Program Highlights by Type
Electrically charged polymer colloids attracted to the surface of water droplets of different shapes crystallize on their curved surfaces.
The curvature frustrates perfect crystallization and defects must be added such as the familiar twelve pentagons which decorate the otherwise hexagonal faces of a soccer ball. The pentagons/disclinations strongly relax curvature.
We have found a gentler way to relax curvature - Pleats. Pleats add width as one moves down their length both in skirts and in crystals. Our experiments indicate that pleats in crystals correspond to grain boundaries (lines of dislocations) which vanish on the surface. Allowed off the surface they form a set of steps which decrease circumference with height as on the crown of the Chrysler building in NY. 
Following up on their recent creation of light-sensitive fibers,
Professors Yoel Fink and Joannopoulos and their research teams have
developed fibers that can detect ("hear") and produce sound ("sing").
November 16, 2010
It
is well known that particulate suspensions (such as cornstarch suspension) can
rapidly harden
when strained. We show that this
hardening can be precisely controlled when the straining is applied by a moving
body. Hardened volumes of different sizes can be dynamically created by
changing the speed of the moving body, allowing the transmission of a focused force to a receiving
substrate.
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. 
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
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. 