| 1. |
- Qazi, S. Junaid S., et al.
(author)
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Alignment of Dispersions of Plate-Like Colloidal Particles of Ni(OH)(2) Induced by Elongational Flow
- 2011
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 115:13, s. 3271-3280
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Journal article (peer-reviewed)abstract
- Small-angle neutron scattering, SANS, has been used to study the alignment of a sterically stabilized colloidal dispersion of nickel hydroxide induced by elongational flow. Two different concentrations, 20 and 23 wt %, of well-defined hexagonal platelets have been studied. Significant anisotropy in the scattering patterns has been observed for the higher concentration dispersion that increases with increasing elongational strain rate. The effect of pipe flow (shear flow) on the orientational ordering at the inlets is also described. Near the outlets there is also a shear stress and this tends to increase the alignment. The orientational distribution of the particles under flow is described in terms of an order parameter calculated with respect to an individual director for each scattering pattern. The experimental measurements are supported by comparison with computer simulations which help to explain the effects of local velocity on the alignment induced by elongational flow. A slight decrease in the observed alignment was found after continuous flow for approximately 30 min and possible reasons are discussed.
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| 2. |
- Qazi, S. Junaid S., et al.
(author)
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Alignment of Plate-Like Particles in a Colloidal Dispersion under Flow in a Uniform Pipe Studied by High-Energy X-ray Diffraction
- 2010
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:24, s. 18701-18709
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Journal article (peer-reviewed)abstract
- High-energy angle-dispersive X-ray diffraction has been used to study the alignment of colloidal suspension of kaolinite particles in water as they flow along a pipe. X-rays with energies above 25 keV have a major advantage, as they can penetrate through thick samples and walls of containers and permit investigation of samples under realistic flow conditions. As an example of the method, flow through a circular cross-section pipe with an internal diameter of 5 mm has been studied: this is typical of industrial applications. The angular distribution of intensities of peaks in the diffraction pattern as a function of the location of the pipe in the X-ray beam provides information about the alignment of particles under flow. Order parameters have been calculated to describe the alignment and direction of orientation. It is observed that the particles align in the direction of flow with their flat faces parallel to the flow. The experimental results are compared with the calculations of the local strain rate that help to explain the onset of alignment of the particles.
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| 3. |
- Qazi, S. Junaid S., et al.
(author)
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Dispersions of plate-like colloidal particles : Cubatic order?
- 2010
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In: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 348:1, s. 80-84
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Journal article (peer-reviewed)abstract
- Experimental evidence for the existence of 'cubatic' order in a colloidal dispersion of plate-like particles is presented. In a 'cubatic' phase, disk-like particles self-assemble with domains of a few parallel plates and the director tends to be orthogonal in adjacent domains. This phase has been predicted previously by computer simulation. The domains are approximately equiaxial and are predicted to exist only within a limited range of aspect ratios and volume fractions. This locally ordered structure cannot be identified readily using scattering techniques, since the patterns are expected to be similar to those of isotropic liquid phases. For this reason, we have used a real-space technique of cryo-transmission electron microscopy that directly probes such locally ordered structures to study dispersions of nickel hydroxide particles. Polydispersity of particle size is expected to require some local tilting in order to include larger particles in a dense structure and this is discussed with respect to the concentration range for which cubatic order is observed. This new structure offers the possibility of novel materials that could be prepared by self-assembly and have applications in a wide range of fields.
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| 4. |
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| 5. |
- Qazi, S. Junaid S.
(author)
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Flow of Colloidal Mesophases
- 2011
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Doctoral thesis (other academic/artistic)abstract
- This dissertation presents new work and results in the flow of complex fluids and experimental methodologies for their investigation. Plate-like colloidal particles of kaolinite and nickel hydroxide are studied. A study of lamellar fragments and their mixture with the nickel hydroxide particles is also presented. The lamellar fragments are self-assembled structures of surfactant molecules that approximate disks. Particles are seen to align with their large faces parallel to the flow direction under shear and elongational strains. Order parameters have been calculated to quantify the extent of preferential alignment and direction of orientation. The experimental data are supported by comparisons with finite-element fluid mechanics calculations that provide estimates of the flow patterns and the strain rates. Elongational strain rates in the range of 5 − 20 s−1 are required to induce a high degree of alignment with the various sizes of the particles whereas about two to three order of magnitude higher shear strain rates are required. The combination of both elongational and shear strain is an effective means to provide a uniform alignment. Comparison of the Peclet numbers calculated for both the shear and elongational flow are presented and this explains that alignment occurs when the energy per particle of the strain is larger than the thermal energy. Mixtures have shown complex behavior: significant changes in the structure are observed that are not seen to the same extent in samples at rest. X-ray diffraction and small-angle neutron scattering techniques are used to characterize the samples and determine the structure in flowing systems. Laboratory X-ray diffraction can be used to characterize dispersed samples. The combination of dynamic light scattering and X-ray diffraction was used to estimate the thickness of the stabilizing layers of the polymer on the colloidal particles. Scattering of synchrotron radiation and neutrons are powerful complementary techniques to provide information about flow and the potential to apply them to systems that are beyond the scope of simple simulations has been demonstrated.
