjet printing, fuel injection and microfluidics. In all of these applications, a small liquid droplet is placed on a surface or ejected from a nozzle and vibrated. In the case of fuel injection and inkjet printing, the droplets are made to vibrate with large amplitudes in such a way that liquid is ejected from the drop creating a fine spray of material. This is used to generate a fine, easily combustible aerosol or a fine spray of ink respectively. Droplet vibration has also been used to drive the motion of liquid droplets along patterned surfaces and has potential applications as a method for directing drop motion in lab-on-a-chip applications.

The vibration of small liquid droplets can also be used as a way of measuring the properties of small liquid volumes. When a drop is perturbed mechanically e.g. by blowing on it, it begins to oscillate at its resonant vibrational frequency. The flow of liquid within the droplet causes damping of the oscillations and the droplet will eventually come to rest. The oscillations of liquid droplets have fascinated scientists since the time of Lord Rayleigh who, in the 1870s showed that the resonant frequency of a levitated drop depends only upon the surface tension of the liquid and the mass of the droplet. Further work showed that the damping in levitated droplets is determined by the viscosity of the fluid. However a severe limitation with these and other studies of levitated drops is that the technology required to levitate liquid droplets is not readily available as it requires the use of large electromagnetic fields or acoustic levitation equipment. A more convenient arrangement involves placing a droplet on a surface to produce a so-called sessile drop. If a droplet can be placed on a surface and vibrated, the oscillations of the droplet can be monitored and used to extract information about the surface tension and mechanical properties of the droplets.

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A sequence of images of a vibrating sessile water droplet supported on the surface of a diffraction grating. The images were acquired at 200 frames per second using a high speed camera.

This study aims to automate the process of droplet vibration and to use a novel light scattering technique to extract information about the properties of fluids. In this technique, a laser beam is passed through a droplet and allowed to fall on a photodetector. As the droplet vibrates, the intensity of light falling on the detector changes and a time dependent trace of the motion of the drop is obtained. Information about the vibrational frequency and damping of the liquid drops can then be extracted from these time dependent signals.

Dr James Sharp
University of Nottingham

James was awarded a Research Project Grant in December 2012.