Optical Binding in Air

Abstract

Optical binding between micron-sized oil droplets in air has recently been observed. The experimental setup, consisting in two vertical, counter propagating and diverging laser beams, builds up a three dimensional trap. The cloud of oil droplets, enclosed in a glass cell, progressively fills in the trap where droplets interact one with another. Scattered intensity is observed on a video camera. Interactions involve optical, electrostatic, radiometric and capillary forces. Orders of magnitude are discussed.Chains up to four droplets have been observed, the most stable structure being the doublet and not the single drop. In air, viscosity being one thousand times smaller than in water, mean free path of a micro-sphere is much bigger. That is why mean residence times in metastable states are of the magnitude of a few seconds and that brownian motion quickly drives the trapped droplets in the very minimum of potential energy: the doublet structure. Two stable states have also been obtained for the doublet. Observation of interference indicates that oil droplets are phase-locked onto each other every λ/2.The spraying technique we use, gives droplets smaller than the micron in radius. This is the intermediate case of the Mie range between the small and large wavelength cases. Those new experimental results exhibit the role of the short and long range interactions in optical binding. They are then theoretically discussed both in the ray model and in the Rayleigh approximation, and compared with previous works on optical binding in water. Moreover, in our case, the index contrast is much bigger. It implies stronger scattered intensities, bigger interaction forces with light and therefore, bigger binding forces.