Intriguing structures formed by tiny particles floating in liquids. Image: Vienna University of Technology
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 Vienna University of Technology (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 were published in Physical Review Letters.
Regular structures in liquids
If small particles 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 nanomaterials 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.