Optical Levitation of Charged Droplets
The development of an experimental system used to investigate charged droplets is presented. The experimental setup combines optical levitation with Millikan's classical oil drop experiment and has also been used as a mean to demonstrate fundamental physical phenomena.
Oil or water droplets are trapped in a vertical laser beam where their sizes can be determined. A vertical electric field is used to move the droplet in the laser beam and a feedback loop for the laser power repositions the droplet.
Oil droplets can be levitated for up to eight hours and their absolute charge and mass can be measured. Water droplets are held a few minutes and their sizes and evaporation rates were studied. Even the polarity of their charge can be determined. Decreasing amplitude of the oscillations is seen when oil droplets oscillated by an electric field are exposed to α-particles. With continued exposure to α-particles a change in the phase of the oscillations, due to change in the polarity of the charge, can be achieved.
Droplets generated, through pipetting or spraying, become electrically charged. The experimental setup is a valuable tool for investigating charged droplets and the magnitude of the charge can be determined without losing the droplet. The equipment is also suitable for educational purposes. It has been used to demonstrate topics such as radiation pressure, determination of size by diffraction and ionization by radioactive sources.
Millikan Oil Drop experiment