Flowing structures in soft crystals

August 8, 2011
Colloids - unordered, in a crystalline structure and as strings under mechanical strain

What is common to blood, ink and gruel? They are all liquids in which tiny particles are suspended – so called “colloids”. In some of these liquids, the particles form groups (clusters), which form regular structures, much like atoms in a crystal. A team of researchers from TU Vienna and Vienna University has now managed to study the remarkable properties of these crystal-like substances in computer simulations. Under mechanical strain, the crystalline pattern can change into a different structure, or it can vanish completely.

The researchers anticipate a broad range of practical applications for these effects. The results of their calculations have now been published in the scientific journal Physical Review Letters.

Regular Structures in Liquids

If small accumulate, they can form clusters. Within a cluster, the particles may overlap and mingle, similar to a densely packed shoal of eels, gliding past each other. Remarkably, these clusters are not situated at random positions, but they spontaneously form a regular structure – a “cluster crystal”. The distance between two neighboring clusters is constant. “Increasing the density of particles adds more and more particles to each cluster – but the distance between them stays the same”, says Arash Nikoubashman, PhD-student at TU Vienna. He made the calculations together with Professor Gerhard Kahl (Institute for Theoretical Physics, TU Vienna) and Professor Christos Likos (University of Vienna).

Crystal Structure Turning into Strings

“Previous results had already led us to believe that these particles could exhibit strange behavior under certain external conditions”, the physicists explain. And their hopes were not unfounded: in computer simulations they managed to calculate how the crystal-like structure behaves under mechanical strain that causes shears stress – which means that surfaces within the liquid are shifted relative to each other. At first, the crystal structure starts to melt, the connections between the clusters are broken. From these molten particle clusters, a new regular order starts to emerge spontaneously. Long, straight strings of particle are formed, neatly aligned in parallel. 

Thin and Thick

While these strings are created, the liquid gets thinner, its viscosity decreases. This is due to the strings being able to slide relative to one another. If the material is subject to even more strain, the strings break up too, a “molten” unstructured ensemble of particle clusters remains, and the viscosity of the liquid increases again. More and more particles are washed away from their original positions and inhibit the flow. This behavior is the same for all kinds of cluster crystals. With a simple theoretical model, the critical strain, at which the ordered structure vanishes completely, can be predicted very accurately.

Under shear strain, crystals made of soft, penetrable particles can exhibit new kinds of self-organization. Geometric structures emerge, governed by the kind of forces acting between the particles. This research in the field of “soft matter” in the micro- and nanometer regime is not only interesting from a theoretical point of view. These materials play an important role in our everyday life – such as blood or large biopolymers like DNA. They are important in biotechnology, and also in petrochemistry and pharmacology – wherever tailor-made nano materials are being used. A liquid which can change its viscosity under mechanical stress promises a broad spectrum of possible applications – ranging from vibration dampers to protective clothing.

Explore further: Crystal to glass cooling model developed

More information: Arash Nikoubashman, et al., Cluster Crystals under Shear, Phys. Rev. Lett. 107, 068302 (2011). DOI: 10.1103/PhysRevLett.107.068302

Related Stories

Crystal to glass cooling model developed

February 22, 2006

University of Tokyo scientists have discovered why cooling sometimes causes liquid molecules to form disordered glasses, rather than ordered crystals.

Repulsive interactions

February 5, 2010

(PhysOrg.com) -- The recent state of the roads is a clear illustration of what happens when water freezes into crystals of ice. But despite its frequent occurrence, the crystallisation of water is remarkably difficult to ...

Tangling the microscopic ladder

December 17, 2010

If a ladder had more than one rung at each step, it would look awkward and would be a bit dangerous to climb. Ladders in the microscopic world were thought to be similar in structure, having only one particle, or rung, in ...

Vienna physicists create quantum twin atoms

May 2, 2011

At the Vienna University of Technology, sophisticated atomchips have been used to create pairs of quantum mechanically connected atom-twins. Until now, similar experiments were only possible using photons.

Recommended for you

Magnetism at nanoscale

August 3, 2015

As the demand grows for ever smaller, smarter electronics, so does the demand for understanding materials' behavior at ever smaller scales. Physicists at the U.S. Department of Energy's Ames Laboratory are building a unique ...

Study calculates the speed of ice formation

August 3, 2015

Researchers at Princeton University have for the first time directly calculated the rate at which water crystallizes into ice in a realistic computer model of water molecules. The simulations, which were carried out on supercomputers, ...

Small tilt in magnets makes them viable memory chips

August 3, 2015

University of California, Berkeley, researchers have discovered a new way to switch the polarization of nanomagnets, paving the way for high-density storage to move from hard disks onto integrated circuits.

Scientists bring order, and color, to microparticles

August 3, 2015

A team of New York University scientists has developed a technique that prompts microparticles to form ordered structures in a variety of materials. The advance, which appears in the Journal of the American Chemical Society ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.