JANUS-FACED FLOATING FILM: Schematic representation of the steps involved in the fabrication of an NCF@PH solar–thermal interfacial evaporator
A group of researchers have reported a breakthrough in desalination that could dramatically change the economics of producing freshwater from the sea and brackish water.
The innovation centres on an engineered material — an ultra-thin floating film called NCF@PH, described as “Janus-faced” because it has two distinct sides, like the Roman god. One side contains nano-carbon florets (NCF) — nano-carbon structures shaped like tiny marigold flowers — which are optimised to trap sunlight and minimise reflection. The other side is made of a special porous polymer (porosity-tuned high internal phase emulsion polymer).
Imagine a tank of seawater covered by a glass sheet. The NCF@PH film floats on the water, covering it. When sunlight strikes the system, the NCF on the sun-facing side absorbs large amounts of light.
The underside of the film — in contact with the water — acts as a scaffold for the NCF coating. The NCF heats the water, causing it to evaporate. Water vapour passes through the film into the space between the film and the glass lid, from where it is directed to a Peltier cooler for condensation.
Evaporation boost
Researchers from the departments of chemistry and mechanical engineering at IIT-Bombay collaborated with Monash University in Australia to build a prototype system called SunSpring. The core of the system is the NCF material.
Two years ago, Prof Subramaniam Chandramouli of IIT-Bombay, who is part of the SunSpring team, had synthesised these nano-carbon florets using silica “moulds”. As reported in Quantum on February 10, 2023, he demonstrated that when coated on porcelain or copper and exposed to sunlight, the material could heat up to 160 degrees C within minutes.
In a recent paper in Advanced Science, the researchers describe how the NCF is integrated onto a porosity-engineered, hydrophobic polymer to create an ultra-thin (200 micrometre), unsinkable solar-thermal evaporator. This design boosts the water evaporation rate to 4.5–6.5 kg per sqm per hour, compared with 1.29 kg in conventional systems.
As a result, SunSpring can produce 18 litres of freshwater per sqm per day — more than double the 7 litres typical of standard evaporation-based desalination systems. The combination of NCF and polymer channels the solar-thermal energy to the water for evaporation and prevents heat loss to the environment.
Tests at IIT-Bombay showed that SunSpring could convert seawater containing 35,000 ppm of salt into freshwater with less than 10 ppm of salt. According to Mohammed Aslam and Amrutha Suresh, the lead authors of the work, the device can run continuously for up to 225 hours.
Efficient condensation
A major improvement is in the way SunSpring handles condensation. In conventional solar stills, the same glass surface is used both to admit sunlight and to condense vapour. Once condensation begins, droplets form on the glass, scattering light and reducing heating — and therefore reducing efficiency.
SunSpring avoids this problem by separating the evaporation and condensation chambers. The condensation surface is a Peltier cooler, a thermoelectric device that becomes cold on one side and hot on the other when powered. “An important aspect of the SunSpring design lies in the decoupling of the sunlight-admitting surface from the water-collection surface,” the paper notes. In addition, salt accumulates only on the water-facing underside of the film, from where it can be easily washed away.
The research is supported by The Green Energy and Sustainability Hub at IIT-Bombay and Anusandhan National Research Foundatuon (ANRF).
Cost factor
Prof Chandramouli estimates that SunSpring can produce freshwater at thrice the cost of a typical RO system. However, costs will fall when the NCF@PH film is produced at an industrial scale.
The team, after two years of development work, now plans to set up a pilot plant in the Rann of Kutch, Gujarat, where groundwater salinity is extremely high and affects local health. The pilot, to be installed in a school, will provide 300 litres of pure water per day for the children.
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Published on November 17, 2025
