softmatter:Confocal Microscopy

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To study colloidal suspension, the confocal microscope offers invaluable data in tems of structure and dynamics [1, 2] due to its planar resolution. This resolution is due to a tiny pinhole that blocks out-of-focus light from reaching the detector. By collecting many planar images within a sample, a 3 dimensional reconstruction of the sample can be created. It is this 3D data that is invaluable to studying colloidal supsensions.

Sample Preparation

In order to probe deep into the sample, the solutions must be refractive index matched to prevent scattering.

Image Processing

In general all the data that is used to characterize complex fluids systems can be generated using confocal imaging techniques and image processing. Algorithms are based upon the following processes [3]:

1.) Image restoration - Most images suffer from various imperfections which can compound error in particle location and tracking. Depending on the system used, different convoulutions may be used to reconstruct an optimum image displaying the system in a near-perfect way.

2.) Particle location - Image processing relys on the fact that a particle center will be the local maxima of intensity within a sphere. This location is given as the voxel location within the 3-D structure.

3.) Sub-pixel refinement - Because the center of a particle is not governed by the pixel size of the imaging technique, true centers should be determined from a sub-pixel refinement algorithm. This can be calculated by offsetting the image center in the direction of higher intesity.


These same processes can be used to identify crystal growth in sperical systems [Gasser, Weeks, Schofield, Pusey and Weitz, Science, 292, 258 (2001), Hoogenboom, Vergeer and van Blaaderen, J Chem Phys. 119, 3371 (2003)], anisotropic particle backbones [4] and to link trajectories in dynamic processing of spheres[3] in glass transition [Weeks Crocker Levitt Schofield and Weitz, Science, 287, 627 (200) and Kegel and van Blaaderen Science, 287, 290 (2000)] as well as colloidal gels [2 and Varadan and Solomon, Langmuir 19, 509 (2003)] and rod-like particles [5].

References

[1] Solomon MJ and Kogan M, Confocal Optical Microscopy, Encyclopedia of Condensed Matter Physics (2005)

[2] Dibble CJ, Kogan M and Solomon MJ, Physical Review E 74, 041403 (2006)

[3] Crocker JC and Grier DG, J. Colloid Interface Science 179, 298-310 (1996)

[4] Mohraz A and Solomon MJ, Langmuir 21 (12), 5298-5306 (2005)

[5] Mukhija D and Solomon MJ, J. Colloid Interface Science 314(1), 98-106 (2007)

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