No description available.

The behavior of colloidal liquid crystals in confinement is addressed on the single particle level using laser scanning confocal microscopy. We seek to disentangle how equilibrium director fields are controlled by the complex interplay between confinement, elasticity and surface anchoring. First, we study the nematic phase confined to wedge structured channels. Varying the wedge opening angle leads to a splay to bend transition mediated by a defect in the bulk of the wedge. Our results are in quantitative agreement with lattice Boltzmann simulations, and we show that comparison between experiments and simulation yields a new method to obtain the splay-to-bend elasticity ratios of colloidal and biological liquid crystals. Next, we extend our study of the wedge structured channels to the cholesteric phase, and measure a splay to twist transition with increasing wedge angle. We directly visualise the 3D nature of the twisted state, and explain how the transition is intricately determined by the anchoring strength and the splay, bend, and twist elasticities. Next, we investigate the effect of rectangular confinement on the nematic phase. The rectangle aspect ratio is systematically varied and we observe five distinct director fields. Comparison with computations of the Frank-Oseen energies yields the extrapolation length, which we find to be of the order of the rod length. Next, we confine the nematic phase to annular geometries of varying dimensions, and observe the novel director fields that are adopted. We approach a level of confinement which is of the order of the particle size. Interpreting our observations with Monte Carlo simulations, which take into account the finite size of the particles, illuminates the applicability of continuum theories down to microscopic lengthscales. We finish with a study of the isotropic-nematic interface in bulk and confinement. We show that parallel anchoring occurs at the interface, and measure the width of the interface to be of the order of the rod length.

Uniformly dimpled colloidal particles are studied using laser scanning confocal microscopy and optical tweezing, alongside real-space image analysis. In particular, a comparison is made between the behaviour of these particles with that of isotropic spherical particles of the same size and polydispersity. We begin by detailing the synthetic techniques as well as the methods used for both experiments and simulations in this work. We then probe the structures and ordering exhibited by the dimpled particles and their spherical counterparts at a single featureless wall. Only the first layer at the wall is observed, and pronounced differences in both the translational and orientational order between the two types of particle is found. Furthermore, we show that the presence of the dimple leads to disordered structures that develop over time. Next, we reduce the dimensionality of the system to quasi-two and study the depletion induced interactions in a monolayer of colloidal particles at a single featureless wall. Using both confocal microscopy and Monte Carlo simulations, we illustrate the selectivity of the depletion interaction with regard to particle shape and polymer size. A level of complexity is then added to the problem by introducing a second colloidal system of small spherical particles. The resulting binary mixture, still with additional non-adsorbing polymer and in a monolayer at a wall, allows us to investigate so-called “lock and key” binding. We show that the inclusion of a lock particle cavity, whose shape and size is complementary to those of the key particle, significantly favours binding behaviour, which is further improved by using a depletant consisting of a small polymer and charge-screening salt. Finally, the depletion induced force between lock and key particles at contact is directly measured using optical tweezers. The dependence of the force due to depletion upon the overlap volume between the particles is illustrated, and we find that the strongest force is produced when the key is held within the cavity of the dimpled lock, demonstrating semiquantitative agreement with theoretical predictions.

