SCIENCE AT THE EDGE SEMINAR

                         Friday, February 14, 2003
      11:30 a.m., Room 1400 Biomedical Physical Sciences Building
                    Refreshments at 11:15 in 1400A


Speaker:       Prof. Seth Fraden
               Complex Fluids Group
               www.elsie.brandeis.edu
               Brandeis University
               Waltham, MA  02254


Title:         Engineering Entropy: Progress and Puzzles


Abstract:

        Although the idea that entropy alone is sufficient to produce an
ordered state is an old one in colloidal science, the notion remains
counter-intuitive and it is often assumed that attractive interactions are
necessary to generate phases with long-range order.  We study colloidal
suspensions of the rodlike virus fd alone and in mixtures with polymers such
as polyethylene oxide.  We begin with simple conditions approximating hard
particle fluids, which are impenetrable objects with no other interaction
potential and in several cases find excellent agreement between theory and
experiment.  However, even these simple, hard particle fluids exhibit
unexpectedly rich phase behavior.  We genetically engineer the viruses to
produce a homologous series varying only in length.  We then modify the
diameter of the particles by permanently attaching water soluble polymers to
the surface of the virus.  While much of the behavior of these biopolymers
can be explained on the basis of entropy, there are also effects resulting
from the electric charge of the viruses that are inexplicable.

        Through experiment, simulation, and theory we explore how phase
behavior is controlled by the shape, concentration, charge, flexibility, and
composition of the particles.  Because all molecules have a hard particle
core, much of the phenomena is likely to be quite general, applying also for
example to low-molecular mass liquid crystals, amphiphiles and block
copolymers, to bioseparation methods and DNA partitioning in prokaryotes, to
protein crystallization, and the manufacture of composite materials.

More information available at: www.elsie.brandeis.edu