Varifocal liquid lens actuated by laser-induced thermal Marangoni forces

This paper proposes a simple and easily implemented configuration of a varifocal liquid lens actuated by laser-induced thermal Marangoni forces. The lens consists of a sessile droplet of low-volatile liquid placed on a transparent solid surface. The focal length tunability is achieved by changing a local curvature of the droplet surface due to the thermocapillary displacement of liquid from the core part of the droplet to its edge caused by heating with a laser beam. Depending on the power of the laser beam, the droplet can operate as a varifocal convergent or varifocal divergent lens. In the convergent mode, the curvature of the droplet surface varies from an initial positive value to a near zero value (flat surface), whereas in the divergent mode, the curvature changes from a positive value to a negative value due to the formation of a thermocapillary dimple in the droplet.