Microscopically textured surfaces could significantly boost the performance of condensers that are vital to the functioning of water purification systems and power plants, according to researchers at the Massachusetts Institute of Technology (MIT).
Using a new technique that allows the behaviour of liquids on solid surfaces to be studied in more detail than has been possible previously, the team has designed a surface that water droplets can move across extremely fast, which could increase the rate that desalination plants produce fresh water and improve energy efficiency in electricity production.
The team, part of a research group led by Kripa Varanasi, MIT professor of ocean utilisation, has designed a surface with features around 10 microns across — around the size of a red blood cell. This surface is coated with a very thin layer of an oily lubricant that is held in place by the capillary action of the tiny spaces between the features — effectively, they pin the lubricant down, Varanasi said. Water droplets zip across this surface like pucks on an air-hockey table with ‘crazy velocities’, he added — some 10,000 times faster than droplets on an unpatterned surface.
This speed of movement means that the droplets fall quickly off the surface, allowing more droplets to form. This means that the condenser works far more efficiently, whether it’s being used to produce fresh water from seawater or to recondense steam that has been used to spin a turbine in a power station. Further research is intended to quantify the improvement in efficiency in power-station condensers.
In a paper published in the journal ACS Nano, Varanasi and his team explain that the form of the pattern is unimportant, as long as the features are the right size. ‘It can be manufactured easily,’ Varanasi said. Only around half a teaspoon of lubricant is needed to coat a square yard of material.
The team used a new technique to study the behaviour of the drops on the surface; the interface is usually difficult to look at because it is hidden by the drop itself. Varanasi developed a technique with another MIT research team, led by Konrad Rykaczewski, which involves freezing the droplets of liquid with liquid nitrogen slush, dropping their temperature so fast that the liquid doesn’t have time to crystallise.
This preserves the shape of the droplets, which can then be sliced with an ion beam to reveal the interface with the surface; this can then be imaged with a scanning electron microscope. ‘It’s a completely new technique,’ Varanasi said. ‘For the first time, we’re able to see the details of these surfaces.’