Running Droplets Captured

Spontaneous motion on surfaces has long been a topic of particular fascination. The ‘camphor dance’ of a camphor particle on water was discussed as early as the 19^th Century by Lord Rayleigh and James Thomson, the elder brother of Lord Kelvin. More recently there has been particular interest in the motion of liquid droplets on surfaces. Motion can be driven by thermal or chemical gradients and usually special preparation of the surface is required. However, in a recent development in the School of Physics, Prof. David Jesson, Dr. Wen Xin Tang and Dr. Jerry Tersoff (IBM, USA) have discovered a new type of droplet motion which is intrinsic and does not require any surface gradients or surfactants. Using the Monash Physics Low Energy Electron Microscope (LEEM), Ga droplets were observed to spontaneously run across an evaporating GaAs (001) surface. This surface is the most technologically important semiconductor surface after Si (001) and has been intensively studied for decades. It is therefore remarkable that droplet motion during evaporation has been missed until now.

During evaporation, the Ga droplets are out of equilibrium with the surrounding surface, and this provides a very general thermodynamic driving force. The droplets may be viewed as micro-motors, converting this disequilibrium into work. A fascinating aspect of the dynamics is that the droplet motion ceases at a certain temperature. However, just above and below this temperature the average droplet velocity increases. This non-monotonic temperature dependence has never been previously reported in the literature and yet can be shown to be a natural consequence of the surface thermodynamics.

Gallium droplets are currently of great interest for potential applications in quantum computing and quantum cryptography because they can be recrystallised under an arsenic molecular beam into quantum structures such as rings, double-rings, dots and molecules. The Monash Physics LEEM allows in situ observation of droplet behaviour in ultra-high vacuum and the intrinsic motion observed during evaporation was entirely unexpected. This research was reported in the April 10 issue of Science 324 (2009) 236.

Mirror electron microscope image of Ga droplets on GaAs (001) taken from a movie of droplet dynamics obtained in the LEEM. In this imaging mode, the droplets appear as a uniform dark disc surrounded by a concentric bright ring. The image is 10 µm across.