244 research outputs found
Ptychographic X-ray computed tomography of extended colloidal networks in food emulsions
As a main structural level in colloidal food materials, extended colloidal
networks are important for texture and rheology. By obtaining the 3D
microstructure of the network, macroscopic mechanical properties of the
material can be inferred. However, this approach is hampered by the lack of
suitable non-destructive 3D imaging techniques with submicron resolution.
We present results of quantitative ptychographic X-ray computed tomography
applied to a palm kernel oil based oil-in-water emulsion. The measurements were
carried out at ambient pressure and temperature. The 3D structure of the
extended colloidal network of fat globules was obtained with a resolution of
around 300 nm. Through image analysis of the network structure, the fat globule
size distribution was computed and compared to previous findings. In further
support, the reconstructed electron density values were within 4% of reference
values.Comment: 19 pages, 4 figures, to be published in Food Structur
Reconfiguration, Interrupted Aging and Enhanced Dynamics of a Colloidal Gel using Photo-Switchable Active Doping
We study light-activated quasi-2d gels made of a colloidal network doped with
Janus particles. Following the gel formation, the internal dynamics of the gel
are monitored before, during, and after the light activation. We monitor both
the structure and dynamics, before, during and after the illumination period.
The mobility of the passive particles exhibits a characteristic scale-dependent
response. Immediately following light activation, the gel displays large-scale
reorganization, followed by progressive, short-scale displacements throughout
the activation period. Albeit subtle structural changes (including pore opening
and widening and shortening of strands) the colloidal network remains
connected, and the gel maintains its structural integrity. Once activity is
switched off, the gel keeps the memory of the structure inherited from the
active phase. Remarkably, the motility remains larger than that of the gel,
before the active period. The system has turned into a genuinely different gel,
with frozen dynamics, but with more space for thermal fluctuations. The above
conclusions remain valid long after the activity period.Comment: 4 pages, 5 figure
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
Strand plasticity governs fatigue in colloidal gels
Repeated loading of a solid leads to microstructural damage that ultimately
results in catastrophic material failure. While posing a major threat to the
stability of virtually all materials, the microscopic origins of fatigue,
especially for soft solids, remain elusive. Here we explore fatigue in
colloidal gels as prototypical inhomogeneous soft solids by combining
experiments and computer simulations. Our results reveal how mechanical loading
leads to irreversible strand stretching, which builds slack into the network
that softens the solid at small strains and causes strain hardening at larger
deformations. We thus find that microscopic plasticity governs fatigue at much
larger scales. This gives rise to a new picture of fatigue in soft thermal
solids and calls for new theoretical descriptions of soft gel mechanics in
which local plasticity is taken into account.Comment: 5 pages, 4 figure
Nanoparticle Network Formation in Nanostructured and Disordered Block Copolymer Matrices
Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native colloidal silica (CS) into a nanostructured block copolymer yields hybrid nanocomposites whose mechanical properties can be tuned by nanoparticle concentration and surface chemistry. In this work, dynamic rheology is used to probe the frequency and thermal responses of nanocomposites composed of a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer and varying in nanoparticle concentration and surface functionality. At sufficiently high loading levels, FS nanoparticle aggregates establish a load-bearing colloidal network within the copolymer matrix. Transmission electron microscopy images reveal the morphological characteristics of the nanocomposites under these conditions
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