Cellulose nanocrystals (CNCs) are crystalline nanoparticles that are extracted from renewable sources such as trees or bacteria through mechanical or chemical treatments of their source. CNCs are of interest to several research communities concerned with sustainable technologies. Specifically, CNCs have attracted great interest in the polymer composite community given their high theoretical specific strength and modulus. Two key obstacles surround the use of CNCs in polymer composites, namely their comparatively lower thermal stability and hydrophilicity render their dispersion, and therefore mechanical reinforcement, in polymer matrices challenging. This research considered a waterborne epoxy and a polyurethane elastomer for CNC/polymer composites since these composites are seldom reported in literature or often suffer from degraded mechanical properties. In the epoxy/CNC composites, samples were prepared by two methods, first an epoxy emulsion was mixed with an amine crosslinker and an aqueous based CNC suspension (1-step mixing), and second, the epoxy emulsion was premixed with the aqueous based CNCs and the amine crosslinker was added some time later (2-step mixing). Both composites were mixed by magnetic stirring, however the samples prepared by the 2-step mixing method exhibited enhanced dispersion and mechanical properties, specifically the storage modulus (E’), tensile strength, and work of fracture. Zeta potential measurements and chemical analysis by FTIR indicated that the dispersion mechanism was physical in nature, rather than chemical. In the second composite system, CNCs were chemically modified with an isophorone diisocyanate (IPDI) monomer having unequally reactive isocyanate groups. The goal of the modification step was to react only one isocyanate group with the CNC surface and have a free isocyanate group available for further modification. The chemical structure of one linked isocyanate (urethane bond) and one free isocyanate was confirmed by FTIR and 13C NMR. The particles modified by IPDI (m-CNC) and the neat particles (um-CNC) were incorporated into a polyurethane matrix based on IPDI and a triol crosslinker. Upon visual inspection of the cured composites, it was clear that the modification step produced homogeneously dispersed nanoparticles in the polyurethane while the um-CNCs were aggregated. When the mechanical properties were tested by uniaxial tensile testing, it was determined that the m-CNC composites resulted in improvements in the tensile strength and work of fracture without degradation of the elongation of break property when compared to the neat matrix. Overall the findings in this research highlight important considerations for designing CNC/thermoset composites with enhanced dispersion and mechanical performance.

Different scenarios for assembling rod-like and spherical colloidal particles using binary mixtures of partially miscible liquids were investigated experimentally. Suitable rod-like colloids were developed first. The subsequent studies of colloids in binary liquids consisted, on one hand, of systems where particles were partially wetted by both phases and, on the other hand, of systems where particles were completely wetted by the minority phase. A simple method to prepare large quantities of micrometer-sized akagan eite-silica core-shell rods was developed. These were proven to be very versatile, with the possibility of modifying their properties on different levels. The aspect ratio is simply controlled by a gradual growth of the silica shells. From them, hollow silica rods and rods with an increased responsiveness to a magnetic field could be obtained in straightforward ways. Bijels were prepared by trapping rod-like particles on a percolating liquid-liquid interface. The familiar bicontinuous organization of liquid domains was observed after structural arrest. At a fixed volume per particle it is demonstrated that for rod-like particles the domain size decreases faster with increasing quantity of particles than in the case of spherical particles. Additionally, the packing of the rods at the interface was elucidated, revealing several characteristic features. In particle-stabilized droplet emulsions rapid evaporation of the continuous phase and eventual full mixing of the liquid phases can leave a cellular network of particles. The formation and eventual stability of these networks were investigated in detail with confocal microscopy. When colloids are completely wetted by the minority component of an asymmetric binary mixture there can be substantial temperature and composition regimes outside the binodal where shear-induced aggregation can take place. This happens as adsorbed layers present at the particle surfaces coalesce and bind particles through a liquid bridge. Depending on particle concentration, percolating networks can form of rods wetted by the minority phase after temperature quenching such a system just across the binodal.

