New Two-Dimensional Film Could Lead to Shape-Shifting Fluids and Soft Robotics

An innovative two-dimensional film made of nanoparticles and polymers was developed by Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

This 2D film can direct two different non-mixing liquids into a range of exotic architectures. This finding could result in shape-shifting fluids, liquid circuitry, and soft robotics, and a host of new materials that use soft, instead of solid substances.

The study, reported on September 25^th, 2017 in the journal Nature Nanotechnology, presents the latest entry in a class of substances known as bijels or bicontinuous jammed emulsion gels, which is a prospective malleable liquid that can support energy conversion, electrical conductivity and catalytic reactions.

Typically, bijels are made of non-mixing or immiscible liquids. Such liquids are familiar to people who shake their bottle of vinaigrette prior to pouring the dressing on their salad. Once the shaking stops, the liquids begin to separate once again, with the lower density liquid – usually oil – rising to the top.

Jamming or trapping particles where these immiscible liquids meet can prevent the liquids from completely separating by stabilizing the substance into a bijel. What makes bijels significant is that, instead of just making the spherical droplets that is usually visible when oil and water are mixed, the particles at the interface shape the liquids into intricate networks of interconnected fluid channels.

Bijels are extremely difficult to make, however, involving exact temperatures at precisely timed stages. Moreover, the liquid channels are generally more than 5 μm across, making them extremely large to be productive in catalysis and energy conversion.

Bijels have long been of interest as next-generation materials for energy applications and chemical synthesis. The problem has been making enough of them, and with features of the right size. In this work, we crack that problem.

Caili Huang, Study Lead Author

Huang began the work as a Graduate Student with Thomas Russell, the Main Investigator of the study, at Berkeley Lab’s Materials Sciences Division, and he carried on with the project as a Postdoctoral Researcher at DOE’s Oak Ridge National Laboratory.

Creating a New Bijel Recipe

The technique described in this latest study simplifies the bijel process by first using specifically coated particles about 10-20 nm in diameter. The smaller-sized particles line the liquid interfaces much more rapidly than the ones employed in conventional bijels, making the smaller channels extremely valued for applications.

We’ve basically taken liquids like oil and water and given them a structure, and it’s a structure that can be changed. 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.

Thomas Russell, Main Investigator of the study, Materials Sciences Division, Berkeley Lab

It was possible for the Researchers to prepare innovative bijels from a range of common water-insoluble, organic solvents, such as deionized water, which contained the nanoparticles and toluene, that had ligands dissolved in it. The emulsion was subjected to a vortex spinning at 3,200 revolutions per minute in order to ensure complete mixing of the liquids.

“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, a Postdoctoral Fellow at Berkeley Lab’s Materials Sciences Division. “You’re synthesizing a lot of this material, which is in effect a thin, 2-D coating of the liquid surfaces in the system.”

Even after one week, the liquids remained a bijel, a sign of the stability of the system.

In addition, Russell, who is also a Professor of Polymer Science and Engineering at the University of Massachusetts-Amherst, said that these shape-shifting characteristics would be valuable in soft actuators, microfluidic devices and microreactors.

Nanoparticle Supersoap

Nanoparticles had not been critically considered in bijels before due to their small size that made them hard to capture in the liquid interface. The Researchers coated nano-sized particles with carboxylic acids and put them in water in order to resolve that problem. Consequently, they took polymers with an added amine group – a derivative of ammonia – and dissolved them in the toluene.

This configuration took advantage of the amine group’s affinity to water, a characteristic that is similar to surfactants, like soap. The Researchers said that their nanoparticle “supersoap” was exclusively developed so that the nanoparticles join ligands, forming an octopus-like shape with a polar head and nonpolar legs that get jammed at the interface.

Bijels are really a new material, and also excitingly weird in that they are kinetically arrested in these unusual configurations. The discovery that you can make these bijels with simple ingredients is a surprise. We all have access to oils and water and nanocrystals, allowing broad tunability in bijel properties. This platform also allows us to experiment with new ways to control their shape and function since they are both responsive and reconfigurable.

Brett Helms, Study Co-author and a Staff Scientist, Berkeley Lab’s Molecular Foundry

Although the nanoparticles were made of silica, the Researchers noted that in previous studies they used carbon nanotubes and graphene to form nanoparticle surfactants.

“The key is that the nanoparticles can be made of many materials,” said Russell. “The most important thing is what’s on the surface.”

Weiyu Wang, Kunlun Hong, and Gregory Smith are the study Co-authors from Oak Ridge National Laboratory.

The DOE Office of Science supports this work. The Molecular Foundry at Berkeley Lab and the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory are DOE Office of Science User Facilities.