Program Highlights
Rodlike fd viruses in an aqueous
solution, along with polymers that produce an attractive force between the
virus particles, have been observed in laboratory experiments to self-assemble
into a variety of geometric structures including twisted ribbons (see the schematic
illustration and microscope picture to the right). We have developed a
theoretical model which explains the properties of these ribbons on the basis
of very general features of the fd rods. The theory yields
predictions in good agreement with experiment, namely, (a) a phase diagram with a first-order transition
from flat membranes to twisted ribbons, (b) the ratio of the ribbon's
pitch to width, (c) the tilt angle of the rods at the edge of the
ribbon. The theory has also demonstrated
the importance of molecular chirality ("twisting-handedness") in the formation of
the ribbons, as well as the tendency of fd rods to assemble into
structures with negative Gaussian curvature (as in a saddle shape).
August 25, 2010
Cilia
and flagella are tiny waving filaments with important functions in humans, such
as clearing our airways. A 3D electron microscopy study of the normal (WT) and
mutant varieties of flagella has revealed new details of the structure, protein
composition, and connections between neighboring components formed by a crucial
structural complex of this biological nanomachine,
the "nexin-dynein
regulatory complex" (N-DRC), which is shown in color on the left. In the poorly
moving mutants, this structure is damaged or almost missing. This opens the way
for building a new mechanical model of this device, and how it functions, along
with mechanical testing of individual cilia and flagella, normal and mutant, in
the Brandeis multi-mode optical microscopy laboratory. 
During the academic year, Fall09 - Spring10, Dr. Kim,
the facility director ,designed, built and tested devices for a number of MRSEC
and outside users.
Stabilized
emulsions containing the oscillating Belousov - Zhabotinsky chemical reaction (BZ) show interesting dynamics. Each drop acts as an independent chemical clock.
However, they chemically communicate and exhibit collective behavior. In (a)
six BZ drops are contained in a
capillary tube. The white bars
are light, which set the oscillators in the reduced state. Drops 1 & 6 are
always exposed to light, setting the boundary conditions. Drops 3 & 4 are
exposed to light for 0.5 periods. (b) A space time plot of the oscillations. White
corresponds to the oxidized state; black to reduced. The drops adopt a dynamic
oscillatory state predicted by theory. (c,d) Photograph and schematic, respectively, of
experimental set-up. Study of these systems will elucidate a variety of
chemically dynamic systems, ranging from neurons to Active Matter – polymeric
systems which can convert chemical energy to mechanical motion.
August 23, 2010
Buehler and co-workers of the MIT MRSEC IRG-II have
found that the key to silk's pound-for-pound toughness, which exceeds
that of steel, is its beta-sheet crystals, the nano-sized cross-linking
domains that hold the material together.
August 23, 2010
August 18, 2010
We have made the first experimental observation of the Aharonov-Bohm effect in neutral excitons. In classical mechanics the motion of an electrical charged particle is only affected if the particle enters a region of space with a magnetic or electric field. Meanwhile, in quantum physics, an electrically charged particle is affected by the magnetic potential in regions even if the magnetic and electric fields are zero in the region (Aharonov-Bohm effect). Several theory papers have discussed the plausible existence of the this effect even for neutral particles! We have confirmed this speculation.
August 18, 2010
Expanding outreach with portable SEM for K-12 math/science. 