Magnetic Probes in Colloidal Glasses

P. Habdas, D. Anderson, B. Andrews, B. Bluth, G. Cianci, R. Courtland, X. Du, A. Franciscovich,
L. Goel, J. Hay, A. Levitt, D. Schaar, S. Wu, & E. Weeks

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microscope setup 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.       big Nd magnet

Rotating a Micron-Sized Magnetic Stir Bar 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.

Click here to see an animated GIF movie showing the response of colloidal particles when a magnetic bead is pulled through them.

Click here to see an animated GIF from actual confocal data. Each frame represents 300s; the sample is very dense.

For more information, you can download our papers:

This work has been primarily funded by NASA.