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Physical and Theoretical Chemistry
Polymers and Soft Condensed Matter
Dr Edith Sevick
http://rsc.anu.edu.au/research/sevick.php
In the past decade, Atomic Force
Microscopy (AFM) and Optical Tweezers (OT) have revolutionised
molecular science by measuring picoNewton forces over lengthscales
from 1 to 104 Angstroms. In our laboratory, we have a
Cell Robotics Optical Tweezer Apparatus which has been extensively
modified for our experiments in polymer and colloids science. The
apparatus consists of an optical trap which weakly "holds"
a micron-sized bead. The trap is formed by a focused laser beam
which is refracted through the transparent bead. The refracted rays
differ in intensity over the volume of the colloidal bead and exert a
force on the bead, drawing it towards the region of highest light
intensity. The optical trap is harmonic near the focal point: the
optical force acting on a colloidal particle position at x within a
trap centered at x0 is Fopt=-k(x-x0),
where k is the trapping constant which can be tuned by adjusting the
laser power. In this way, the optical trap generated by the OT
serves to both localise a colloidal particle and to measure the
small, sub-picoNewton scale forces acting on the particle. With
substantial modifications, our OT apparatus probes forces at small
lengthscales and over small timescales which are necessary in studies
in nonequilibrium statistical mechanics and polymer/biopolymer
science.
Probing Thermodynamics of Small Systems over Small Timescales
using Optical Tweezers
In past decades there have been few, if any, experimental
demonstrations of new concepts in Equilibrium and
Non-equilibrium Statistical Mechanics, primarily because this
scientific area is considered to be 'mature' with few unsolved
fundamental problems. However, starting in 1993, Denis Evans and
his colleagues formulated a set of new non-equilibrium theorems
which has practical implications for the behaviour of nano-systems.
Despite the many applications in both fundamental science and
engineering, the results of this theorem had not been demonstrated in
any real or experimental system. Force measurement techniques such
as AFM and OT probe very small energies, forces, and distances and
can be used in experiments designed to test and demonstrate these
theorems. We have been using a modified OT to demonstrate
experimentally some of Evans' non-equilibrium theories by measuring
picoNewton forces acting on a colloidal bead over nanometre to micron
distances, allowing us to monitor the bead's energy fluctuations on
the order of thermal energy. In particular we have demonstrated that
the Transient Fluctuation Theorem (TFT) describes the trajectories of
an optically trapped bead subject to solvent flow field. In addition
to this and other experiments initiated in 2002, we have used
Langevin dynamics to analytically re-derive experiment-specific TFT.
(with G.M. Wang, D. Carberry, E. Mittag, D.J. Evans, and
D.J. Searles, [Griffith U.])
Optical Tweezers for Biopolymer Studies
By attaching streptavidin coated latex beads to the ends of modified
DNA, we are able to use an optical trap and micropipette to stretch a
single bead-DNA-bead assembly and to study the interactions of
specific binding proteins on DNA. In 2002, we have modified the OT
apparatus to carry out these experiments. We have incorporated new
piezo translators and a control system for automated stretching of
the chain, modified the fluid cell for solution exchange and
electrophoresis, and automated the control of laser power, data
acquisition, and stage translation. In conjunction with the Protein
Synthesis & Evolution group headed by Nick Dixon, we are
investigating the chemical protocols for attaching the beads to the
ends of double-stranded DNA chains. (with G.M. Wang, D. Carberry,
N.E. Dixon)
PhD student David Carberry, and Drs Edie Sevick and Genmiao Wang discuss
research, cricket, cars, . . . at tea-time.
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