Research Plans and Interests of David Williams
Over the five years I have built up a small, but active research
group
(1 postdoc and 2 students) focusing on Soft Matter Physics and
Biophysics. I also have secondary interest in Granular Materials and
Surface Growth Problems (formation of quantum dots and sandwiches).
By far the most emphasis (90 %) has
been placed on the soft matter area. The techniques I have used and
will continue to use are analytic theory and computer simulation.
In more detail my research themes are:
Biophysics
Soft matter physics has traditionally dealt mainly with man-made systems:
synthetic polymers; surfactants and gels. These systems often have a
single component or a few components and our understanding of them is
now reasonably advanced. Although there continue to be many first-order
problems available in this area it is rather clear that a large fraction
of the future of this subject (and of physics in general) lies in
problems inspired by biology. The reasons for this are fairly simple.
Biology and medicine are highly data-rich fields - the experiments are
reasonably inexpensive and can be conducted on the laboratory scale. There
are substantial funding possibilities in this area - both from
government agencies and from private companies. The number of people carrying
out simulations or providing theoretical support in these areas is
negligibly small, hence the wide opportunities available.
My current and future work in this area is in 3 major projects:
(1) Formation of DNA condensates:
These form when DNA in solution is
mixed with a condensing agent. This agent acts as a glue, and a number
of novel morphologies form, in particular, flowers and toroids. This
work is conducted in collaboration with people in biophysics and physics
at the University of Michigan
(F.C. MacKintosh and B. Schnurr
) and with
Jan Hoh in the Department of Physiology at Johns hopkins University.
AFM Image of DNA Condensates
(courtesy of Ye Fang and Jan Hoh
, Department of Physiology, Johns
Hopkins University). The scale bars in the images are 125 nanometres.
Images of a Computer Simulation of DNA Condensation
Dynamics (work done in collaboration with B. Schnurr and
F.C. MacKintosh
, University of Michigan)
Stages in the folding of a semiflexible chain. Note
that the scale changes with each image and the scale bar is of constant
size.
Intermediate State for the folding of a long
semiflexible chain.
Computer simulation of the Intermediate Tennis Racquet Configuration
(left) and the analytic solution to the classical elastica problem
(right).
(2) Electrophoresis of DNA and Proteins:
This is a commonly used
technique to separate charged chains by driving them through a gel using
an electric field. Recently, Bob Austin at Princeton has proposed and
demonstrated a novel electrophoretic medium - a series of obstacles
etched into a silicon chip. This has opened up a very wide
avenue of research since it is not clear what obstacle array will
optimize chain separation. Furthermore, the technique can also be used
to sort objects on a much larger scale such as cells.
The main approach used here is computer
simulation. This work is carried out in collaboration with Edith Sevick
in the Research School of Chemistry at ANU. We recently received a large
ARC grant to fund this work.
(3) Properties of Semiflexible Biopolymers:
Almost all of polymer
science has focussed upon very flexible chains which suffer negligible
free energy penalty upon bending. Most biopolymers are like a garden
hose - to bend them involves a high energy penalty. Some of my current
and future research involves understanding both the statics and
dynamics of these class of polymers. This was also the topic of my
first PhD student, Joanne Bright, who finished in 1999.
(4) Budding from Cells:
I have recently begun
discussing possible collaborations with people
in the John Curtin School of Medical Research (JCSMR) at ANU, and in particular
with the group of Professor Gage and Dr Gary Ewart. One initial point of contact has been
to try and understand how the viruses (such as influenza or HIV) bud
from cells. They need to bud in order to reproduce, and if one can stop
them from budding then one has found a cure. There is a significant
literature in the physics community on this subject
, and one aim is to try and apply this to
specific problems.
Photos of Cell Budding for Cells with HIV Gag
protein (courtesy of Dr Gary Ewart, JCSMR, ANU)
Early Stages of Budding in the presence of Gag
protein 40 000 x maginfication.
Later Stages of Budding in the presence of Gag
protein and VPU, 60 000 x maginfication.
Fully Formed Shells in the presence of Gag
protein and VPU, 60 000 x maginfication.
Lower Magnification Picture in the presence of Gag
protein and VPU, 4 000 x maginfication.
Polymer Physics
Alongside the specific biophysics problems I to continue to work on
a number of more traditional polymer problems. Two examples
are:
(1) Block copolymers:
A large fraction of my work has been devoted to
understanding the properties of "self-assembled" block copolymer
systems, which can form a number of novel phases. The simplest of these
are diblocks, and their bulk behaviour is now very well understood from
both a theoretical and experimental point of view. However, two major
areas of research remain. One is the interactions of block copolymers
with surfaces. This has applications to "templating" where one would
hope to template solid-state structures using the well-organised
surfaces of block copolymers. This has been a major area of research for
myself and my post-doc Gerald Pereira (funded on an ARC large grant).
Simulations of Self-Assembled Diblock Copolymer
Systems, from joint work with Gerald Pereira
Diblock conformation for a thin film near a
stripe showing the presence of stress-induced defects.
Well ordered diblock conformation for a thin film near a
stripe.
Computer Simulation of the Cylindrical Phase.
), have proposed a method of simulating these systems
which does not require any assumptions about the morphology. This particular
area has overlap with that of Stephen Hyde, since some of these polymer
morphologies can be approximated as minimal surfaces.
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(2) Single Polymer Chains:
Much of traditional polymer science has
focussed on bulk properties of polymers, where many chains are present.
This is often the regime which is important for applications and has
been the focus of a number of experimental techniques (classical
rheometry, neutron scattering, electron microscopy, the surface forces
apparatus). Over the past decade a number of new techniques have come to
the fore, which have allowed the imaging and manipulation of individual;
chains. These include the atomic force microscope, fluorescence
microscopy and optical and
magnetic tweezers. For instance, using fluorescence microscopy, it is
now possible to make real-time movies of a single DNA chain moving in an
electric field. These techniques have opened up a number of new
avenues for theory and simulation. They are particularly appealing from
the point of view of biology, where many of the processes such as
muscle contraction and DNA replication need to be understood at the
level of the single chain. I have done a considerable amount of work on
single chains over the past five years and will continue to do so. Some
of this work is conducted in collaboration with Edith Sevick in RSC
(ANU), who has an optical tweezers setup, and from 2000 will have a two
post-docs to collaborate on this work. There is also strong overlap
here with the work of Tim Senden in Applied Maths who has pioneered work
on manipulating single chains with the atomic force microscope. My
work in this area resulted in a recent award of an ARC
Senior Research Fellowship.
Click Here For Some Work on Single
Polymer Chains
My basic philosophy (and one which I think which
dominates the whole Department) is that it is important to me that my
work is of interest to people outside the narrow confines of "physics" -
to chemists, chemical engineers, biologists, materials scientists and
medical researchers.