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Abstract
In this work, an adaptive liquid lens using a novel transparent electrically responsive fluid, dibutyl adipate (DBA), is demonstrated. The DBA liquid lens with a hemispherical plano-convex shape can change its curvature according to the application of various input voltages. More specifically, when an external direct current (DC) electric field is applied to the DBA liquid, the charges that are injected from the cathode move along with the DBA molecules toward the anode and accumulate on the surface of the anode. When the DC electric field is removed, the shape of the DBA liquid is recovered to its original state. This electrostatic force induces the deformation of the DBA liquid lens within a concentric annular anode electrode. In addition, the focal length of our system is increased from a value of approximately 7.5 mm to 13.1 mm when the voltage is changed from 0 to 100 V. Interestingly, the resolution of our DBA liquid lens can reach a value of ∼28.5 lp/mm. The proposed DBA liquid lens exhibits high optical transmittance (∼95%), good thermal stability (20–100°C), simple structure, and an excellent imaging property, which implies that the DBA liquid is a promising candidate for fabricating novel adaptive liquid lenses.
© 2022 Optica Publishing Group
Introduction. The adaptive liquid lens [1,2] can adjust its focal length by changing the shape of the liquid–liquid interface or the liquid–air surface. By considering the advantages of low-cost fabrication, high optical performance, incorporation of fewer mechanical elements, and enhanced versatility, the adaptive liquid lens possesses several potential applications in the fields of optical waveguides [3,4], imaging systems [5–8], mobile electronics [9,10], endoscopes, and machine vision systems [11]. According to the underlying driving mode, an adaptive liquid lens can be divided into the mechanical operation [12,13] and the direct electrical actuation [14]. Mechanical operation requires the involvement of external driving systems to control the movement of liquid. The response time, driving effect, and stability of this kind of liquid lens rely on the external mechanical driving system. Moreover, the respective cost of the mechanical operation is generally higher, and the driving system is bulky [15–17]. Compared with the mechanical operation, direct electrical actuation is more attractive because it has the advantages of compact structure, low power consumption, low cost, and accurate device control.
A typical adaptive liquid lens that is actuated by electrical variation is the conventional electrowetting lens [18,19]. In general, an electrowetting lens is fabricated by dripping a small amount of conductive liquid on the surface of an electrically insulating dielectric layer. When an external voltage is applied between the droplet and the electrode, the electric charges accumulate on the surface of the dielectric layer. As a result, the contact angle of the liquid droplet can be modified by the developed electrostatic force. The altered contact angle will subsequently modify the surface profile of the droplet. Electrowetting liquid lenses are widely used in various aspects of our life, including cell phone cameras, optical modulators, optical fiber switches, machine vision, and displays. Although the electrowetting liquid lenses have many advantages, the necessary liquid that is used in the electrowetting liquid lens is conductive, such as salt water, which is prone to volatilization. Furthermore, the contact of the electrode and the conductive liquids could lead to the production of microbubbles by the electrolysis process during the driving process.
To overcome these difficulties, liquid lenses based on the dielectrophoresis (DEP) effect [20,21] have been proposed. DEP is a phenomenon that occurs when a neutral particle is placed within an electric field that is spatially non-homogenous. The liquids used in DEP liquid lenses are mainly two kinds of immersible transparent liquids with a large difference in the dielectric constant and the refractive index. Furthermore, evaporation and the microbubbles generation effect will not occur owing to the intrinsic properties of these liquids. However, the employed driving electrode of the DEP liquid lenses must be patterned and coated with an insulting layer, which increases the development as well as manufacturing cost.
