Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 309

aliquot of Nile Red solution (0.01 wt.%) was immediately added to give a final
concentration of 0.4 mg of dye per 1 ml of the solution. The solution was
thoroughly mixed at a shear rate of 1 rad s 1 for 5 min at 201C. The stained
emulsion was then immediately placed into a laboratory-made welled slide,
filling it completely. A coverslip was placed on top of the well, and it was
ensured that there were no bubbles trapped between sample and coverslip. The
edge of the coverslip was sealed to prevent moisture loss and exposure to the
atmosphere.
Trajectories of fluorescently labelled COOH-PS microspheres were recorded
at 201C in real time using the CLSM. The probe particles were added at a
concentration of 0.2-0.4 vol.%. The position of each fluorescent microsphere
was identified by finding the brightness-averaged centroid position with a sub-
pixel accuracy of approximately 20 nm spatial resolution. 21 While the micro-
scope was capable of tracking hundreds of particles simultaneously, the
number of tracked microspheres per field of view was actually kept relatively
low (between 10 and 30) to eliminate complications due to particle-particle
interactions. A voxel size of 183 183 668 nm was chosen for most of the
experiments (image dimensions 93.75 23.44 nm), but the pinhole size was
sometimes increased to record thicker slices and facilitate more extended
tracking of microspheres in the focal plane. The acquisition time for each pixel
slice in the horizontal x y plane was typically 0.134 s. Images were scanned
approximately 10-20 mm below the level of the coverslip to minimize hydro-
dynamic (and other) interactions with the coverslip.
Movies recording the changing positions of the diffusing microspheres in the
x - y plane were analysed according to the procedures of Crocker and Grier. 21
A tracking macro-routine was applied between successive frames. The trajec-
tories were analysed with a slightly modified version of the IDL tracking
software series (Research Systems, Boulder, CO) comprising a series of macros
developed originally by Crocker and Weeks. 22 Microsphere positions were
matched frame by frame, using a routine incorporated into the IDL software,
identifying each individual particle and then generating its trajectory. Another
macro was employed to diagnose and eliminate any background drift due to
unwanted convective flow or movement of the microscope stage. Systematic
drift was found by examining the average values of dx and dy for all the visible
particles from one frame to the next, and then subtracting the computed
background drift component from the raw data for each individual particle.
Further experimental details can be found elsewhere. 17
20.3 Results and Discussion
We describe first the behaviour of COOH-PS microspheres of diameter 0.5 mm
dispersed in aqueous solutions of 60 wt.% glycerol (0.010 Pa s) and 88 wt.%
glycerol (0.147 Pa s). It can be seen from Figure 2 that the ensemble-averaged
MSD of the microspheres is a linear function of lag time t for these simple
Newtonian liquids. The two straight lines in Figure 2 correspond to theoretical