Imma Perfetto
Cosmos science journalist
Scientists from the Philippines have come up with a clever way to make adjustable water-based liquid lenses, which they say could be used as accessible and low-cost improvised lens systems in classrooms and laboratories.
“This was inspired by the natural shape of a water droplet on a flat surface, which resembles a plano-convex lens,” write the researchers in a paper describing their the method in the journal Results in Optics.
“We vary the size and curvature of the droplets by changing the droplet volume, resulting in a variable focusing effect.”
Traditional glass lenses have a set curvature, so they bend light in a characteristic way. Only by stacking and altering the spacing between its lenses can the focal length of a camera or microscope be altered.
But liquid lenses can be adjusted without any moving mechanical parts, which means they can be small and light. This is often achieved by manipulating the shape of a sealed pocket of liquid – using pressure or electricity – to alter the way light refracts through it, and therefore its focal length.
But traditional approaches to fabricating these dynamic liquid lenses are often complex and expensive.
The simple new approach involved coating an ordinary glass slide with polyvinyl chloride (PVC) plastic using a process called “electrospinning”. This involved melting PVC in an electric field, which stretched it out into fine microfibres that were deposited onto the glass slide.
This made the surface more water repellent (hydrophobic), which helps a droplet stay in a spherical dome shape rather than flattening out. The researchers shone a laser light through the droplet and found that they could change its focal length by simply adding more water.
“The focal length increases linearly as the droplet size increases,” they write.
As the droplet volume increased from 5 microlitres (μL) to 60μL, the focal length also increased from 1.3mm to 7.6 mm.
The researchers say their simple system can produce an array of convex lenses with varying focal length by simply changing the volume and shape of the droplet.
“While the presented system herein may have a limited dynamic tunability and usability, [it] can be used in simple experiments in class discussions and open optical systems,” they write.
“The proposed design can be used as an accessible and low-cost improvised lens system in simple and laboratory-scale optical experiments.
“This could also serve as a baseline proof-of-concept in producing a more advanced and sophisticated volume-based dynamic water-based liquid lenses.
