Nature’s own tools help mimic the natural world., says S.Ananthanarayanan.

Eyes and eyesight are marvels of evolution and specialization. Cells that were sensitive to dark and shade were refined to form images that were focused by lenses. The light-sensitive cells formed into screens that are arrays of miniature receptors of unmatched sensitivity and colour differentiation. And then, the processing, to make sense out of the information that the system collects.

Animals go a step further and have pairs of eyes, a feature that helps them perceive the dimension of depth. And many of the arthropod class of animals, or animals which have exoskeletons, carry this to the extreme, with pairs of compound eyes, or division of the eye into thousands of separate units, which cover a wide field of view, all at once.

Technology has harnessed sensitivity to light in the form of cameras. Camera lenses have become specialized and the complexity of the retina is matched by fine-grained photo film, or the pixels of the camera screen. And to follow the path of compound eyes, scientists have developed arrays of lenses on polymer sheets that can be shaped into a hemisphere. The lenses throw separate images on an array of silicon photo-detectors with electrical connectors. A device with 180 active lenses was less than 2 cm across and held out the promise of greater things. Achieving the nanometer dimensions of the segments of the eyes found in nature, and building enough of them to create a larger compound eye, however, has proved out of reach.

Donglee Shin, Tianxu Huang, Denise Neibloom, Michael A. Bevan, and Joelle Frechette, from Johns Hopkins University, Baltimore, in ACS Applied Materials & Interfaces, a journal of the American Chemical Society, report a method that overcomes these limitations. Rather than fashion the microscopic components, Joelle Frechette and her team use nanometer sized oil droplets to work as micro-lenses, and these are deposited as a single layer covering another droplet of oil, to serve as the flexible body of a synthetic compound eye

The model that the Baltimore group used is the eye of the mosquito, “a source of inspiration for both its optical and surface properties”, the group says in the paper. The nanoscale features of the micro-lenses provide antifogging and antireflective properties and, as the lenses have very short focal length, all objects that are in view are in focus. ‘The hemispherical arrangement of micro-lenses captures images from all around, which the brain integrates, to achieve nearly peripheral vision while staying motionless’, the paper says.

The picture shows the compound eye bulging out of a mosquito’s head. We can see the segments, which are called ommatidae, of the compound eye, and that the eyes form the largest part of the mosquito head

The structure of the ommatidae is shown in the second picture. A feature of the tiny compound eye is that the units are of low resolution and economical in terms of data. This enables the brain to deal efficiently with scanning for food or danger, while other senses, like smell take care of detail. “The simplicity and multi-functionality of compound eyes make them good candidates for miniaturized vision systems, which could be used by drones or robots to rapidly image their surroundings”, says a press release from ACS.

Mimicking the structure, however, has been challenging, the paper says. While the focus has been on replicating the structure using fabricated lenses deposited on flexible substrates, the nanoscale features of the real object to replicate have been missing, as also a method to produce many artificial compound eye lenses at once. In the current work reported, the group addresses the challenge with the help of the curvature inherent to nanoparticles that are in the form of droplets, as a structure called the liquid marble.

The liquid marble is a drop of liquid whose surface has been coated with material that keeps the liquid away from other materials. A drop of water, which is naturally spherical, would spread out when placed on a glass sheet. On way of preventing this is to grease the sheet. Another would be to coat the drop with material that prevents contact with glass. The drop would then be a flexible object, which returns to being spherical, like a marble, when no forces are acting on it. When oil and water are mixed, again, the oil forms droplets, but small droplets can be dispersed uniformly, as an emulsion. The droplets, however, will progressively merge into larger droplets, unless this is prevented by adding particles that coat the droplets and keep them from direct contact.

The Baltimore group used a capillary device to generate nanometer-sized droplets of an oil, and surrounded the droplets by nanoparticles of silicon. These droplets, as they do not coalesce, thanks to their coating, form themselves into regular, patterned raft, a single layer. The droplets are assembled as such a single layer of droplets on the surface of another larger droplet – and the result is a liquid marble, of a spherical, liquid body surrounded by droplets of the same substance, in the form of nano-meter size spheres – just the structure of the compound eye.

And finally, the paper says, the lenses are treated so that the assembled structure stabilizes into a mechanically robust material – in the form of compound eye lenses that consist of a multitude of lenses of the same material. “As a result, the assembly process demonstrates a facile fabrication strategy to create a monolayer of micro-lenses encapsulating a liquid marble without an extra process,” the paper says. Apart from being an accurately formed artificial compound eye lens system, the process overcomes a major drawback in earlier methods, of not being scalable

“The process circumvents traditional fabrication challenges and reproduces both the optical and antifogging properties of the mosquito eye.” The process allows for the form of the liquid marble lens to be configured, which permits mobility and deformability to produce other types of lenses of this kind, the paper says.

“Further development of this process will advance the miniaturization of vision system applications, such as for medical imaging, reconnaissance devices, and robotics,” the paper says.

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