This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.
Scientists knew that they could generate electricity by running salt water across an electrically charged surface. But they could never get the process to make enough energy to be useful. Now engineers have figured out a way to do that. Their trick: Make the water flow over that surface much more quickly. They achieved this by making the surface super water repellent.
Prab Bandaru is a mechanical engineer and materials scientist at the University of California San Diego. His team’s innovation grew out of frustration. None of the other things they tried had worked. One “spur of the moment thing … just happened to work,” he says with a laugh. It was hardly planned.
Scientists describe a surface that repels water as hydrophobic (HY-droh-FOH-bik). The term comes from the Greek words for water (hydro) and hating (phobic). The UCSD team describes the material it uses as super-hydrophobic.
Their new energy system starts with table salt, or sodium chloride. As its name suggests, this salt is made from bonded atoms of sodium and chlorine. When the atoms react to make salt, an electron from a sodium atom breaks off and attaches to a chlorine atom. This turns each neutral atom into a type of charged atom called an ion. The sodium atom now has a positive electrical charge. Opposite charges attract. So that sodium ion now is strongly attracted to the chlorine atom, which now has a negative charge.
When the salt is dissolved in water, the water molecules cause the association between the sodium and chlorine ions to loosen. As this salt water flows over a surface with a negative charge, its positively charged sodium ions will be attracted to it and slow down. Meanwhile, its negatively charged chlorine ions will keep flowing. This breaks the bond between the two atoms. And that releases the energy that had been stored within it.
The challenge was to get the water to move quickly enough. “When the chlorine flows away fast, then the relative velocity between the slow sodium and the fast chlorine is enhanced,” Bandaru explains. And that will increase the electric power it generates.
The team described its innovation on October 3 in Nature Communications.
This use of a super-water-repellent surface to generate energy is “really, really exciting,” says Daniel Tartakovsky. He’s an engineer at Stanford University who was not involved in the research.
Other researchers have tried to use water repellency to boost the energy production of a salt-water electric generator. They did it by adding tiny grooves to the surface. When the water flowed over the grooves, it encountered less friction as it traveled over the air. Yet even though the water flowed faster, the energy production didn’t increase very much. And that, Bandaru says, is because the air also cut the water’s exposure to the negatively charged surface.
His team tried different ways to get around this problem. They tried making the surface more porous. Their idea was to speed the water’s flow by providing even more air at the surface. “We were in the lab, thinking, ‘Why isn’t this working?’” he recalls. “Then we said, ‘Why don’t we put liquid inside [the surface]?’”
It was just a brainstorming idea. The researchers hadn’t done any calculations to figure out if it might work. They just tried replacing the air in the surface’s grooves with oil. And it worked! “We were very surprised,” Bandaru says. “We got a very, very high result for the [electrical] voltage.” To probe whether they’d made some mistake, Bandaru says, they quickly realized “‘We have to try this again!’”
They did several more times. And each time, the results came out the same. “It was reproducible,” Bandaru says. This reassured them that their initial success was no accident.
Later, they examined the physics of the liquid-filled surface. Recalls Bandaru, “It was one of those ‘Duh’ moments when we realized, ‘Of course it had to work.’”
Why it works
Like air, oil repels water. Some oils are far more hydrophobic than air — and can hold a negative charge. Bandaru’s team tested five oils to find which offered the best mix of water repellency and negative charge. Another advantage to using oil: It doesn’t wash away when the water flows over it because a physical force known as surface tension holds it to the grooves.
The team’s newly reported tests offer proof that the concept works. Other experiments will need to test how well it might work on a larger scale — one that might deliver a useful amount of electricity.
But the technique might find use even in small-scale applications. For instance, it might be used as a power source for “lab-on-a-chip” assays. Here, tiny devices perform tests on very small amounts of fluid, such a drop of water or blood. On a larger scale, it might be used to generate electricity from ocean waves, or even using the wastes moving through water-treatment plants. “It doesn’t have to be salt water,” Bandaru explains. “Maybe there is wastewater that contains ions. As long as there are ions in the liquid, one can use this scheme for generating voltage.”
Using a liquid like oil to speed up the flow of water while also conducting electricity might greatly improve the efficiency of such power systems. “If it works,” Tartakovsky says, it might even offer “a big breakthrough in battery technology.”
application A particular use or function of something.
assay A tests used to look for and measure the amount of some looked-for substance. It could be amount of some metal in an ore or perhaps the amount of some virus in a sample of blood.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
battery A device that can convert chemical energy into electrical energy.
bond (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.
chlorine A chemical element with the scientific symbol Cl. It is sometimes used to kill germs in water. Compounds that contain chlorine are called chlorides.
conductor (in physics and engineering) A material through which an electrical current can flow.
electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
friction The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another.
generator A device used to convert mechanical energy into electrical energy.
hydrophobic Repelling (or not absorbing) water.
innovation (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.
ion (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.
liquid A material that flows freely but keeps a constant volume, like water or oil.
materials science The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application. A scientist who works in this field is known as a materials scientist.
mechanical engineering A research field in which people use physics to study motion and the properties of materials to design, build and/or test devices.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.
porous The description of a substance that contains tiny holes, called pores , through which a liquid or gas can pass. (in biology) The minute openings in the skin or in the outer layer of plants.
salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.
sodium A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.
surface tension The surface film of a liquid caused by the strong bonds between the molecules in the surface layer.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
velocity The speed of something in a given direction.
voltage A force associated with an electric current that is measured in units known as volts. Power companies use high-voltage to move electric power over long distances.
wastewater Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
Journal: B. Fan, A. Bhattacharya and P.R. Bandaru. Enhanced voltage generation through electrolyte flow on liquid filled surfaces. Nature Communications. Vol. 9, Oct 3, 2018. doi: 10.1038/s41467-018-06297-9.