In this work, a transparent liquid, dibutyl adipate (DBA), is chosen to fabricate a robust electrical actuated liquid lens. DBA is widely used as a plasticizer for polyvinyl chloride (PVC). The employed transparent PVC gel [22], which consists of 10 wt.% PVC and 90 wt.% DBA, is a “smart” material for preparing adaptive lenses. When a DC voltage is applied to the PVC gel, the charges injected from the cathode will move to the anode together with the electronegative DBA molecules and will accumulate on the anode. After removing the external electric field, the PVC gels will recover to their original shape. The creep deformation of PVC gels results in focal length changes. By considering that the driving voltage of PVC gel-based lenses is usually very high (300–800 V) [23–25], the induced creep deformation is relatively small. Here, the DBA liquid is just employed as the material for the preparation of the adaptive lens. Even though there is no PVC network, the DBA liquid can also be actuated by an applied DC electric field. Therefore, the DBA liquid will move toward the anode and accumulate onto the surface of the anode. The underlying actuation mechanism of the DBA liquid is thoroughly studied and an adaptive liquid lens with a diameter of 1 mm is demonstrated. The proposed DBA liquid lens configuration exhibits an electroactive and reconfigurable behavior with the applied voltage. More specifically, immediately after removing the input voltage, the DBA liquid returns to its initial shape, demonstrating a reversible deformation. In contrast to the electrowetting liquid lenses, our DBA liquid lens exhibits the merits of no evaporation or electrolysis, which leads to a more stable optical performance. In addition, the DBA liquid is in direct contact with the electrode, which does not need to be treated with any insulting layer. Thus, the related fabrication procedures are greatly simplified compared to the electrowetting liquid lens and the liquid lens based on the DEP effect.
Mechanism and device fabrication. DBA (Tboiling point ∼305°C, nDBA ∼ 1.434, Tmelting point∼ −37.5°C, ρ ∼ 0.96 g/ml at 20°C, Sigma-Aldrich) was selected as the optical liquid to fabricate the liquid lens. Figure 1(a) reveals the molecular formula of DBA. From the viewpoint of the molecular structure, DBA is dipolar owing to the existence of the C=O bond [26]. Under the application of an external DC electric field, the DBA liquid exhibited a creeping motion between the two operating electrodes. To further investigate this mechanism, monodispersed polystyrene (PS) microspheres with a diameter of ∼60 µm were incorporated into the DBA and placed between two parallel electrodes under an optical microscope (OM), as is depicted in Fig. 1(b). The distance between the two electrodes was 5 mm.
Fig. 1. (a) Molecular formula for DBA. (b) Set-up for the electric response test. Optical images of the motion of spacers in DBA liquid under DC electric field: (c)–(e) 180 V and (f)–(h) −180 V.
Figures 1(c)–1(e) illustrate the optical images of the motion of the PS microspheres in DBA liquid under a DC electric field of 180 V. The induced PS microspheres were randomly and statically suspended within the DBA liquid in the absence of the electric field. In Fig. 1(b), a red line is inserted to highlight the initial position of the cluster of PS microspheres. The symbol “+” means that the anode is connected to the left side, whereas the symbol “−” connects to the right side. When an electric field was applied, the PS microspheres moved toward the anode (left). It is interesting to notice that when a 180 V electric field was applied continuously for a total period of 60 s, the PS microspheres were moved approximately 355.3 µm [Fig. 1(d)]. As the voltage was continuously applied, the PS microspheres migrated further and further away from the cathode. In addition, when the voltage was applied for 110 s, the PS microspheres were moved 697.4 µm [Fig. 1(e)].
After the electric field was applied for 140 s [Fig. 1(f)], the polarity was reversed. Consequently, the PS microspheres began to depart from the cathode (left) and move towards the anode (right) again. The inserted blue line highlights the initial position of the specified PS microspheres. More specifically, when the reversed electric field (V = −180 V) was applied for 40 s, the PS microspheres moved 618.4 µm [Fig. 1(g)]. After migrating for 60 s under the reversed voltage, the PS microspheres moved 907.9 µm [Fig. 1(h)]. The motion of the PS microspheres within the DBA liquid implied that the DBA flowed from the cathode to the anode when the electric field was introduced. Owing to the movement of the DBA liquid by the electric field, the surface profile of the DBA liquid could be changed.
