60 research outputs found
Measuring every particle's size from three-dimensional imaging experiments
Often experimentalists study colloidal suspensions that are nominally
monodisperse. In reality these samples have a polydispersity of 4-10%. At the
level of an individual particle, the consequences of this polydispersity are
unknown as it is difficult to measure an individual particle size from
microscopy. We propose a general method to estimate individual particle radii
within a moderately concentrated colloidal suspension observed with confocal
microscopy. We confirm the validity of our method by numerical simulations of
four major systems: random close packing, colloidal gels, nominally
monodisperse dense samples, and nominally binary dense samples. We then apply
our method to experimental data, and demonstrate the utility of this method
with results from four case studies. In the first, we demonstrate that we can
recover the full particle size distribution {\it in situ}. In the second, we
show that accounting for particle size leads to more accurate structural
information in a random close packed sample. In the third, we show that crystal
nucleation occurs in locally monodisperse regions. In the fourth, we show that
particle mobility in a dense sample is correlated to the local volume fraction.Comment: 7 pages, 5 figure
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
Poly(dimethylsiloxane)-Stabilized Polymer Particles from Radical Dispersion Polymerization in Nonpolar Solvent: Influence of Stabilizer Properties and Monomer Type
Particles used in electrophoretic display applications (EPD) must possess a number of specific properties ranging from stability in a nonaqueous solvent, high reflectivity, low polydispersity, and high charge density to name but a few. The manufacture of such particles is best carried out in the solvent of choice for the EPD. This opens up new interests in the study of nonaqueous dispersion polymerization methods, which deliver polymer particles suspended in low dielectric constant solvents. We explore in this article the use of a poly(dimethylsiloxane) macromonomer for the stabilization of poly(methyl methacrylate) polymer particles in dodecane, a typical solvent of choice for EPDs. The use of this stabilizer is significant for this method as it is directly soluble in the reaction medium as opposed to traditionally used poly(12-hydroxystearic acid)-based stabilizers. Additionally, the present study serves as a baseline for subsequent work, where nonaqueous dispersion polymerization will be used to create polymer particles encapsulating liquid droplets and solid pigment particles. In this article, the influence of the macromonomer molecular weight and concentration on the properties of the synthesized particles is studied. In addition, we investigate the possibility of synthesizing polymer particles from other monomers both as a comonomer for methyl methacrylate and as the only monomer in the process. The influence of macromonomer concentration is also studied throughout all experiments
A Landau-Squire nanojet
Fluid jets are found in nature at all length scales, from microscopic to
cosmological. Here we report on an electroosmotically driven jet from a single
glass nanopore about 75 nm in radius with a maximum flow rate ~15 pL/s. A novel
anemometry technique allows us to map out the vorticity and velocity fields
that show excellent agreement with the classical Landau-Squire solution of the
Navier Stokes equations for a point jet. We observe a phenomenon that we call
flow rectification: an asymmetry in the flow rate with respect to voltage
reversal. Such a nanojet could potentially find applications in
micromanipulation, nanopatterning, and as a diode in microfluidic circuits.Comment: 20 pages, 4 figure
Confocal microscopy of geometrically frustrated hard sphere crystals
Confocal microscopy has been used to study the crystallization of two colloidal model systems that are geometrically frustrated in a completely different way: (I) hard colloidal polyhedrals, where crystallization is frustrated due to the incommensurate particle shape and (II) large spherical impurities in a sea of monodisperse colloidal hard spheres, where crystallization is frustrated by the introduction of impurities. As a reference system, we analyzed the crystallization of pure monodisperse colloidal hard spheres. We show that although the crystal structures of both systems are highly dissimilar on the individual particle level, both sources of geometrical frustration have a similar effect on the structure on the grain level. We quantitatively characterize the polycrystalline structures and study the crystallization process in time. Whereas grain boundaries persist in the frustrated systems due to structural arrest, the majority of grain boundaries anneals out quite rapidly in the reference system. Therefore, we argue that both sources of geometrical frustration cause a polycrystalline structure
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