The Indian Express (Delhi Edition)
The last drop
How to empty the ketchup bottle every time
FOR anyone (and that is almost everyone) who has shaken and thumped a bottle of ketchup to squeeze the last dollop out of it, or flattened and then rolled up a tube of toothpaste to eject one final squirt onto their brush, help may soon be at hand. For more than a decade Kripa Varanasi and his colleagues at the Massachusetts Institute of Technology (MIT) have been creating and studying slippery surfaces for use in industrial equipment such as steam turbines and desalination plants.
More recently, they have found ways to apply their ideas to create internal coatings for containers so that their contents will flow out easily and completely, with no shaking, thumping or squeezing. And now they think they have discovered a way to adapt these super-slippery coatings to steer liquids across flat surfaces, opening up the possibility of pumping fluids around without the need for pipes.
The lotus position
Dr Varanasi’s work started with what are known as super-hydrophobic watershedding surfaces, a classic natural example of which is a lotus leaf. It repels water so effectively that droplets simply tumble off. The reason is that the leaf’s surface is covered with microscopic structures which contain air pockets. This reduces the surface tension that would otherwise cause a water droplet to cling on. By coating the condensing areas used in steam turbines with similar surfaces his team believes it will be possible to speed up the shedding of water droplets. That would boost efficiency and, as most of the world’s electricity is still generated by coal, gas and nuclear plants that rely on steam turbines, it would also save an awful lot of money.
The same idea has since been adapted to help move other substances, such as toothpaste, paint and ketchup. These have a gooeyness that means they can get into the air pockets and take a grip. To counter that, the researchers replace the air with liquids such as oils. The resulting surfaces are, in effect, self-lubricating—so that even the stickiest substances flow across them easily. The trick, says Dr Varanasi, is to have the right combination of surface structure and lubricating fluid, so that the oiling liquid does not get swept away by what is flowing over it.
Open sauce operating system
To create a completely emptyable container for a substance, be it ketchup, toothpaste, shampoo or face cream, means matching that substance to a specific surface structure and a bespoke lubrication fluid. The best way to do this, says Dr Varanasi, is to design the texture of the surface to trap a lubricant which is itself derived from the substance with which the container will be filled.
That also has the benefit of not contaminating the product should some of the fluid escape. A lubricant for a food product, for instance, might be derived from a natural oil which it contains.
Dr Varanasi’s team have developed a database of recipes that can be used to lubricate containers for a wide range of materials. In 2012 he and one of his students, Dave Smith, founded a company called Liquiglide, which is working with a number of consumer-goods firms such as Elmer’s, an American gluemaker, to create easy-to-pour, squeeze and shake containers for their products.
Liquiglide has also devised a variant of the system that can be applied to the vast number of vessels and pipes in factories. This, the company claims, could reduce production losses considerably. At the moment, the tendency of things like paints to stick to piping, mixing tanks and so on means that as much as 30% of the material may be lost, especially during clean-ups in batch production, as when switching to a different colour of paint.
One feature of Dr Varanasi’s liquid-impregnated surfaces is that droplets forming upon them tend to have a large area of contact. It increases the effects of the surface’s temperature on a droplet. And that got Dr Varanasi and David Quéré of ESPCI, a research university in Paris, and their colleagues thinking about how to exploit one of those effects, known as thermocapillary motion. A change in temperature can alter the surface tension of a droplet, causing it to move. Usually, very large temperature differences are needed even for a droplet to move slowly. Out of curiosity they devised an experiment using a surface texture impregnated with oil. As they report in Physical Review Fluids, the researchers applied a temperature gradient and recorded the movement of water droplets. Even with low changes in temperature the droplets skipped along their slippery surfaces. The group have subsequently upped the speed at which they can propel water droplets to a heady ten millimetres a second. That is ten times quicker than has been reported on conventional surfaces.
Dr Varanasi and Dr Quéré have several ideas for making use of this discovery by selectively heating and cooling different areas of the surface to steer the droplets around. One is to create new types of microfluidic devices—or labs-on-a-chip, as they are known colloquially. These oneshot machines, about a centimetre across, are being increasingly used for things like analysing blood. To work, they have to be able to move reagents around inside themselves through tiny pipes and valves. This movement is hampered by surface tension, the effects of which increase as the dimensions of the pipework diminish. Dr Varanasi and Dr Quéré think that by selectively heating and cooling different areas of a liquid-impregnated surface, they could move and mix fluids without such intricate plumbing.
Steering fluids around might also help with the group’s work in developing more powerful condensers. It may even solve one of the problems of space flight. Much conventional equipment depends on gravity to move liquids around inside it. That does not work in orbit. But thermocapillary motion would.
Meanwhile, back on Earth, most people might settle for saving even a little of what every year amounts to a massive lake of wasted condiments, bathroom products, creams and just about anything else that comes in bottles, containers or tubes. If Dr Varanasi has his way the days of shake and thump are numbered.