If the anode is connected to a circular electrode, the deformation of the DBA liquid would be symmetric. The symmetric deformation of the transparent liquid is a good candidate for variable focus lens configurations. Figure 2(a) illustrates a schematic illustration of a hemispherical plano-convex liquid lens. The liquid lens consists of a printed circuit board (PCB) substrate with an annular Cu electrode, the DBA liquid, the mylar film, and an indium tin oxide (ITO)-coated glass substrate. The surface of the Cu electrode was coated with a hydrophobic layer, perfluorinated poly(butenyl vinyl ether) (known as CYTOP with a surface energy of ∼12.7 mN/m). Additionally, the diameter of the hole was 1 mm and the volume of the injected DBA liquid was 0.7 µL. The schematic presents a 3-dimensional (3D) cross-sectional view of an electrically tunable hemispherical plano-convex liquid lens driven within a circular aperture electrode of a PCB frame with and without applying external voltages. The operating mechanism of the electrically tunable liquid lens was attributed to the electric field-induced creep deformation of the DBA liquid under the enforcement of the external electric field. When no bias was applied, the DBA liquid formed a hemispherical plano-convex shape in the hole owing to surface tension, as is depicted in Fig. 2(b). However, when a certain electric field was applied to the liquid lens, the DBA molecules were charged and polarized. More specifically, as is shown in Fig. 2(c), the charged DBA molecules moved toward the anode and accumulated onto the surface of the anode. This creep deformation mechanism of the DBA liquid induced by the electric field can change both the shape and the curvature of the hemispherical DBA liquid lens, and thus a variable focal length can be attained with the proposed liquid lens configuration.
Fig. 2. (a) Schematic illustration of the device structure and actuation mechanism of the liquid lens (b) without applying any voltage and (c) applying a certain voltage.
Results and discussions. An ultraviolet-visible (UV-Vis) spectrophotometer (Cary 5000, form Agilent Technologies, USA) was employed to detect the transmittance of the DBA liquid. As is revealed in Fig. 3(a), the transmittance of the DBA liquid reached 95% in the wavelength range of 390–780 nm. Such a high optical transmittance implied that the light loss of the DBA liquid was very small. Furthermore, the thermal stability of the DBA liquid was evaluated by performing thermogravimetric analysis (PE TGA8000). The thermal weight losses of the DBA liquid in the temperature range of 20–600°C are displayed in Fig. 3(b). During the heating procedure, the heating rate was kept constant at 10°C/min. In the provided curve, it can be observed that there was little change in the temperature range from 20°C to 100°C, which implied that the DBA liquid was stable within this temperature window. When the temperature was higher than 100°C, the weight of the DBA liquid decreased quickly. Moreover, at a temperature of 250°C, the DBA liquid was almost decomposed completely. This result indicated that the DBA liquid exhibits good stability around the temperature range of 20–100°C. Because of the equipment limitations, the stability of DBA liquid below 20℃ was not measured. The melting point of DBA was −37.5℃ and could operate steadily for a long period at room temperature, which implied that the DBA liquid lens had good stability.
Fig. 3. (a) Transmittance curve, (b) TG curve of DBA liquid. Dielectric properties of DBA as a function of the frequency at room temperature: (c) dielectric constant, (d) dielectric loss.
To further understand the underlying operation mechanism, the dielectric properties of the DBA liquid were measured, as is disclosed in Figs. 3(c) and 3(d). In general, the dielectric properties of a material are defined by considering the complex dielectric constant ɛ* function, which consists of the dielectric constant ɛ′ and dielectric loss ɛ″ ,
where ɛ′ is the real part related to the electrical energy converted to stored energy within a material and is associated with the displacement of positive electric charges along the applied electric field, as well as that of negative electric charges in the opposite direction to generate a polarized dielectric material. In addition, ɛ″ refers to the imaginary part, which is also called dielectric loss, which represents not the stored energy but that dissipated within the material in the form of heat. The dielectric constant ɛ′ of the DBA liquid was estimated to be ∼4.5, whereas the extracted value exhibited slight fluctuations between 4 and 5 at all frequencies. As far as the dielectric loss ɛ″ of the DBA liquid is concerned, it was approximated to be 0 at all frequencies. The dielectric behavior of the DBA liquid from this measurement revealed that the polarization phenomenon would occur during electrical actuation. The DBA molecules were charged and polarized, then the polarized DBA molecules were able to follow the electric field polarity change.The structure of the DBA liquid lens is schematically illustrated in Fig. 1(b). The DBA liquid exhibited a convex shape in the holes owing to the surface tension. Because the refractive index of the DBA liquid (nDBA ∼ 1.434) is larger than that of the air (nair ∼ 1), the DBA liquid lens behaves as a converging lens. To characterize the optical performance of the DBA liquid lens, the respective image of an object through the lens when it is actuated by an external voltage was observed. A “Christmas snowman” toy was selected as the object and placed under the DBA liquid lens. A digital camera is set above the lens cell to record the final image. In the relaxed state (V = 0), a clear upside-down image was observed in white light, as is depicted in Fig. 4(a). When the voltage exceeded V = 25 V, the image became slightly blurry, which indicated that this voltage was the threshold voltage of the liquid lens, as can be ascertained from Fig. 4(b). By increasing the voltage, the image became increasingly blurry, as is shown in Figs. 4(c), 4(d), and 4(e), where the corresponding voltages were 50 V, 60 V, and 80 V. The blurry image implied that the focal length of the liquid lens had been changed. When the value of V = 100 V was enforced, the image was highly blurry and could not be recognized owing to the large defocus effect. After removing the electric field, the image recovered to its initial state. A dynamic video (Visualization 1) at 10 times speed presents the image change induced by the DBA liquid lens. At V = 100 V, τrise and τdecay were measured to be ∼25 s and ∼32 s, respectively.