Thesis (MSc)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: In this thesis the formation and properties of a polymer gel on and at a surface are investigated. The gel under investigation is defined as a three-dimensional network of macromolecules that form permanent links with one another and also with confining planar surfaces. The precise location of the crosslinks on the wall or on another macromolecule is not known prior to linking, and will differ from sample to sample. However, once the crosslinks are formed, they are assumed to be permanent. This random linking is the source of the disorder in the system, over which a quenched average has to be taken. An existing model [9] of network formation, with polymer-polymer crosslinks, is extended to incorporate a surface and polymersurface crosslinks. Within the framework of replica theory, statistical averages and physical properties of the system are calculated by means of a variational approach. Macroscopic information, in terms of the free energy of deformation, is obtained by using two different potentials to simulate the erosslinks mathematically. / AFRIKAANSE OPSOMMING: In hierdie tesis word die vorming en eienskappe van 'n polimeergel, wat teen 'n oppervlak gevorm word, ondersoek. Die gel word gedefinieer as 'n drie-dimensionele netwerk van makromolekules wat permanente bindings met mekaar, maar ook met twee inperkende, platvlakke, vorm. Die presiese ligging van die bindings op die muur en op ander makromolekules is nie vooraf bekend nie, en sal verskil van een gel-monster tot die volgende. Sodra die konneksies egter gevorm is, word aanvaar dat hulle permanent is. Die lukrake bindingsproses is die oorsprong van wanorde in die sisteem, waaroor 'n wanorde-gemiddelde bereken moet word. 'n Bestaande model [9]van netwerkvorming, met polimeer-polimeer bindings, word uitgebrei om 'n oppervlakte en polimeer-oppervlak bindings in te sluit. Statistiese fisika gemiddeldes en fisiese eienskappe van die sisteem word binne die raamwerk van replika-teorie en 'n variasie benadering bereken. Makroskopiese inligting, in terme van die vrye energie van vervorming, word verkry deur twee verskillende potensiale te gebruik om die konneksies wiskundig voor te stel.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: This thesis investigates questions surrounding the role that iron-rich colloids (nominally sized between 0.02 μm and 0.2 μm) and particulates (>0.2-0.45 μm) play in the context of the greater iron biogeochemical cycle. To this end, this study complements a review of reported size-fractionated iron (Fe) measurements with chemical and mineralogical data derived from synchrotron-based xray measurements. From an extensive literature review, the global surface ocean colloidal iron (cFe) pool is found to be highly dynamic, frequently exhibiting seasonal trends and nutrient-like behaviour. Spatial variability in surface ocean colloidal iron concentration is primarily a function of total iron supply, although the concentration and strength of iron-binding ligands, and inorganic thermodynamic constraints are additional influential factors. The size-fractionated study of colloidal Fe has rendered considerable evidence pointing towards direct or indirect biological utilization of this cFe pool; however, a more complete understanding of cFe-biological interaction necessarily requires better knowledge of cFe chemistry and mineralogy. To address these issues, this thesis documents the development of a novel x-ray microscopy and spectroscopy technique for determining the Fe speciation of individual Fe-rich particles under environmental conditions. Variations in the peak splitting in iron L3-edge XANES (X-ray Absorption Near-Edge Structure) spectra reflect changes in the local coordination environment surrounding the metal centre. Specifically, the energy splitting ( ΔeV) and intensity ratio of the split peaks at the L3-edge vary as a function of the Fe valence state, the number and chemistry of coordinating ligands and polyhedral distortion effects; and combinations of the two parameters are found to be characteristic of individual Fe minerals. To understand Fe speciation, the Δ eV versus intensity ratio plot was successfully applied to a variety of environmental Fe particles (greater than 20 nm diameter) collected from two ocean basins; the Southern Ocean and the south western Pacific Ocean. Speciation differences in Fe particles collected from the Southern Ocean show distinct compositional trends between the coasts of South Africa and Antarctica, with different Fe pools associated with the different oceanographic frontal zones. Despite the oxygenated nature of the seawater sampled, the presence of significant particle-hosted Fe(II) was observed in both the Southern Ocean at high latitudes, and at sampling sites proximal to the Kermadec Ridge in the Pacific Ocean. Ferrous iron particles at the latter study area were shown to be strongly associated with carbon functional groups, notably alcohol and carboxamine moieties. These findings, relating to particle chemical differences and associations with organic matter, have significant implications for our understanding of particle behaviour, their surface interactions and the role that they play in primary productivity and global elemental cycles. / AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek kwessies met betrekking tot die rol wat ysterryke kolloïede (van nominale groottes tussen 0.02 μm en 0.2 μm) en partikels (>0.2-0.45 μm) in die konteks van die groter ysterbiogeochemiese