Multifunctional Liquid Marble Compound Lenses | ACS Applied

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Multifunctional Liquid Marble Compound Lenses Donglee Shin, Tianxu Huang, Denise Neibloom, Michael A. Bevan, and Joelle Frechette ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12738 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Multifunctional Liquid Marble Compound Lenses Donglee Shin, Tianxu Huang, Denise Neibloom, Michael A. Bevan, and Joelle Frechette* Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218, USA

KEYWORDS Liquid marble, bio-inspired, mosquito eye, droplets, antifogging, Janus lenses, multiphase fluid

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ABSTRACT Mosquito compound eyes are elaborate multifunctional hierarchical structures. The presence of ordered curved features spanning length scales of nanometers to millimeters provides the mosquito eye with a wide field of view, an infinite depth of field, as well as antifogging properties. Developing bio-inspired compound lenses is challenging because of the need to mimic all characteristic curvatures along with their functionalities. Herein, we show how the curvature inherent to nanoparticles, emulsion droplets, and liquid marbles can be employed to mimic the hierarchical structure and functionality of mosquito compound eyes. At the nanometers to micron length scale we employ nanoparticle-stabilized emulsion droplets of photocurable oil to form microlenses with nanoscale surface features. After polymerization, the microlenses form a monolayer on an oil droplet to create an optically clear, millimeter scale, liquid marble that functions as a compound lens. We characterize the optical and surface properties of the compound lenses and find that they reproduce the functionality of the mosquito eye. Additionally, we exploit the mobility and reconfigurability of liquid marbles to create arrays (centimeter scale) of compound lenses and other types of functional lenses such as the Janus lens that magnifies the image acquired by the compound lens. Simple and scalable methods to create compound lenses could aid in the development of miniaturized advanced vision systems.


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INTRODUCTION Development of autonomous vehicles and robots necessitates efficient and cost-effective vision systems to image surroundings rapidly and under harsh environmental conditions.1 The compound eye found in most arthropods2-5 consists of individual microlenses organized on a curved array, and decorated with nanoscale protuberances (called nanopillars or nanonipples).6-7 The mosquito compound eye is a truly hierarchical, multiscale, and multifunctional structure, and is a source of inspiration for both its optical and surface properties. Nanoscale features are present on the microlenses (nanonipples, ~100 nm)8 to provide antifogging and antireflective properties (instead of a lubricated lid and pupil),7-10 whereas the microscale dimension of the microlens (ommatidium, 26 µm) provides a nearly infinite depth of field without focusing elements.2, 5, 11 The hemispherical arrangement of the ommatidia forms the compound eye (0.5 mm) and provides a large field of view (FOV).3 Each microlens captures an individual image, and the brain integrates all images from the compound eye to achieve nearly peripheral vision without requiring eye or head movement, and with minimal aberration.11 The compound eyes are also efficient in terms of data; the ommatidium has a lower resolution but allows imaging without overloading the brains of insects.12 Based on their simplicity and multifunctionality, compound eyes are good candidates for miniaturization of vision systems, either for autonomous vehicles, medical, military, or robotics applications.3, 5, 13 Challenges in synthetically reproducing the structures, characteristic length scales, and multifunctionality of the compound eye highlight the need to develop scalable methods to fabricate materials with multiple spherical features spanning length scales from nanometers to millimeters.8 Current approaches include multi-step processes relying first on lithography followed by mechanical deformation.2, 13-14 Prior work replicated the curvature of both the ommatidium and of


