Biomimetic liquid lenses actuated by a laser beam: effects of evaporation and orientation to gravity
Liquid lenses actuated by thermocapillary and solutocapillary forces via heating with a laser beam demonstrate a high level of adaptability. The focal length and the optical aperture are tuned by the variation of the refracting surface curvature and the base diameter of a liquid droplet in response to the change of the laser intensity. Depending on the properties of liquids and the actuating forces, the lenses can operate in both converging and diverging modes. However, evaporation caused by heating, and high sensitivity to gravity may restrict applicability of the lenses in complex optical devices. This paper presents an experimental study of the optical characteristics of lenses consisting of ethylene glycol and a binary mixture of ethylene glycol and ethyl alcohol under conditions of long-term operation, performing multiple focal adjustments and arbitrary orientation of the optical axis. We have shown that stable continuous lens operation with a constant focus and continuous switching between two values of the focal length lasts for several tens of minutes after accommodation depending on the laser power and the driving forces. Then, evaporation leads to an irreversible change in the optical characteristics of the lens. We have also established that liquid lenses retain their optical characteristics and stable position when tilted to the horizon to several tens of degrees. The results obtained give a reason to believe that the studied lenses have high potential for applications in consumer and industrial optical devices.