How would you like a kitchen surface that cleans itself? Technological advances such as this could be one step closer after a breakthrough by Northumbria University and Nottingham Trent University.
Using experimental techniques, researchers have made the first ever direct observation of the elusive dewetting process, which takes place when a liquid film retracts to form a bead-shaped drop. The achievement could now spark a new line of research and lead to breakthroughs involving the use of liquids, such as better coatings and more effective self-cleaning surfaces.
Dewetting is the opposite of ‘spreading’, a familiar process which can be observed day to day, such as when a drop of oil is placed on the surface of a pan. The liquid initially has a bead-like shape, and it slowly spreads to form a thin film. The opposite process, called dewetting, occurs when a liquid film retracts from a solid to form a bead-shaped drop, which can be observed when a wet window is left to dry up.
The details of dewetting are extremely important to any situation involving the removal or drying of a liquid. Despite its apparent simplicity, the direct observation of the full dewetting of a droplet into a single drop had remained elusive and difficult to achieve until Northumbria and Nottingham Trent’s recent experiment.
In a recent paper in the journal Science Advances, the research team came up with an ingenious solution to this problem. Using a novel method known as dielectrowetting, they exploited the electric properties of liquids to force a liquid to coat a solid surface using an applied voltage.
Professor Glen McHale, Pro-Vice Chancellor (Engineering and Environment) at Northumbria University and Professor of Applied and Material Physics, said: “Our experimental setup opens-up the possibility of preparing liquid shapes in a very controlled manner, which then dewet. This can lead to new methods for liquid manipulation in technologies such as coating and self-cleaning surfaces.”
By embedding very thin patterned electrodes in the solid and carefully arranging them into a circular pattern, the team achieved the formation of a thin circular liquid film. By switching off the voltage, they revealed, for the first time, the full dewetting process of the liquid film back to a bead-like drop shape.
Professor Carl Brown, Coordinator of the Nottingham Trent University Engineering Research Unit, and Professor of Physics in the School of Science and Technology, said: “At first sight, one might have expected that dewetting is just the time-reversal of spreading. Surprisingly, we found that dewetting not spreading in reverse. Instead of a smooth sequence of drop-like shapes, the dewetting film forms a rim at its own edge which retracts at constant speed for most of the dewetting process.”
To understand this behaviour, the team used a combination of theory and numerical simulations to rationalise the experiments. Dr Rodrigo Ledesma-Aguilar, from Northumbria, said: “Both the simulations and the theory support that the liquid tends to adopt the closest local equilibrium shape it can during dewetting. This explains the smooth rim shape which survives for most of the process.”
Nottingham Trent University’s Andrew Edwards, first author of the paper, said: “Unveiling the dynamics of a dewetting film in all its detail has been a mind-blowing experience. This is my first original contribution as a PhD student and has allowed me to apply a range of knowledge gained in my first degree as a physicist. It is extremely pleasing to see how our experiments are so well described by the theory and the simulations.”
Dr Michael Newton, Reader in Experimental Physics in the School of Science and Technology at Nottingham Trent University, added: “Our method can be used to learn more about the underlying physics behind other dewetting phenomena such as condensation, evaporation and droplet rebound. These processes are critical for applications such as fog-collection, coating and lubrication. The technique developed can also be used for characterising liquid properties when only small volumes are available.”
Watch a video about the dewetting process here.
Learn more: It’s delightful, it’s dynamic, it’s dewetting!
Researchers at Duke University and the University of British Columbia are exploring whether surfaces can shed dirt without being subjected to fragile coatings
Scalpels that never need washing. Airplane wings that de-ice themselves. Windshields that readily repel raindrops. While the appeal of a self-cleaning, hydrophobic surface may be apparent, the extremely fragile nature of the nanostructures that give rise to the water-shedding surfaces greatly limit the durability and use of such objects.
To remedy this, researchers at Duke University in Durham, North Carolina and the University of British Columbia in Vancouver, Canada, are investigating the mechanisms of self-propulsion that occur when two droplets come together, catapulting themselves and any potential contaminants off the surface of interest. They ultimately hope to determine whether superhydrophobicity — a surface that is impossible to wet — is a necessary requirement for self-cleaning surfaces.
“The self-propelled catapulting process is somewhat analogous to pogo jumping,” said Chuan-Hua Chen, an associate professor in the Department of Mechanical Engineering and Materials Science at Duke University. He and his colleagues present their work this week in Applied Physics Letters, from AIP Publishing.
When the droplets coalesce, or come together on a solid particle, they release energy – analogous to the release of biochemical energy of a human body on a pogo stick. The energy is then converted through the interaction between the oscillating liquid drop and the solid particle – analogous to the storage and conversion of energy by the spring mechanism of the pogo stick.
“In both cases, the catapulting is produced by internally generated energy, and the ultimate launching comes from the ground that supports the payload – the solid particle or the pogo stick,” Chen said.