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the compound eye, with limitations either on the size of the structure or the curvature (and the FOV),14-16 without the nanoscale surface features. The focus of prior work has been on replicating the curvature of both the ommatidia and of the compound eye, but critically lacking its nanoscale surface features (nanonipple). An additional challenge in the development of processes to mimic the mosquito eye involves scalability. Often processes produce a single compound eye and cannot readily be scaled up to produce multiple eyes at once. Therefore, it remains a challenge to develop a general process to produce a bio-inspired compound eye that is both scalable and reproduces the key multiscale features and functionality of the mosquito compound eye. The hierarchical structure and curvature of the mosquito compound eye is a good target for the development of novel fabrication methods. A potential and versatile avenue to produce compound lenses would be to design a liquid manufacturing process, where necessary curvatures and assembly occurs between liquid droplets acting as precursors for individual ommatidium. The fluid nature of droplets makes them readily deformable and dynamic. When interfacial stabilization is present, droplets can assemble into 2D arrays or 3D structures.17 The curvature of droplets can also be controlled precisely. Recent developments in microfluidics have led to the large-scale production of monodispersed liquid droplets with a wide range of functionalities, and where the droplets produced serve as microscale container of nanoparticles or other cargo.18 For optical applications, monodispersed droplets containing a large volume fraction of nanoparticles have been used to produce photonic crystal devices such as films,19 domes,17 and balls.20 Moreover, liquid droplets can be stabilized by micro- or nanoparticles to form either Pickering emulsions21 or liquid marbles.22 Liquid marbles are droplets consisting of a liquid core that is protected by a coating of particles.23-24 Liquid marbles display elastic properties and robustness against coalescence, which allow them to roll and move on surfaces. Liquid marbles have been


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employed towards optical, chemical, biological, and microfluidics applications. For example, liquid marbles have been used as a transformable water-lens,25 microcontainers,23 sensors,26 in cosmetics and pharmaceutical formulation,27 and they interact with light to create optically responsive reactors,28-29 or light-driven delivery vehicle.30-31 We hypothesize that compound lenses inspired by the mosquito eye can be created by combining the mobility and stability of liquid marbles with the monodispersity and large-scale production of droplet-based microfluidic devices. In addition, a chemically patterned substrate can host and localize multiple sessile droplets where each could serve as the hemispherical dome structures of compound lenses, leading to a scalable process.32-34 Here we introduce a liquid manufacturing process based on the assembly of liquid droplets to introduce naturally the necessary curvature and deformability that alleviate fabrication challenges of compound lenses. Previous approaches showed that an individual liquid droplet could act as an optical element.35-37 However, our work is the first to rely on droplets to form functional hierarchical optical lenses with the key features spanning length scales of nanometers to millimeters. Our building blocks are droplets stabilized by nanoparticles and produced via microfluidics for large-scale production and monodispersity. The droplets act as polymerizable microlenses and assemble as a monolayer onto larger sessile droplets to form optically transparent liquid marbles. The formation of a monolayer of microlenses with the same refractive index as the droplet leads to first demonstration of liquid marbles that function as compound lenses. As a result, the assembly process demonstrates a facile fabrication strategy to create a monolayer of microlenses encapsulating a liquid marble without an extra process, such as a cleaning38 or evaporation.39 We then rely on the mobility of liquid marbles under shear to create arrays of


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compound lenses. Finally, we polymerize the lenses to arrest the assembled structure and form a mechanically robust hierarchical material.

EXPERIMENTAL SECTION Materials. We use the Stöber method to synthesize monodispersed silica nanoparticles (NPs) with diameters ranging between 120-600 nm.40 The fabricated NPs are washed five times with ethanol and mixed with ethoxylated trimethylolpropane triacrylate (ETPTA, Sigma-Aldrich) containing 1 vol% photoinitiator (2-Hydroxy-2-methylpropiophenone, Sigma-Aldrich). After the mixing, the ethanol is selectively evaporated in an oven for 12 h at 70°C, and the remaining final oil phase solution is ETPTA with 5 wt% silica NPs. Before the microfluidic experiments, the polymer solution is sonicated for 30 min. The aqueous phase solution consists of deionized (DI) water with 1 wt% surfactant (Pluronic F108, Sigma-Aldrich). Microlens Fabrication via Microfluidic Devices. For droplets over 100 µm diameter, we utilize a glass capillary microfluidic device.19 We fabricate the device with a rectangular glass capillary (400 µm, VitroCom, 8240) as an outer capillary, with its inner surface rendered hydrophilic through a surface treatment with 3-aminopropyltrimethoxysilane (Gelest). A circular glass capillary (VitroCom, CV2033) with 200 µm inner diameter (ID) is tapered to have a 130 µm orifice and inserted into the square capillary to form the inner capillary. The typical outer flow rates (OF) of the aqueous phase are in the range between 400-1500 µL min-1, and the oil phase inner flow rates (IF) are between 2-10 µL min-1 to create droplets with radii ranging between 100250 µm. Droplets with 110, 124, 149, and 203 µm diameters are obtained with OF of 1500, 1000, 500, 500 µL min-1 and IF of 2, 2, 4, 10 µL min-1, respectively. To produce droplets with diameters less than 100 µm, we use droplet generator devices molded from poly(dimethylsiloxane) (PDMS)


