Careful tracking of the kinks in microscopic chains helped Professor Sibani Lisa Biswal, left, and Rice graduate student Dichuan Li quantify Brownian-induced axial spin of the particles. Photo: Jeff Fitlow/Rice Univ.
Rice Univ. researchers have created a method
to measure the axial rotation of tiny rods.
Sibani Lisa Biswal, an assistant professor in chemical and biomolecular
engineering, said it's easy to view microscopic rods as they wiggle and weave
under the influence of Brownian forces. But it's never been easy to see one
spin along its axis, let alone measure that motion.
The technique created by Biswal and her team involves micron-scale rigid
chains in liquid that act like perfect cylinders as they exhibit Brownian
motion. But the rod-like chains incorporate the slightest of imperfections.
These nearly invisible "kinks" are just big enough to the measure the
chain's rotation without influencing it.
Knowing how elongated molecules move in a solution is important to those who
study the structure of liquid crystals or biological processes like the
dynamics of lipid bilayers, Biswal said.
The research follows her lab's creation of a technique to build stiff chains
of particles that mimic rod-like polymer or biological molecules. Using them
like Legos, the lab assembles chains from DNA-grafted paramagnetic polystyrene
particles, which line up when exposed to a magnetic field and link together
where the strands of DNA meet.
The result looks like a string of beads. Depending on the length and type of
the DNA linkers, the rods can be stiff or flexible. Slight variations in the
paramagnetic properties of each particle account for the kinks. "We can
make them robust; we can make them stable," Biswal said. "Now we're
actually using them as a model for polymer chains."
It's long been known that stiff rods in a solution rotate as they dance and
are pushed by the atoms around them. Nearly two centuries ago, Robert Brown observed
the rotation of flat arsenic trioxide flakes but had no way to characterize
that motion. While Albert Einstein and others have since made progress in
applying formulas to Brownian motion, the particulars of rotation have remained
a relative mystery.
Dichuan Li, a graduate student in Biswal's lab, was inspired to look at
rotation after reading Brown's 1827 report in a classic-paper reading club.
"He noticed what he thought must be axial rotation, but he wasn't able to
measure how fast it was moving," Li said.
The new method is the first systematic approach to measuring the axial
rotation of particles, he said. Once chains are formed, the magnetic field is
released and the chains are free to move in a solution between two cover
plates. Li isolated and filmed the structures as they twisted, and he later
analyzed the kinks to quantify the chains' motion.
The finding opens a door to further study of longer or more complex polymer
or biological chains, Biswal said. She said the paramagnetic beads could be
used to model rods of varying stiffness, "even more flexible structures
that can actually curve and bend, just like DNA, or branch-like structures.
Then we can apply forces to them and see what happens."
Biswal hopes to take a closer look at how polymers entangle in materials of
varying density. "How they're stabilized by entanglement is not well understood,"
she said. "We're moving toward being able to create not just single chains
for study, but large collections of these chains to see if they provide good
models to look at things like entanglement."
The technique is detailed in a paper that appears in Physical Review Letters.