Fig. 4. Images observed through the liquid lens under the application of various voltages: (a) V = 0 (Visualization 1); (b) V = 25 V; (c) V = 50 V; (d) V = 60 V; (e) V = 80 V; (f) V = 100 V.
To estimate the resolution of the DBA liquid lens, a resolution target (USAF 1951, Thorlabs) was used to replace the toy. Figure 5(a) shows the imaging system that was used for performance testing. In the relaxed state (V = 0), our eye could only distinguish group 4 and element 6 of the target. Hence, the corresponding resolution of the DBA liquid lens was ∼28.5 lp/mm. Moreover, the image of the resolution target that was observed through the DBA liquid lens at V = 0 is shown in Fig. 5(b). The observed resolution was relatively low, which was proposed to arise from the chromatic aberration and non-parabolic lens profile.
Fig. 5. (a) Imaging system used for the performance testing, (b) resolution of the DBA liquid lens in the relaxed state.
The focal length of the DBA liquid as a function of the applied voltage was measured by a home-made experimental set-up. A collimated He–Ne laser beam (λ = 633 nm) was employed to illuminate the DBA liquid lens, whereas the focused beam was analyzed by using a charge-coupled device (CCD) beam profiler (BC106 N-VIS, Thorlabs). The beam passing through the lens was focused to a spot. The distance from the liquid lens surface to the focal spot was defined as the focal length. The light beam passing through the DBA liquid lens is revealed in Fig. 6(a), while the measured focal length is revealed in Fig. 6(b) and the bars in the figure denote the experimental error of the focal length arising from the uncertainty in determining the beam waist. In the relaxed state (V = 0), the focal length of the DBA liquid lens was ∼7.5 mm. When the voltage changed from 0 to 100 V, the focal length varied from ∼7.5 mm to 13.1 mm.
Fig. 6. (a) Illustration of the light beam focused by the DBA liquid lens, (b) focal length change under the enforcement of different DC electric fields.
Conclusions. In this work, we present an adaptive liquid lens by employing a novel liquid, DBA plasticizers. The proposed DBA liquid exhibits a high transmittance ∼95% in the wavelength range of 390–780 nm, as well as a good thermal stability in the temperature range of 20–100°C. The DBA liquid lens structure consists of an annular Cu electrode coated with CYTOP, a common ITO electrode substrate, and pure DBA liquid. The employed Cu electrode is connected to the anode and the ITO electrode is connected to the cathode. The meniscus of the DBA liquid lens can be tuned by applying an external DC electric field. For a DBA liquid lens with a 1 mm aperture, the focal length varies from ∼7.5 mm to 13.1 mm by tuning the voltage from 0 to 100 V. Accordingly, the resolution of the DBA liquid lens is ∼28.5 lp/mm. Compared with the conventional electrowetting liquid lens, the DBA liquid cannot be easily evaporated or electrolyzed. In addition, the surface of the electrode does not need to be coated with an insulating layer, which largely simplifies the device structure of the liquid lens. For the proposed DBA liquid lens structure, the driving voltage is substantially decreased and the deformation is greatly enhanced compared with the PVC gel-based lenses. Our proposed DBA liquid lens with simple structure, high optical performance, good stability, and excellent imaging property exhibits various potential applications in the fields of waveguides, imaging systems, mobile electronics, endoscopes, and machine vision systems.
Funding
National Natural Science Foundation of China (61805066); China Postdoctoral Science Foundation (2019M652168).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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