Bicontinuous jammed emulsion gels. Bijels, for short.
It’s OK if you haven’t heard of them before, yet if you’ve ever shaken a bottle of vinaigrette you’re probably familiar with the concept. The vinaigrette is made of two liquids that are immiscible (that’s a fancy way of saying that they don’t mix). You shake them up to form your dressing; as soon as you stop shaking, the two liquids begin to separate.
But if particles are trapped at the interface where the two liquids meet, they shape complex networks of interconnected fluid channels and stabilize the substance into a bijel. These malleable liquids hold promise for supporting catalytic reactions, electrical conductivity and energy conversion. Only trouble is, they are notoriously difficult to make. They need exact temperatures at precisely timed stages. And those fluid channels are normally wider than 5 micrometers—too large to be useful in energy conversion and catalysis.
Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), however, have developed a new two-dimensional film made of polymers and nanoparticles that can direct two different non-mixing liquids into a variety of exotic architectures. That could lead to applications for soft robotics, liquid circuitry, shape-shifting fluids—and a host of other new materials that eschew solids in favor of the soft.
As reported in the journal Nature Nanotechnology, the researchers simplified the process of making bijels by using specially coated particles about 10-20 nanometers in diameter. These line the liquid interfaces much more quickly and create the narrow channels that could be highly valuable for applications.
"Bijels have long been of interest as next-generation materials for energy applications and chemical synthesis," said the study’s lead author, Caili Huang. "The problem has been making enough of them, and with features of the right size. In this work, we crack that problem."
"We've basically taken liquids like oil and water and given them a structure, and it's a structure that can be changed," said Thomas Russell, the study’s principal investigator. “If the nanoparticles are responsive to electrical, magnetic or mechanical stimuli, the bijels can become reconfigurable and re-shaped on demand by an external field."
The researchers prepared bijels from a variety of common organic, water-insoluble solvents, mixing them at an extreme rate of 3,200 revolutions per minute to ensure lasting stability. "This extreme shaking creates a whole bunch of new places where these particles and polymers can meet each other," said study co-author Joe Forth. "You're synthesizing a lot of this material, which is in effect a thin, 2-D coating of the liquid surfaces in the system.”
Nanoparticles had not been seriously considered for bijels before, because their small size made them hard to trap in the liquid interface. But the researchers took advantage of a surfactant-like quality in their chemical process to create a nanoparticle “supersoap.” It was designed so that the nanoparticles joined ligands, forming an octopus-like shape with a polar head and non-polar legs that got jammed at the interface.
"Bijels are really a new material, and also excitingly weird in that they are kinetically arrested in these unusual configurations," said study co-author Brett Helms. "The discovery that you can make these bijels with simple ingredients is a surprise...This platform also allows us to experiment with new ways to control their shape and function, since they are both responsive and reconfigurable."
Although the researchers formed their nanoparticle surfactants with silica, previous studies showed that graphene and carbon nanotubes could also be used.
"The key is that the nanoparticles can be made of many materials," said Russell. "The most important thing is what's on the surface."