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!
Martian colonists could use an innovative new technique to harvest energy from carbon dioxide thanks to research pioneered at Northumbria and Edinburgh Universities.
Dry ice may not be abundant on Earth, but increasing evidence from NASA’s Mars Reconnaissance Orbiter suggests it may be a naturally occurring resource on Mars as suggested by the seasonal appearance of gullies on the surface of the red planet.
If utilised in a Leidenfrost-based engine dry-ice deposits could provide the means to create future power stations on the surface of Mars.
One of the co-authors of Northumbria’s research, Dr Rodrigo Ledesma-Aguilar, said: “Carbon dioxide plays a similar role on Mars as water does on Earth. It is a widely available resource which undergoes cyclic phase changes under the natural Martian temperature variations.”Perhaps future power stations on Mars will exploit such a resource to harvest energy as dry-ice blocks evaporate, or to channel the chemical energy extracted from other carbon-based sources, such as methane gas.
Northumbria University, officially the University of Northumbria at Newcastle, is a university located in Newcastle upon Tyne in the North East of England. It is a member of the University Alliance.
In the Research Assessment Exercise 2008 a small amount of research in nine of twelve areas submitted was described as “world leading”.
Notable research awards in 2009/10 included funding from the Department of Health’s Policy Research Programme for a Northumbria-led national assessment of dementia care, in collaboration with the Universities of Edinburgh, Newcastle and Glamorgan. The Engineering and Physical Sciences Research Council awarded £1.4 million to a Northumbria University research team working alongside the Universities of Birmingham, Central Lancashire, Swansea and London (Birkbeck) on energy consumption. RTC North and a private company, Nonlinear Dynamics – a world leader in its field – announced a research collaboration with Northumbria University which could lead to a major breakthrough in the production of bio-fuels. The three year project will bring together traditional scientific laboratory analysis techniques and some of the world’s most advanced data analysis software. A new company established by the University in 2010 will give manufacturers the chance to use computational chemistry to create “designer molecules” for the first time in an industrial setting. The process, Quantum Directed Genetic Algorithms (QDGA), is a unique solution for identifying new catalysts and reactants.