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using the Sylgard 184 (Dow Corning) PDMS kit, and mixed in a 10:1 weight ratio. The PDMS mixture is degassed in a vacuum oven, poured over SU-8 patterned silicon wafers, and cured at 80 o

C for at least 2 h. Once cured, the PDMS devices (fluid-focusing outlet with 40 µm width and 50

µm height) are cut and bonded to clean glass slides via exposure to oxygen plasma at 45 W for 45 s. Immediately after, the devices are rendered hydrophilic using a layer-by-layer technique41 in which solutions of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) are flushed alternatingly through the microfluidic device. These treatments are repeated twice more to make the channel walls sufficiently hydrophilic. Then, the devices are flushed with DI water and stored in saturated humid air until use. The 25 µm droplets are produced at IF of 50 µL h-1 and OF of 400 µL h-1. The droplets are collected at the outlet of the microfluidic device and exposed to UV-light (UVP, UVGL-55) for 30 s (365 nm wavelength, 6 W) for polymerization. Assembly of the Microlenses to Form Compound Lenses in Water. The polymerized microlenses are washed more than 7 times in DI water using a vortex instrument to remove the Pluronic surfactant prior to deposition (otherwise the microlenses will not anchor at the oil-water interface). A sessile droplet of ETPTA containing the photoinitiator (1 vol%) is prepared on a plain microscope glass slide (Fisherbrand) in DI water, and the washed microlenses are collected and released above the sessile droplet via micropipette. Anchoring of the microlenses at the oil-water interface is driven by a reduction in interfacial energy. The microlenses organize into an array of close-packed microlenses via capillary assembly to form a compound lens that is then UVpolymerized (UVP, UVGL-55) for 1 min to yield the final structure in water. Preparation of OTS Patterned Glass Substrates. We create hydrophobic circular patterns through microcontact printing of octadecyltrichlorosilane (OTS) on glass slides using PDMS stamps (Figure S1). The PDMS stamps are fabricated using a soft lithography process


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analogous to the one employed to create the PDMS microchannels for drop fabrication. After the elastomer is cured in the oven, a PDMS slab is cut into individual stamps and cleaned with isopropanol for storage. The ink used is 6 mM OTS in toluene. To print the patterns onto the slides, the glass slide is first exposed to 100 W oxygen plasma for 2 min. After plasma treatment, the slide is placed on a hot plate set at 70 oC. A cotton swab is used to apply a thin layer of the OTS ink to the stamp surface. The stamp is then pressed firmly on the slide for 1 min, followed by a rinse with acetone and drying with nitrogen. Characterization. The micro and macroscale structure of the compound lens are imaged using a stereomicroscope (Olympus, SZX12) and an optical microscope (National, DC-128). The nanoscale surface features of the compound lens are imaged with an ESEM (Holand FEI Company, FEI Quanta 200 FEI) after gold sputtering. The wetting properties and water contact angle hysteresis are analyzed by using a video-based contact angle and surface tension meter (First Ten Angstroms, FTA 125). Measurements of the Focal Length. To measure the focal length (f), which is the distance between the vertex and the focal point of a microlens, we first focus on the core of the microlens (set z (z-position of microscope stage) = zc). Then, we adjust the focus knob until we achieve the sharpest output image, which is the focal point (set z = zf). The difference between the two zpositions (zc−zf) gives the distance between the core to the focal point, and the summation of the microlens radius (r) and z-position difference (zc −zf) is equivalent to the focal length of the microlenses, which is the distance from the front of the lens to the focus.

