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Focal length is an important parameter to evaluate the performance of the varifocal liquid lens. To study the effect of the pre-stretched ratio of the dielectric elastomer on the focal length, two varifocal liquid lenses driven by dielectric elastomer with different pre-stretched ratios (250% and 300%) are developed in this paper. When an actuation voltage is applied to the compliant electrodes of the dielectric elastomer, the surface curvature of the droplet alters, thus changing the focal length of the varifocal liquid lens. The focal lengths of the two varifocal liquid lenses are measured by the magnification method. The results show that the varifocal liquid lens with a smaller pre-stretched ratio has a larger focal length when the actuation voltage is lower than 3.0 kV. When the actuation voltage is larger than 3.0 kV, the varifocal liquid lens with a larger pre-stretched ratio has a larger focal length. The effect of the diameter of the DE on the focal length of the varifocal lens is also studied. The results also indicate that the varifocal liquid lens actuated by DE with a smaller diameter has a larger focal length when the actuation voltage is lower than 3.0 kV. When the actuation voltage is larger than 3.0 kV, the varifocal liquid lens actuated by DE with a larger diameter has a larger focal length. The study is beneficial to the development of a varifocal liquid lens with a wide focal length range.
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