## 1 Introduction  
Adaptive-focus lenses have attractive applications in machine vision, mobile phone cameras, eyeglasses, image surveillance systems, target tracking, biometrics, and electronic displays. Unlike a glass lens whose focal length is fixed, the focal length of an adaptive lens can be tuned by changing the refractive index of the employed medium or the shape of the surface profile. For the former mechanism, liquid crystal (LC) is an effective medium because its optical anisotropy and dielectric anisotropy can be varied by an electric field. By controlling the LC directors of the LC lens cell with an electric field, a gradient refractive index distribution across the lens aperture can be changed. As a result, the focal length of the LC lens can be tuned. An LC lens offers a relatively fast response time, good stability, and low power consumption. However, most of the LC lenses are polarization dependent, and the fabrication process is rather complicated [1; 2; 3; 4; 5; 6]. To overcome these problems, polymer-dispersed liquid crystal [7] or polymer-stabilized blue phase LC [8; 9] can be considered. However, the phase change of these optically isotropic nanostructures is only one-third of a homogeneous aligned LC, thus, their dynamic range is rather limited, and the required voltage is usually very high. In comparison with the abovementioned LC lens, which is based on refractive index change, the adaptive lens based on surface profile change presents a large dynamic range, scalable aperture, and better optical performances. Various approaches, such as an elastic membrane [10; 11; 12], electrowetting effect [13; 14; 15], dielectrophoretic effect [16; 17; 18], ferrofluidic piston actuation [19; 20; 21],and soft electroactive actuators [22; 23; 24; 25; 26; 27; 28; 29; 30], have been demonstrated. Each approach has its own strengths and weaknesses. For instance, in an elastic membrane lens based on fluidic pressure, at least one elastic membrane is employed as its boundary. The shape of the membrane can be changed by pumping liquid in and out of its chamber using a piezoelectric actuator [12] or other mechanical methods [10; 11; 31; 32]. Depending on the volume variation of the liquid in the lens chamber, a large focus power change can be easily obtained. This type of lens usually faces the challenges of shape distortion, a gravitational effect, and mechanical actuation. Additional concerns come from the elastic membrane itself. Most elastic membranes, such as polydimethylsiloxane (PDMS), are physically stable in a dry condition. However, many liquids can cause them to shrink or swell under direct contact. These factors will degrade the optical performances of the lenses. Adaptive lenses based on an electrowetting and dielectrophoretic (dielectric) effect share some common features. For example, they belong to a liquid-liquid system, in which one liquid forms a droplet, while the other liquid fills the surrounding of the droplet. The purposes of these lenses with such a liquid-liquid system are mainly for minimizing the gravitational effect, lubricating the substrate surface, and enhancing the mechanical stability in case of shaking, vibrating, or shocking. In an electrowetting lens, salt water is the key liquid employed. The shape of the water is deformed by the generated electrostatic force [33; 34]. In a dielectric lens, both liquids are nonconductive but with different dielectric constants. The shape of the liquid-liquid interface is controlled by the generated dielectric force [16; 35]. A lens with a liquid-liquid system has some inherent issues: (1) the nonconductive liquid in the lens cell can be easily charged during actuation, leading the lens to work inefficiently; (2) a bulky structure because of two liquids and two substrates; and (3) multiple reflection and scattering due to substrate/liquid and liquid-liquid surfaces, degrading the optical performances of the lenses. A liquid lens based on ferrofluidic piston actuation can exhibit a large dynamic range and good optical performances [19; 20; 21].