Determination of the FOV. We image numbers from ‘1’ to ‘20’ printed on a hemispherical transparent sheet. The curved sheet was 15 mm in diameter, and the number


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between ‘12’ and ‘13’ is located on the center indicating 0°. Each space between single digit numbers from ‘3’ to ‘9’ indicates the increment of 8°, and each space between numbers from ‘10’ to ‘18’ is 13°. The angles were calibrated by using a protractor prior to imaging. Hence the number ‘4’ and ‘6’ indicate -76° and -60°, and the number ‘17’ and ‘18’ indicate 60° and 73°. Therefore, the measured overall viewing angle (from ‘4’ to ‘18’) is 149° Surface Functionalization. We use a home-built induction plasma reactor to clean and activate the surface (superhydrophilic, water contact angle (CA) < 5°). The samples are pumped down to a pressure of 10 mTorr, then O2 flows in the chamber to maintain a pressure of 300 mTorr. Plasma is induced at 40 W for 30 s. To render the compound lens superhydrophobic, we place the lenses in toluene with 6 mM octadecyltrichlorosilane (OTS, Sigma-Aldrich) for 30 min after the plasma treatment. After the treatment, the measured water CA is 165°. A longer immersion in the toluene solution (30 min) does not increase further the CA, so we selected 30 min immersion to avoid the risk of any possible structural deformation. We estimate the contact angle hysteresis (< 5°) by placing a 9 μL water droplet on a slightly tilted (4°) compound lens and observe that the droplets roll-off.


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RESULTS AND DISCUSSION

Figure 1. Overview of the droplet-based multiphase process to create a hierarchical structure inspired by the mosquito compound eyes. (a) Schematic of experimental setup for microlens fabrication. (b,c) SEM images of a UV-cured microlens with nanoscale surface topography (b) and hexagonal arrangement of the silica nanoparticles (300 nm) on the surface of the microlens (c). (d) Schematic of assembly of the microlenses into a compound lens in water. (e,f) Stereomicroscope images of the microlens-decorated dome (convex-planar shape) (e) and hexagonally close-packed microlenses on the dome (f). The oil droplets contain a dye for imaging.


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Compound Lens Assembly. The fabrication process revolves around two main serial steps that leverage the dynamic nature and curvature of fluid interfaces (Figure 1). Each step consists of droplet formation, particle assembly, and arrest of the target structure (for details see the Experimental Methods). First, capillary microfluidic devices produce individual droplets of a photocurable oil (ETPTA, ethoxylated trimethylolpropane triacrylate) decorated with silica nanoparticles (Figure 1a-c). The diameter of the droplets is uniform and controlled by the flow rates and the diameter of the capillary. Nanoparticles present in the oil phase partition to the oilwater interface to add nanoscale features during drop formation (Figure 1c). These nanoscale features mimic the non-close packed hexagonal array of nanonipples present on the ommatidia of mosquito eyes. At the outlet of the capillary, we photopolymerize the droplets and collect them. The polymerized droplets will be the microlenses in the final compound lenses. In the second step, we disperse the microlenses in water and deposit them on a larger sessile oil droplet via capillary assembly (Figure 1d). The microlenses organize into a close-packed array on the oil droplet to form a compound lens (Figure 1f) that we polymerize to yield the final structure. The polymerization steps create mechanically robust hierarchical structure that can be easily removed from water for future use. The overall fabrication process leads to control of the nanostructure (nanonipples, 120-600 nm), microstructure (microlens, radius of 25-250 μm), and macrostructure (compound lens, radius of 0.5-5 mm). At the end of the process, the structure of the final bioinspired compound lens is compared with the reported structure of the mosquito compound eye8 as shown in Figure S2. The fabricated compound lens has characteristic length scales that closely mimic those in compound eyes found in nature such as those of mosquito8, arthropods,5 moth,10 dragonfly,2 and fly.7


