Penn engineers invent two-dimensional liquid

Where water and oil meet, a two-dimensional world exists. This interface presents a potentially useful set of properties for chemists and engineers, but getting anything more complex than a soap molecule to stay there and behave predictably remains a challenge.   

Recently, a team from the Department of Chemical and Biomolecular Engineering in the School of Engineering and Applied Science has shown how to do just that. Their “soft” nanoparticles stick to the plane where oil and water meet, but do not stick to one another. That means they can freely move past one another while being confined to that interface, effectively acting as a 2-D liquid.

The team, consisting of postdoctoral researcher Valeria Garbin, graduate student Ian Jenkins, and professors Talid Sinno, John Crocker, and Kathleen Stebe, also devised clever ways of measuring the properties of this unique system. Their data will better inform computer simulations and potentially lead to applications in fields like nanomanufacturing and catalysis.

“We understand how particles work in 3-D,” Crocker says. “If you put polymer chains on the surface that are attracted to the solvent, the particles will bounce off each other and make a nice suspension, meaning you can do work with them. However, people haven’t really done that in 2-D before.”

Even when particles are able to stay at the interface, they tend to clump together and form a skin that can’t be pulled back apart into its constituent particles.

The team’s technique for surmounting this problem hinged on decorating their gold nanoparticles with surfactant, or soap-like, ligands. These ligands have a water-loving head and an oil-loving tail, and the way they are attached to the central particle allows them to contort themselves so both sides are happy when the particle is at an interface.

This arrangement produces a “flying saucer” shape, with the ligands stretching out more at the interface than above or below. These ligand bumpers keep the particles from clumping together.  

To get a picture of how the particles packed in their 2-D layer, the researchers dripped a particle-containing an oil droplet out of a pipette into water. Over time, particles attached to the oil-water interface, at which point the researchers could change their packing density by sucking some of the oil back into the pipette. By measuring the optical properties of the particles when overcrowding pushed some out, they could work backwards to the number of particles on the interface. From there, they could determine some universal rules that govern the physics of such systems.  

“This is a very beautiful system,” Stebe says. “The ability to tune their packing means that we can now take everything we know about the equilibrium thermodynamics in two dimensions and start to pose questions about particle layers. Do these particles behave like we think they should? How can we manipulate them in the future?”

2-D Liquid