The focal length of the variable-focus optical system can be quickly changed within a certain range, so as to realize the clear imaging of the same objects at different positions or levels, and its function is beyond that of the fixed-focus system. The traditional variable-focus system has a mechanical movement, which restricts its application range. In order to realize zoom without mechanical movement, the variable-focus lens is the key, and an electrowetting liquid lens is an electrically controlled variable-focus lens with excellent performance.

At present, many researchers both in China and abroad have conducted a lot of in-depth research on the mechanism, performance, and application of electrowetting liquid lenses. However, due to the diversity of specific lenses and the complexity of the problems, the theory of electrowetting needs to be perfected, the performance needs to be improved, and new applications need to be developed. In terms of application, the performance of the liquid lens is the key. The two most important performance indexes of the variable-focus lens are the focal length and zoom time after zooming, which depend on the response characteristics of the electrowetting liquid lens. From the transient state to the steady state, the focal length after zooming is determined by the steady-state value, and the response time is determined by the transient process. Whether in theory or the experiment, the research on steady-state responses is much less difficult than that on the transient process, so the current research on the response characteristics of electrowetting liquid lenses mainly focuses on the steady-state response, while the research on the transient process is rarely reported. As a result, there is not a completely clear and unified understanding of the transient process.In this paper, the transient process of electrowetting liquid lenses is measured and analyzed, which is helpful to perfect and enrich the electrowetting theory and expand new applications.

A measurement system of the zoom process of electrowetting liquid lenses is designed and built (Fig. 1). The change process of the output voltage measured by the photoelectric detector with time reflects the zoom process of the liquid lens and indicates the change in the interface state of the liquid lens with time, that is, the response characteristics of the liquid lens. The response processes of an ARCTIC liquid lens (A-25H) under different driving voltages, including the response process of loading and unloading voltage, are measured by the self-designed system. According to the response process, the response times of loading and unloading under different driving voltages are obtained, and the measured results under various conditions are analyzed.

In this paper, it is proposed that the response of a variable-focus liquid lens driven by voltage includes inherent response and forced response. The functional form of the inherent response signal is determined by various parameters of the liquid lens, and it is not related to the driving voltage. However, its coefficient is related to the driving voltage. The forced response has the same functional form as the driving voltage. The change law of the transient process of liquid lenses depends on the inherent response and is independent of the driving voltage. Therefore, changing the driving voltage for the same liquid lens only changes the amplitude of the transient process response but does not change the changing law of the transient process, and thus the response time of the liquid lens is not affected. The experiment verifies that the response of liquid lenses can be decomposed into inherent response and forced response (Fig. 2). The response process and response time of the liquid lens driven by different voltages are measured [Fig. 5(d)]. It is found that the measured results are consistent with the theoretical analysis results. Moreover, the experimental result that the response signals of the liquid lens intersect at the same point A under different driving voltages [Fig. 4(b)] is analyzed, which further verifies the theoretical analysis of the proposed transient process of the liquid lens.In addition, the recoil phenomenon of electrowetting liquid lenses is observed. This phenomenon occurs in the initial stage of loading voltage and unloading voltage.

In this paper, it is proposed that the response of electrowetting liquid lenses includes inherent response and forced response, and the changing law of the transient process depends on the inherent response and is independent of the driving voltage. Therefore, the response time of electrowetting liquid lenses is independent of the driving voltage. This conclusion enriches the related theory of electrowetting liquid lenses and has direct guidance for its application. The recoil phenomenon of the electrowetting liquid lens is observed. This finding makes it possible to reasonably utilize or avoid this phenomenon and gives new factors to be considered in the design and preparation of electrowetting liquid lenses, which is helpful to enrich and perfect the working mechanism of electrowetting liquid lenses.