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The success of the fabrication process relies on controlling over interfacial interactions. Bare silica nanoparticles (NP, 5 wt% in oil phase) spontaneously partition to the oil-aqueous interface to reduce its interfacial energy during the formation of the microlens. At the oil-water interface, electrostatic repulsion between the negatively charged silica NPs likely leads to the formation of a hexagonal array (Figure 1c).42 Matching between the refractive indices of the oil (noil = 1.47) and of the silica NPs (nsilica = 1.45) ensures near optical transparency43 and also diminishes the van der Waals attraction between the particles,19 favoring their colloidal stability. In the second step, anchoring of the microlenses at the oil-water interface is again driven by a reduction in interfacial energy. Based on the SEM image analysis of the microlens surface (Figure 1c), their surface consists of approximately 64 % of polymerized oil (the remainder being the silica NPs). After polymerization, the presence of negatively charged silica NPs on the surfaces of the microlenses prevents their aggregation in the aqueous phase. The lateral capillary force (immersion force)44 driven by wetting (Figure S3) between the microlenses lead to the formation of a hexagonal close-packed array (monolayer) at the oil-water interface (Figure 1e,f).


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Figure 2. Reconfigurable structure and assembly of the bioinspired compound lenses. (a) Schematic of the fabrication process to prepare multiple compound lens on a pre-patterned surface. (b) Deposition of a compound eye liquid marble onto thin circular ETPTA patches. (c) Polymerized compound lens arrays (8×8). (d) Optical microscope images demonstrating control of the shape of the deposited compound lens through patterning of underlying ETPTA patches. (eg) Cross-sectional (e,g) and side (f) views of alternate lens types. The curvature of the lenses is manipulated via control of the interfacial properties: a highly wettable concave mold for the inversed contact lens shape with the microlenses on the inner radius (e), a low wettability concave mold for the Janus compound lens shape (f), or introduction of immiscible third fluid phase for the contact lens shape with the microlenses on the outer radius (the third fluid phase is removed after photopolymerization) (g).


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Reconfigurable Liquid Marble Compound Lenses. A unique aspect of the process is the capability to move, deform, and relocate the compound lenses. External vibration prior to the final polymerization step converts the deposited compound lens into liquid marbles (Figure S1a and Supporting Video S1).22 Armored by the microlenses, the liquid marbles are mobile and can readily roll without leaving residue on a solid substrate (the energy required to remove a microlens is much higher than the shear stress). We can then deposit and arrest the marbles at pre-defined locations on a surface patterned with thin patches of liquid (ETPTA) to create an array of compound lenses (Figure 2a). To deposit and arrest the liquid marbles, glass surfaces are first patterned with hydrophobic patches of octadecyltrichlorosilane (OTS) using conventional microcontact printing with a polydimethylsiloxane (PDMS) stamp (Figure S1b) as described in Experimental Methods, and the patches are covered with thin films of ETPTA through dewetting (Figure S1c). When a rolling marble meets an ETPTA patch, it coalesces with the patch and returns to its lens shape (Figure S1d and Supporting Video S2). Importantly, throughout the deposition process, the microlenses remain as a monolayer at the oil-water interface, and a microparticlestabilized liquid marble does not coalesce if it contacts other liquid marbles (Figure 2b and Supporting Video S3). Therefore, through the ease to form mobile and reconfigurable liquid marble lenses we show the feasibility to fabricate of arrays of bio-inspired compound eyes, as shown in Figure 2c (for details see Figure S4). Control over both the surface wetting properties and liquid volume allows us to create different types of compound lenses. If a compound marble encounters an oil patch, it will coalesce with the oil and maintain the shape of the underlying patterned surface. Therefore, it is straightforward to create non-circular compound lenses by changing the feature shapes on the