siklus speel. Vir hierdie doel bou die studie voort op ’n oorsig van aangemelde grootte-gefraksioneerde yster- (Fe-)metings met behulp van chemiese en mineralogiese data wat uit sinchrotrongebaseerde x-straalmetings verkry is. Na aanleiding van ’n uitvoerige literatuurstudie, blyk die globale poel kolloïdale yster (cFe) op die see-oppervlak hoogs dinamies te wees en toon dit dikwels seisoenale tendense en voedingstofagtige gedrag. Ruimtelike veranderlikheid in die cFekonsentrasie op die see-oppervlak is hoofsaaklik ’n funksie van totale ystervoorsiening, hoewel die konsentrasie en sterkte van ysterbindende ligande sowel as anorganiese termodinamiese beperkings ook ’n invloed kan hê. Die grootte-gefraksioneerde studie van kolloïdale Fe het beduidende bewyse opgelewer wat op die direkte of indirekte biologiese benutting van hierdie cFe-poel dui. Tog verg ’n vollediger begrip van cFe- biologiese interaksie noodwendig meer kennis van die chemie en mineralogie van cFe. Om hierdie kwessies te ondersoek, dokumenteer hierdie tesis die ontwikkeling van ’n innoverende X-straalmikroskopie- en X-straalspektroskopietegniek om die Fe-soortvorming van individuele Feryke partikels in omgewingsomstandighede te bepaal. Variasies in die pieksplitsing van yster-L3 rand-XANES- (“X-ray absorption near-edge structure”-)spektra weerspieël veranderlikheid in die lokale koördinasie-omgewing rondom die metaalkern. In die besonder wissel die energiesplitsing ( eV) en intensiteitsverhouding van die splitsingspieke by die L3-rand na gelang van die Fevalensietoestand, die getal en chemie van koördinasie-ligande, en poliëdriese distorsie-effekte, en kombinasies van die twee parameters blyk kenmerkend van individuele Fe-minerale te wees. Om Fe-soortvorming te verstaan, is die stipping van Δ eV versus intensiteitsverhouding suksesvol toegepas op ’n verskeidenheid Fe-omgewingspartikels (groter as 20 nm in deursnee) wat uit twee oseaankomme – die Suidelike Yssee en die suidwestelike Stille Oseaan – bekom is. Soortverskille in Fe-partikels wat uit die Suidelike Yssee bekom is, toon kenmerkende samestellingspatrone tussen die kus van Suid-Afrika en Antarktika, en verskillende Fe-poele word met die verskillende oseanografiese frontsones verbind. Ondanks die suurstofhoudende aard van die seewatermonsters, is beduidende Fe(II) in partikels opgemerk in die Suidelike Yssee by hoë breedteliggings sowel as op studieterreine naby die Kermadec-rif in die Stille Oseaan. Ysterhoudende partikels van laasgenoemde studieterrein het ’n sterk verband met koolstof- funksionele groepe, veral alkohol en karboksamien, getoon. Hierdie bevindinge met betrekking tot die chemiese verskille tussen partikels en die verband met organiese materie het beduidende implikasies vir ons begrip van partikelgedrag, die oppervlak-interaksies van partikels, en die rol wat dit in primêre produktiwiteit en globale elementsiklusse speel.

No description available.

This thesis describes experimental studies of colloidal particles dispersed in solvents which themselves have phase transitions. One common definition of soft matter is: a material characterized by a mesoscopic length scale. This length scale is, for example, the colloid size or the ordered domain size. Here we combine a complex host with one characteristic length scale with dispersed particles that have a different size. It might be anticipated that new behaviour will occur. Two limits of particle characteristics are probed: the case of dilute sterically-stabilized particles and the case of a weak gel of attractive particles. The two systems are polymer particles dispersed in a phaseseparating microemulsion and silica nanoparticles dispersed in a low molecular weight liquid crystal. In each system a temperature driven phase transition plays a crucial role. In the microemulsion case we observe how transitional and pre-transitional phenomena create effective interactions between particles and how new behaviour emerges in the host solvent in the late stage of the phase separation. We show that the pre-transitional clustering of the PMMA particles is due to an adsorbed layer of dodecane. Subsequently heterogeneous nucleation of the gas phase is seen. After phase separation has occurred in off-critical samples the particles remain in either the continuous or dispersed phase depending on the original microemulsion composition. In the late stage of the phaseseparation the coalescence and coarsening behaviour changes significantly, after more material exchange between the phases has taken place. This behaviour is reminiscent of viscoelastic phase separation in polymer based samples. In the liquid crystal case we discover the anisotropy of the liquid crystal persists over large length scales and modifies the local dynamics of the gel. Using electron microscopy and scattering techniques we demonstrate that the silica embedded in the liquid crystal forms agglomerates with a fractal structure. Rheological characterization demonstrates that the resulting composite is a gel. Investigation of the composite’s local dynamics using x-ray photon correlation spectroscopy shows anisotropy and intermittency in the dynamics on significant length scales. In both systems we have studied new behaviour seen due to the influence of one component on the dynamic characteristics of the other The pre- and post- phase transition phenomena are only crucial in the microemulsion case where the particles have purely repulsive interactions. Our results illustrate the subtle balances that occur in soft composite systems.

