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We are trying to figure out how a colloidal
glass responds to localized, fast disturbances. To this end we
have placed a very small amount of paramagnetic beads into the large amount
of fluorescent beads which form the colloidal glass. By exposing
these magnetic beads, which are about the same size as the fluorescent
spheres, to a strong magnetic field gradient, we can pull them through
the sample. By rotating the direction of a strong field, we can rotate
a pair of magnetic beads which are stuck together. The image at left shows
the set up for the dimer rotation experiment; a strong permanent magnet hangs
from a computer controlled stepper motor. The poles are oriented horizontally.
The stepper motor rotates the field in the horizontal plane at a controlled
rate.
The effect of the magnetic beads on the sample is unknown. Does the sample react elastically, flexing under the stress of the moving magbead and then returning to its original shape after the magnetic field is removed? Will it simply move along with the bead? How does the distance from the magbead affect the response of individual beads or groups of beads? |
| To do this experiment, we purchased several magnets, large and small, from ForceField magnet merchants. Their Neodynium alloy magnets have strengths at the pole of up to ~1 T. This is extremely strong. It turns out that the force on a paramagnetic particle depends mostly on the field gradient, provided that there is a sufficient constant magnetic field to magnetize the bead. So the geometry of the magnet configuration is of utmost importance. We are exploring the possibility of shaping high magnetic permeability materials into various pointy shapes to maximize the field gradient the magbeads see. |  |
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Images of the colloids were taken with the 3D confocal laser scanning
microscope in Prof. Eric Weeks' lab. Their positions were tracked using
software written in the IDL programming language. Plots of displacements
versus distance from the magbead disturbance were made, showing a pleasant
fall-off suggestive of an exponential decay.
Click on the picture to see a movie of a rotated magbead cluster; the movie
represents 24 minutes of the experiment. The movie repeats after
the bead has made one half rotation.
Below we have another movie showing the response
to a pulled magnetic particle.
We are looking for the dependance of the properties of this fall-off on colloid concentration. Various geometric and topological analyses may also be useful for analyzing our system. |
This work has been primarily funded by NASA.