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| 6. |
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| 7. |
- Qazi, S. Junaid S., et al.
(author)
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Impact of Ni(OH)(2) Platelike Particles on Lamellar Surfactant Mesophases and the Orientation of Their Mixtures under Elongational Flow
- 2011
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 115:35, s. 10413-10424
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Journal article (peer-reviewed)abstract
- This article describes the behavior under elongational flow of mixtures of lamellar fragments and platelike colloidal particles. Mixtures of a dialkyl chain cationic surfactant, DHTAC (15 wt %), and the nonionic surfactant Brij 97 (0.5 wt %) form dispersions of lamellar phase fragments, and these were studied in the presence of smaller platelike colloidal particles of Ni(OH)(2) stabilized with sodium polyacrylate. Small-angle neutron scattering was used to follow the changes in the lamellar phase fragments in the mixture under elongational flow. The addition of a small fraction of the Ni(OH)(2) dispersion resulted in significantly increased viscosity and reduced the structure and size of the lamellar fragments, and this effect was further enhanced because of the flow induced alignment. The behavior of the mixture under elongational flow is described, and the induced orientation is discussed in terms of order parameters at the different positions using spatially resolved small-angle scattering. The effect of flow rate on the orientational alignment in the mixture is also described.
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| 8. |
- Qazi, S. Junaid S., et al.
(author)
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Orientation of a Dispersion of Kaolinite Flowing in a Jet
- 2012
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 28:8, s. 3704-3713
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Journal article (peer-reviewed)abstract
- Orientational alignment in a dilute dispersion of kaolinite particles has been investigated in a flow pattern that combines both shear and elongational stress, namely flow at a jet created by a 2 mm diameter nozzle inserted in a 6 mm diameter pipe. Spatially-resolved X-ray diffraction with synchrotron radiation permits detailed maps of the alignment to be deduced and compared with fluid mechanics calculations of the flow. The angular distribution of diffracted intensity from a given position in the pipe provides information about the orientation distribution of the particles. This is quantified and presented in terms of order parameters. The cone-shaped nozzle provides a jet of liquid giving a high degree of alignment of the particles that is uniform along lines across the conical section and constant in the small straight-sided region at the exit of the nozzle. The vortex motion that arises from the flow with a modest Reynolds number could be determined as well as the tendency for some particles to align with their large faces perpendicular to the overall flow direction at the flat surface of the nozzle outlet.
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| 9. |
- Qazi, S. Junaid S., et al.
(author)
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Self-Assembled Structures of Disc-Like Colloidal Particles
- 2011
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In: Trends in colloid andinterface science XXIV. - Berlin, Heidelberg : Springer. - 9783642190377 ; , s. 61-65
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Conference paper (peer-reviewed)abstract
- Self-assembly is an important phenomenon that leads to formation of interesting and novel structures in colloidal dispersions. We present experimental evidence for the existence of a ‘cubatic’ phase in a colloidal dispersion of disc-like particles of nickel hydroxide colloidal dispersions. In this structure, disc-like particles self-assemble as domains of a few parallel discs and the orientation tends to be orthogonal in adjacent domains. This phase has been predicted previously by computer simulations. The domains are approximately equiaxial and are predicted to exist only within a limited range of aspect ratios and volume fractions. We have used the real space technique of cryo-transmission electron microscopy in our studies as this locally ordered structure could not be identified readily using scattering techniques, since the patterns are expected to be similar to those of isotropic liquid phases.
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| 10. |
- Qazi, S. Junaid S., et al.
(author)
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Use of wide-angle X-ray diffraction to measure shape and size of dispersed colloidal particles
- 2009
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In: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 338:1, s. 105-110
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Journal article (peer-reviewed)abstract
- Laboratory X-ray diffraction is used to investigate the size and shape of dispersed plate-like and spherical colloidal particles. Analysis of the wide-angle diffraction data provides information about the size and shape of crystals from the width of the Bragg peaks according to the Debye-Scherrer formula. The measurements, data analysis, and evaluation are discussed. It is shown that X-ray diffraction with conventional laboratory equipment on dispersed particles is feasible as a tool to determine both particle size and shape. Data for two samples - gold colloids and nickel (II) hydroxide particles are presented. The advantages and limitations of the method are discussed. X-ray diffraction measurements that are made in combination with dynamic light scattering can be used to estimate the thickness of stabilizing layers of polymers. (C) 2009 Elsevier Inc. All rights reserved.
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