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surface (see Figure 2d). For practical importance, we show that the same general process can create very different types of lenses through manipulation of the surface chemistry, topography or even the introduction of an immiscible third-phase fluid on the supporting substrate (Figure 2e-g). For example, if the substrate is patterned with spherical cap cavities, we can create a lens with the curvature of the opposite sign. Consider a PDMS substrate with a cavity shown in Figure 2e,f. The ETPTA oil wets PDMS in water (contact angle 90o. Therefore, we are able to achieve a concaveconvex compound lens (Figure 2e) on the wetting surface and a convex-convex (biconvex) lens on the non-wetting one (Figure 2f). In particular, the biconvex lens, which has a front compound lens and a rear classical lens, is a “Janus compound lens”. As a result, the input image captured by the microlens array is magnified by the rear conventional convex lens, so the output image encompasses both compound and classical lensing properties (Figure S5). Such a Janus compound lens is extremely challenging to fabricate using existing methods for compound lenses. We also create compound contact lenses (convex-concave, Figure 2g) through a similar process but with the introduction of a third immiscible fluid. Here the oil phase (ETPTA) must completely wet the interface between water and the immiscible third phase (which is a fluorocarbon oil, FC-70, in this experiment). Complete wetting requires that the spreading coefficient (S), 𝑆𝑆 = 𝛾𝛾F,W − 𝛾𝛾E,F − 𝛾𝛾E,W ,

to be positive. Here γF,W, γE,F, and γE,W are the interface tension between FC-70 and water, ETPTA and FC-70, and ETPTA and water interface, respectively. We obtain a spreading coefficient of S = 24.3 ± 1.1 mN m-1 based on reported values (γF,W = 45.0 ± 1.5 mN m-1, γE,F = 12.5 ± 0.3 mN m-1, γE,W = 8.2 ± 0.1 mN m-1).45


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Figure 3. Optical properties of the compound lenses. (a) Schematic of the experimental setup to determine the focal length of an individual compound lens. (b,c) Optical microscope image using a 4× objective focused on the vertex of a small group of microlens (125 µm radius) that are part of an individual compound lens (convex-planar) (b), and the number projected by the microlenses focused on the focal point (c). (d) Agreement with the lensmaker equation demonstrating control over the measured focal length as a function of the radius of the microlens. Error bars represent the standard deviation of five measurements. (e) Set-up for the measurements of the field of view with a hemispherical transparent sheet marked with number from ‘1’ to ‘20’ (5 rows) used as an input image for a compound lens (width = 4.6 mm, and height = 1.2 mm). (f) Output images projected by the microlenses (radius 125 µm) of the compound lens at different viewing angles, achieved by tilting the microscope stage from -60° to 60°. The scale bars represent 100 µm.


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Optical Properties of Artificial Compound Lenses Recent bio-inspired compound lenses5, 11, 36, 46-47 incorporate microlenses with focal lengths in the range of 10-1350 μm since the microlenses, through their short focal lengths, have a nearly infinite depth of field.5 The radii (r) and refractive index (here n = 1.47) of a microlens modulates its focal length via the lensmaker equation (f = nr/(n − 1)).13 We can create microlenses with different radii (25-250 μm) by simply manipulating the flow rates of the inner (oil) and outer (aqueous) phases (Figure 1a) as described in the Experimental Methods. We use 180 nm silica nanoparticle (5 wt% in oil phase droplet) to fabricate the surface nanostructure on the microlenses as shown in Figure S6, and the size, periodicity, and spacing were found to be 163 ± 12 nm, 260 ± 19 nm, and 85 ± 9.4 nm, respectively. Limiting the size and spacing of the nanostructure under 190 nm helps to limit light scattering due to the surface roughness or the presence of condensed water droplets.7-8 The average size, periodicity, and spacing statistics are analyzed using ImageJ software. We prepare 4 different compound lenses (convex-planar shape) decorated with microlenses of uniform radii of 110, 124, 149, and 203 μm and then measure the focal length following a reported protocol (Figure 3a),36 see Experimental Methods for details. The hexagonally-close packed microlens array (Figure 3b) projects multiple identical images when the microscope objective focuses on the focal points of the microlenses (Figure 3c). The measured focal lengths are compared to predictions from the lensmaker equation. Based on our measurements, we obtain focal lengths between 325 - 650 μm, in good agreement with predictions (Figure 3d). The measured and estimated focal lengths of the microlenses fabricated by our liquid-based process are comparable to the other bio-inspired compound lenses made through different methods. The hemispherical structure of the ommatidia of an arthropod eye provides an exceptional viewing angle (between 140-180 degrees).5 For a synthetic compound lens, supporting the


