When not confined to a container, liquids will splatter, dribble, and ooze. They shape-shift into their surroundings as puddles and streams, largely beyond human control.
Lauren Zarzar is trying to make more obedient liquids. Zarzar, a materials scientist at Pennsylvania State University, designs liquids that she can tame—fluids that move or change shape on command. That could mean a tube of water that retains its shape inside a second fluid, or a droplet that separates into two different oils when triggered by light. At their most ambitious, these liquids could even become electronic circuit components. Researchers have already made all-liquid wires and antennas.
Zarzar is working with tiny droplets about a tenth of a millimeter in size. These droplets consist of two types of oil, one encased in the other, like caramel in chocolate. When she changes the temperature of the droplet’s surroundings, she can make the droplet turn inside out.
They’re not sure what they want to do with the droplets, but Zarzar imagines that you could use them to trigger chemical reactions: Place a catalyst inside some droplets, dump them into a container of reactants, and when you want the reaction to begin just flip the droplets inside out. Or, she thinks they could be fashioned into liquid lenses for a microscopic camera that change their focus by morphing in real time.
But the applications don’t capture the creative shift now underway in how materials scientists consider liquids. “When people think of a liquid, they think of a chemical in a flask that doesn’t do anything,” says Zarzar. “We’re thinking about liquid as a material, about how to harness its structure and adaptability.”
One goal is to develop liquids that retain a customized shape without containers or molds. To do this, materials scientist Tom Russell and his colleagues at Lawrence Berkeley National Laboratory have modified a 3D printer to make a variety of all-liquid structures. Using a syringe attached to the printer, they can inject spirals of water into a surrounding body of water. The spirals keep their shape because the printer injects nanoparticles in with it, which form an extremely thin membrane around the structure. It’s almost like blowing smoke rings, except in liquid. “We can print a tube of liquid in another,” says Russell.
Using this printer, his colleague Brett Helms, a chemist, has created a liquid with an internal vessel structure. The liquid sits on a microscope slide like a small puddle with a canal running through it, propped up by a nanoparticle membrane.
The blue liquid, which contains nanoparticles, travels through a second liquid. The nanoparticles form a membrane to keep the two fluids from mixing.
To make this structured liquid, Helms and his collaborators first coated the microscope slide in a pattern of a water-repelling plastic, which determines the shape of the canal. Then, they use the 3D printer to deposit two liquids on the slide, each containing a different type of nanoparticle. Where the nanoparticles meet, they form a membrane, allowing a stable channel to form inside the liquid. They want to use these all-liquid structures to study how chemical reactions proceed, says Helms. For example, by changing the geometry of the channel, they can control how quickly chemical reactions occur, potentially letting them slow down the process to observe the dynamics of molecules in more detail.