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Many recent experiments have focused on measuring local viscoelastic properties on small length scales using MICRORHEOLOGY techniques. Two powerful and complementary methods, laser light scattering and single particle tracking have allowed researchers to study the local properties of polymer gels, and other complex fluids with great success over many orders of magnitude in frequency. These experiments measure the mean square displacement as a function of time of embedded probe particles in the solution undergoing random thermal motion (see photo). The mean square displacements are then interpreted in terms of local viscoelastic response. However, applying these techniques to highly heterogeneous systems (like crosslinked gels, or biological cells), has been a challenge. Light scattering techniques average over an ensemble of particles and do not allow for measurements of local variations in mechanical response. Single particle tracking is a very localized measurement, but since single probes are used, many consecutive experiments are required to map out spatially dependent response. |
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We have developed a new technique called multiple particle tracking that
allows us to simultaneously measure the displacements of roughly one hundred
fluorescent particles using video microscopy. We analyze the
individual particle displacements and map out the local viscoelastic
response across the sample. This allows us to begin to
understand and characterize the heterogeneities
in soft systems. This is a image of the trajectories of small probe particles moving in a structured polymer gel. We have color-coded the particles according to the local viscosity they measure. In this particular case, we see no spatial correlation of microenvironments. Click here to find out how you too can do particle tracking! |
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We have studied several inhomogeneous systems with this technique,
including, an agarose gel and F-actin solutions and poly(acrylic acid).
Agarose is an industrially important
poly(saccharide) gel that is used as
separation medium in gel electrophoresis. This gel is very stiff, yet
contains many micron-sized pores though which
particles, or proteins can navigate. It is exactly the heterogeneous
nature
of this gel that makes it such a good
separator! Actin is a protein complex found in the cytoskeleton of
eucaryotic cells. It provides mechanical
stability and helps in cell locomotion and transport. This picture is an
electron micrograph of an actin network taken by Sally Zigmund at the
University of Pennsylvania. We are currently using multiple
particle tracking to study the
local mechanical response of
biological cells. |