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microlens on a curved macrospherical dome is necessary to achieve a wide viewing angle. A prior fabrication process relies on the mechanical deformation of arrays of microscale features to create a curved dome.2 Therefore, the deformation process alters the spacing between the microlens and their shape. In contrast, our sequential process where we create macroscale curvature prior to the microlens deposition allows us to control the dimension (and associated curvature) of the macroscopic dome without affecting the spacing between the microlenses or their dimensions. As a result, we obtain a large viewing angle for the compound eye. For example, we create four compound lenses with different heights (1.2, 1.8, 2.1, and 2.3 mm) but with the same radius (2.3 mm) by increasing the volume of the photocurable oil (10, 12, 17, and 25 μl) on circular hydrophobic patches (Figure S7a). The samples with heights of 1.2 and 2.3 mm represent the compound lens with low and high curvatures, respectively. A change in volume changes the curvature of the macroscopic dome, and the curvature change results in the change of the viewing angle. We can estimate the field of view (FOV) as:11 2𝑅𝑅ℎ

(1)

FOV = 2 arcsin(𝑅𝑅2 +ℎ2 ),

where h and R are the height and the radius of the macrospherical dome, expecting FOV of 110°, 152°, 170°, and 180° with increasing fluid volume. We confirm the predicted wide FOV through a concave-convex shape compound lens (with 1.2 mm height and 2.3 mm radius) by reading numbers from ‘1’ to ‘20’ (5 rows) printed on a hemispherical transparent sheet (Figure 3e and Figure S7b). The concave-convex shaped lens minimizes light distortions at the edge of the compound lens, so is better suited to measure the FOV. The marked numbers indicate the viewing angle, and we captured images through microlenses located on the two opposite side edges (-60° and 60°), the middle (-40° and 40°), and the center (0°) of the compound lens (Figure 3f), and the details are in Experimental Methods. Based on these measurements, we find that the overall


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viewing angle is 149°, with evenly spaced and undistorted images.

Figure 4. Antifogging properties caused by surface nanostructures. (a,c,e) The output images projected by the compound lens with a nanostructured superhydrophilic surface (a), with a nanostructured superhydrophobic surface (c), and without a surface nanostructure (e) under an artificial fogging flow. The scale bars represent 200 µm. (b, d, f) Schematic of water condensation on the compound lens. The continuous water film prevents the growth of individual water droplets on the superhydrophilic microlens surface (b), droplets condensed on the superhydrophobic nanostructure are growing on the edges of the microlenses (d), and the growth of water droplets on microlenses without nanostructure makes the image unreadable (f).


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Nanoparticles Provide Antifogging Properties. Antifogging properties are necessary for the compound lens to function under many environmental conditions, such as a highly humid or a low temperature environment. Fogging on the surface of an optical lens scatters light, and can detrimentally affect optical performance. To function in humid environments, the nanonipples on the surface of the ommatidia render the mosquito eyes naturally antifogging based on their hydrophobicity and on their hierarchical surface structure.8 Previous studies on the antifogging properties of real compound eyes indicate that the high water CA (higher than 155°) and the low contact angle hysteresis (less than 15°) are essential to reproduce the functionality.7-8 In the bioinspired compound lens, the silica nanoparticles on the surface of the microlens (Figure 1c) can provide antifogging properties. In general, fogging can be prevented on surfaces with functionality displaying extreme wetting characteristics such as suphydrophilicity48 and superhydrophobicity.8 After simple surface treatments (details see the Experimental Methods), the water contact angle (CA) of the compound lenses can be either superhydrophilic (CA < 5°) or superhydrophobic (CA = 165°) without affecting the architecture or the optical properties of the lenses (Figure S8a,b). We test the antifogging performance of the compound lenses under an artificial fogging flow (using a commercial humidifier with a specified flow rate of about 2.63 mL/min) created by an ultrasonic humidifier for 80 s. For the superhydrophilic lens, the antifogging properties are due to the spreading of the condensed water droplets on the compound lens (film-like condensation), see Figure 4a. After 40 s, we observe some fogging due to the presence of water droplets on the microlenses, causing some overlapping output images. However, the water droplets rapidly spread and coat the entire surface of the compound lens to produce a thin continuous water film, as shown in Figure 4b. Once the film forms, no further fogging occurs, and the output image remains clear.


