Liquid lenses and driving mechanisms: A review

In this paper, we discuss liquid lenses driven by various mechanisms. By properly designing the device structure and choosing the optimal materials, the liquid lenses offer great potential for practical uses. The driving mechanism dictates the application and performance of the liquid lenses. Here we categorize the driving mechanisms into mechanical and electrical ones. In general, mechanical driving with an elastic membrane and an external pump drives liquids in a cavity by controlling the hydraulic pressure. The mechanical driving method can be applied to most of the liquids, but the application of the electrical driving method would be limited by the conductivity or the permittivity of the liquids. Therefore, the properties of the different liquids, e.g., dielectric liquids, liquid crystal molecules, and conductive liquids, deeply affect the mechanism we may choose to realize a liquid lens. Among various electrical methods, dielectrophoresis (DEP), electrostatic forces, and electrowetting-on-dielectric (EWOD) are emphasized here for driving dielectric liquids, liquid crystal molecules, and conductive liquids, respectively. DEP deforms the liquid lenses when the permittivities are different between the liquid and the medium. Electrostatic force orients the liquid crystal molecules to follow the applied electric field. Electrowetting-driven liquid lenses change their focal lengths by altering the contact angle. Here we show the designs and the structures of liquid lenses to describe their mechanisms, performances and feasibilities. It is worth mentioning that the liquid lenses using electrowetting have been commercialized. No moving parts would be the most important reason to use electrical manipulations rather than mechanical ones.