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Figure 1 Schematic representation of a spherical probe particle in a biopolymer network.
(A) For a particle of radius a much greater than the mesh size
x
, the MSD is
directly related to the bulk linear rheology. (B) For a small non-interacting
particle (a
o
x
), the MSD is related to the solvent viscosity and local repulsive
interactions with the network. (C) For a highly sticky particle (a
o
x
), the
particle diffusion is greatly restricted, but the MSD differs from the bulk linear
rheology because strong particle-protein interactions modify the local biopoly-
mer network structure. (D) For a slightly sticky particle (a
o
x
), its Brownian
motion in the solvent environment is hindered by competing attractive interac-
tions with the biopolymer network (Taken from Ref. 10, and reproduced with
permission of the Biophysical Society.)
than the structural length-scales (a
o
x
), it has been noted
10
that complications
from particle-protein interactions can be very significant. With interactions
that are strong, small particles can become completely immobilized within the
entangled network (Figure 1C). And the presence of even fairly weak protein
adsorption can cause particles to associate preferentially with network strands
or cavity walls, thereby hindering their diffusion and limiting their exploration
of smaller pores (Figure 1D).
In this paper we report specifically on the use of particle tracking to probe
the microrheology in the phase-separating regions of a heterogeneous protein-
stabilized emulsion exhibiting depletion flocculation. The heterogeneity is
induced by the presence of a very small concentration of a non-adsorbing
polysaccharide (xanthan gum). On the macroscopic scale, this type of emulsion
exhibits substantial serum separation
11-14
and strongly shear-thinning rheolog-
ical behaviour.
12,15
Our objective is to monitor the local dynamics of incorpo-
rated probe particles to establish which regions of the evolving heterogeneous
system are predominantly responsible for determining the bulk rheology, and
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