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When the compound lens is superhydrophobic with low CA hysteresis (Figure S8c and Supporting Video S4), antifogging properties are due to the sliding and collection of water droplets in the interstices between the microlenses, as shown in Figure 4c,d, allowing us to observe clear output images. We can compare these two cases to a control experiment where we prepared the compound lenses without a nanostructure (Figure 4e,f). In the control, we observe the presence of microscale water droplets that grow on the microlens and distort the image (for details of the wetting property see Figure S8d,e). The distortion occurs because individual water droplets act as independent small microlenses (Figure 4f), causing overlapping and duplicated output images for a single microlens, rendering the output images unreadable. In short, the hierarchical structure of the compound lens that includes the silica nanoparticles allows us to image under high humidity either through the formation of a continuous water film or via sliding of larger condensed droplets.


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CONCLUSIONS We demonstrated a process based on liquid marbles to create compound lenses inspired by the mosquito compound eyes. Instead of relying on microfrabrication, the compound lenses originate from optically transparent liquid marbles that are protected by a monolayer of microlenses with matching refractive indices. The microlenses are themselves individual droplets laden with silica nanoparticles and produced simply via capillary assembly. Individual compound lenses can form an array by rolling the liquid marbles on a surface with pre-defined surface patterns. The resulting lenses are hierarchical and seamlessly integrate characteristic length scales from nanometers to millimeters. The process circumvents traditional fabrication challenges and reproduces both the optical and antifogging properties of the mosquito eye. The reconfigurable nature of the liquid compound lens provides mobility and deformability to produce other types of compound lenses such as a Janus compound lens, which has optical features of both single lenses and compound lenses. The focal length of the microlenses (325-650 μm) leads to a nearly infinite depth of field and the hemispherical dome to a large FOV (measured 149o). In addition to the optical properties, the presence of nanonipples on the surface provides the compound lens with antifogging properties. Future studies could be carried out to build a complete liquid-based compound eye vision system. Further development of this process will advance the miniaturization of vision system applications, such as for medical imaging, reconnaissance devices, and robotics.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Conversion of a deposited compound lens into a liquid marbles, Structural comparison between a real mosquito eye and the fabricated compound lens, wetting behavior of a microlens at waterETPTA interface, formation of a larger array of compound lenses, optical properties of Janus compound lens, surface nanostructures of microlens, controlled FOV with different fluid volume, wetting properties of the surface treated compound lenses (PDF) Video S1: Conversion of a Compound Lens into a Liquid Marble (MP4) Video S2: Conversion of a Liquid Marble into a Compound Lens (MP4) Video S3: Mobility and Stability of a Rolling Liquid Marble (MP4) Video S4: Water Repellant Property (MP4)

AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions J.F. and M.A.B. devised the project. J.F. and D.S. designed the experiments, worked out the technical details and interpreted the data. D.S., T.H., and D.N. performed the experiments. D.S.,


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D.N. and J.F. drafted the manuscript. D.S., T.H., D.N., M.A.B., and J.F. discussed the results, critically edited the manuscript, and approved the final version of the manuscript. Funding Sources Department of Energy BES DE-SC0017892, National Science Foundation 1562579. D.N, and National Science Foundation Graduate Research Fellowship under Grant No. DGE-1746891. Notes The authors do not declare any competing interests.

ACKNOWLEDGMENT This work was supported by the Department of Energy BES DE-SC0017892 as well as preliminary work on patterning droplets through under NSF 1562579. D.N. would also like to acknowledge the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1746891.


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Multifunctional Liquid Marble Compound Lenses | ACS Applied

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