Here is my CV.

Here is my  Ph.D. thesis in pdf format.
 
 

Recent Publications:
 

C.D. Santangelo and A.W.C. Lau, Effects of counterion fluctuations
in a polyelelctrolyte brush, submitted to Euro. Phys. J. E (2003).

Z. Dogic, J. Zhang, A.W.C. Lau, H. Aranda-Espinoza, P. Dalhaimer,
D.E. Discher, P.A. Janmey, T.C. Lubensky, A.G. Yodh,
Elongation and fluctuations of semi-flexible polymers in a nematic solvent,
accepted to PRL (2004).

A.W.C. Lau, B.D. Hoffman, J.C. Crocker, and T.C. Lubensky,
Microrheology, stress fluctuations and active behavior of living cells,
Phys. Rev. Lett. 91, 198101 (2003).

A.W.C. Lau and P. Pincus, Counterion Condensation and Fluctuation-Induced Attraction,
Phys. Rev. E 66,041501 (2002).

A.W.C. Lau, Keng-Hui Lin, and A.G. Yodh,
Entropic interactions in suspensions of semiflexible rods:
Short-range effects of flexibility, Phys. Rev. E 66, 020401(R) (2002).

A.W.C. Lau, D.B. Lukatsky, P. Pincus, and S.A. Safran,
Charge-Fluctuations and Counterion Condensation, Phys. Rev. E 65 051502 (2002).
 
 

A Brief Research Summary:

My primary research interest is theoretical soft condensed matter physics, the study of
materials characterized by their ease of response to external forces and thermal fluctuations.
I am particularly interested in the exciting area of its overlap with cellular biology. Indeed,
the fundamental building blocks of life - the plasma membrane, the cytoskeleton, microtubule,
DNA, F-actin - are all soft materials. To date, I have worked on a number of  projects involving
electrostatic, entropic, and elastic effects in biomolecular systems.  My research interest has
recently extended  to viscoelastic properties of living cells and nematic gels,  a simple model of
actin networks in cells. Many of these activities,  which were inspired by biological systems,
have stimulated experimental investigations, which in turn may provide important
physical insights.
 

Correlation effects in electrostatics

Of all interactions, electrostatics is arguably the most fundamental for soft systems and it is
ubiquitous in biological materials. The standard mean-field approach to charged systems is
the Poisson-Boltzmann (PB) theory, or its linearized version, the Debye-H\"{u}ckel theory.
Although PB theory provides an adequate description for weakly charged systems,  it fails
for highly charged membranes and biopolymers such as DNA, and in particular, it cannot
account for the counterion-mediated attractions, which are believed to play a major role in
DNA condensation.  I have explored this electrostatic attraction in a unified framework,
based on the Wigner crystal model.  In addition, I have worked out the renormalization of
the bending  constants of a highly charged membrane  produced by charge-fluctuations.
It turns out that, in contradistinction to PB prediction, a highly charged membrane can
become more flexible and likely to bend. Moreover, based on a ``two-fluid'' model, I have
predicted a novel fluctuation-induced condensation transition, in which a large fraction of
counterions is condensed onto a charged plate.  A particularly interesting prediction of this
theory is that at physiologically relevant temperatures, monovalent and divalent counterions
exhibit qualitatively distinct behaviors, as is often observed experimentally.  I have also
extended this condensation framework to two similarly charged plates, and found that,
in addition to attraction, this system may exhibit a first-order binding transition.  My
recent interest in correlation effects extends to other charged systems and polyelectrolyte
brushes, in particular.

Depletion interaction mediated by semiflexible rods

Another interaction that is important in soft matter and particularly in colloidal science is
the depletion attraction.  Its origin is purely entropic: When two spherical particles are
suspended in a solution of order-of-magnitude-smaller particles, the larger particles attract
because of the gain in the available volume -- hence entropy -- of the smaller particles
when the larger particles are close together.  A recent experiment which measures the
interaction potential between two spheres in a solution of semi-flexible rods (fd virus)
did not agree with the ``rigid-rod'' theory which predicts the depletion interaction
mediated by perfectly rigid rods.  To explain this discrepancy,  I proposed a simple
bent-rod model, which relies on the assumption that if the rods are sufficiently stiff,
they might be approximated by two rods of half the length attached together at a fixed angle.
This model is simple enough for an analytical treatment and provides an accurate fit of
the measured interaction potential.

Fluctuations of a bio-polymer in a nematic background
 

The advent of single-molecule manipulation of biopolymer provides an enormous opportunity
to learn about the dynamics and statistical properties of biomolecules on a single-molecule level.
While most in vitro experiments are done in an isotropic background, many biopolymers such
as the actin filaments within the sarcomere and neurofilaments within the axon actually reside
in an anisotropic nematic-like environment.  Together with the experimental group at UPenn, I
have investigated the behaviors of single semiflexible polymers dissolved in a nematic background
of fd virus.  I have contributed theoretically to explain the experimental observations, and in
particular, constructed a rotationally-invariant theory for a single semiflexible polymer in a
nematic matrix, and calculated from it the fluctuation spectrum, which compares well with experiments.

Microrheology of living cells
 

An physical picture of active behaviors of living cells is crucial for a more complete
understanding of cellular processes such as intracellular transport of vesicles and organelles.
The molecular basis of such behaviors are highly specialized protein molecules which
transduce the chemical energy of a fuel molecule, ATP, to mechanical work and motion.
Together with J. Crocker and Lubensky, I am presently pursuing the question of whether
microrheology can be applied to studying active behavior of the cytoskeleton of living cells.
Microrheology  has recently become a powerful tool to measure elastic moduli of complex fluids.
Unlike conventional rheology measurements, it relies on the Brownian (thermal) fluctuations of
micron-sized beads dispersed in the sample to assess the viscoelastic response function.
A natural extension of this novel technique to living cells is to assume that the beads
are not driven by thermal noise, instead by a non-thermal noise arising from the presence
of active elements (motors).  With Tom Lubensky, I have worked out theoretically that if one
can quantify the rheology of a cell using a frequency-dependent shear modulus, the power
spectrum of the non-thermal noise can be sensibly extracted from two-point microrheology
(tracking two particles simultaneously) experiments.  Preliminary experimental results of
Crocker show that this power spectrum exhibits power-law behavior over a few decades
in the ten Hertz regime.