The first author of the study, Washington State graduate student Mojtaba Falahati, holds one of the lenses made by this method. [Picture: WSU]

Plastic lenses are becoming more and more widely used in optical equipment, and this is for good reason-they are light weight, low cost, versatile, and have sports optical properties suitable for many applications. However, a large part of the cost equation of some plastic lenses lies in the expensive molds and related equipment required in the manufacturing process. This makes the decks conducive to mass production, rather than using molds to make small batches or custom plastic lens designs.

A research team from two universities in the United States has developed an ingenious, low-cost alternative to make a special type of plastic lens-plano-concave lens (Appl. Phys. Lett., doi: 10.1063/1.5090511). Instead of making complicated or expensive molds, the team’s method uses magnetically controllable ferrofluid droplets to shape the surrounding resin and make lenses. The researchers believe that this method, which they describe as "low-cost, fast, and straightforward," is particularly useful when developing lenses for small batch purposes, such as rapid manufacturing and testing of lenses of different sizes and shapes.

The team includes engineering researchers from Washington State University and Ohio State University. They focus on the problem of using polydimethylsiloxane (PDMS) to make lenses. Polydimethylsiloxane (PDMS) is due to its good transparency. , Biocompatibility and flexibility, and become a strong candidate for a variety of optical applications. This type of lens is most commonly produced using molding techniques for mass production, although some groups have developed alternative niche manufacturing methods that use photolithography, laser engraving, and other methods.

Researchers in Washington and Ohio are particularly interested in manufacturing lenses for the smartphone-based laboratory equipment they are developing, but want to avoid the cost and complexity of commercially molded lenses. After failing to try 3D printing the lenses they needed in the laboratory, they came up with the idea of ​​creating a liquid mold that can be controlled magnetically.

The core of the system is a drop of fluid—in this case, a commercially available water-based ferrofluid impregnated with superparamagnetic nanoparticles. The researchers put a drop of liquid on a layer of uncured liquid PDMS resin in a petri dish. Since the two liquids have different degrees of hydrophilicity, the PDMS and ferromagnetic droplets will not mix, but remain completely separated. The lower density ferromagnetic fluid is initially located on top of the resin.

The shape of the ferrofluid droplets can be controlled by whether the magnets are set to attract each other (upper left corner) or repel (lower left corner) and the distance between the two magnets. This can be adjusted by using homemade equipment in the laboratory (right). [Image: Reprinted from M. Falahati et al., Appl. Physical Wright. 114, 203701, doi: 10.1063/1.5090511, authorized by AIP Publishing】

Next, the team placed the petri dish on the stage of a self-made instrument equipped with rare earth neodymium cylindrical magnets, with rare earth neodymium cylindrical magnets installed above and below the sample. Depending on the configuration, the upper magnet can be flipped to allow magnetic attraction or repulsion.

When the lower magnet is placed under the petri dish, an attractive force is generated, and the ferrofluid droplets are dragged across the resin and stay at the bottom of the petri dish, forming lenticular spots under the upper resin. The research team found that by adjusting the distance between the upper and lower magnets and the direction of the upper magnet, it can control the shape of the ferrofluid clumps at the bottom of the plastic—effectively turning the clumps into free-flowing clumps. , Magnetron mold.

After adjusting the magnet to the correct shape, the researchers put the entire device in an oven and cured the plastic resin at 75°C for 40 minutes. After curing, the result is a hard plastic plate with a controlled concave curvature at the bottom and a flat surface—a plano-concave lens at the top. The water-based ferrofluid falls off the lens and can be reused to make the next one.

The team used this setup to quickly create a variety of plano-concave lenses with a variety of different contours, from flat models with upper magnets configured to provide repulsive force to sharper, more tapered contours when the upper magnet is set to attract. The concentration of magnetic nanoparticles in the ferrofluid provides another useful control knob for adjusting the shape of the lens. Researchers can make lenses of different sizes by changing the volume of ferrofluid. Tests using negative USAF target patterns have shown that PDMS lenses molded using this method "have performance comparable to commercial lenses."

The engineers plan to advance the project by more fully characterizing the interaction between the polymer and the ferrofluid at the interface, and by establishing a "clear relationship" between the parameters of the lens profile, the magnetic field, and the droplet volume in the experiment. And other parameters. At the same time, the researchers believe that the technology is "suitable for rapid production of small batches and customized" lens designs without the need for expensive molding and casting processes.

Optical manufacturing of free-form surfaces in fluids

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