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August 2018

Scientists 'Squeeze' Nanocrystals in a Liquid Droplet Into a Solid-Like State – and Back Again

A team of Foundry staff and users has found a way to make a liquid-like state behave more like a solid, and then to reverse the process.

They put a droplet of a liquid containing iron oxide nanocrystals into an oily liquid containing tiny polymer strands.

They found that a chemical additive in the droplet can compete with the polymer – like a tiny tug of war – on nanoparticles at the intersection of the liquids.

They were able to cause the nanoparticles assembled here to jam, making it act like a solid, and then to unjam and return to a liquid-like state by the competitive push-pull action of the polymer and the additive.

They watched this movement between the two states simply by looking at changes in the shape of the droplet. The changes provide information about the tension on the surface of the droplet, like observing the surface of an inflating or deflating balloon.

They used an atomic force microscope, which works like a tiny record player needle to move over the surface of the droplet to measure its mechanical properties.

While changing liquid states to solid states typically involve temperature changes, in this latest study researchers instead introduced a chemical compound known as a ligand that bonds to the surface of the nanoparticles in a precise way.

At higher concentrations of ligand, the assemblage of nanocrystals relaxed more quickly from a jammed state to an unjammed state.

Researchers also found that they could manipulate the properties of the liquid droplets in the oil solution by applying a magnetic field – the field can deform the droplet by attracting the iron-containing nanocrystals, for example, and change the tension at the surface of the droplets.

Finding new ways to control such all-liquid systems could be useful for interacting with living systems.The study is also valuable for showing fundamental chemical and mechanical properties of the nanocrystals themselves.

Read the full press release here.