The main goal of this thesis was to develop a multi-technique approach to characterise high molecular weight (colloidal) dissolved organic matter (DOM) occurring in estuarine and coastal marine environments and to investigate how these bulk properties may relate to their ability to influence the physicochemical speciation of metals such as iron and copper. This characterisation was undertaken systematically on the pre-filtered (0.4 μm) estuarine or marine sample, referred to here as the ‘bulk’, as well as on the ultrafiltered (< 5 kDa) and colloidal (> 5 kDa) fractions. Three successive studies were carried out. The first one took place in the estuarine mixing zone of a high pH, low turbidity black water river (River Thurso) and the other two in the receiving coastal waters (Thurso Bay). The optical and size distribution properties of these waters were examined in detail in the first study while their iron and copper-binding properties were examined in the second and third studies, respectively. Size fractionation results showed that the proportion of colloidal to soluble organic carbon (DOC) tended to decrease from the upper estuary (~ 60%) to coastal waters (~ 20%). With respect to trace metals, similar trends were observed as one progressed from the mouth of the river towards the open sea. In relation to their molecular absorption, fluorescence and size fractionation properties, both River Thurso DOM and Nordic Reservoir NOM Reference Material presented the same characteristics and mixing behaviour, indicating the dominance of humic and fulvic substances in the Thurso river-ocean system. The river-borne, humic colloids underwent two types of transformations upon mixing with the seawater end-member. The first one was the coiling or contraction of individual macromolecules ―monomers― with increasing salinity. The second one was the concurrent aggregation of these small monomer units (d = 2 – 4 nm) to form entities that were still colloidal, i.e. smaller than 0.4 μm. As a result of extensive association of iron and copper with the colloidal and soluble fractions respectively, not only organic carbon but also iron and copper behave conservatively in the River Thurso estuary. Throughout the coastal region of freshwater influence (S = 29 - 35) colloidal iron accounted for 30 - 80% of total dissolved iron and was present as iron-humic complexes supplied by the river and showing a uniform stability constant (log KFe’HS’ = 11.3  0.1, i.e. log KFe3+ HS’ = 21.3  0.1). Soluble iron was found to be largely complexed to ligands of marine origin with log K’Fe’HS’ = 11.9  0.1, thus revealing for the first time a difference between the iron-binding strengths of colloidal and soluble ligands. Terrestrial colloidal iron was found to be entirely, if slowly (~ 10 hours), accessible to the added competing ligand 2-(2-thiazolylazo)-pcresol (TAC) used for the determination of K’Fe’HS’. Furthermore, iron appeared to play a role in holding these terrestrial colloids together. Evidence for this came from variations in humic fluorescence intensity over time in response to dissociation of the ironhumic colloidal associations induced by a chelating resin. These results are consistent with the concept that iron derived from a peat-draining river is strongly but reversibly bound to humic substances and remains so under marine conditions. In the same coastal region, two types of high-affinity ligands binding over 99.99% of total copper were detected. The stronger ligand (L1, log KCu2+L1’ = 15.5 - 16.1), of riverine origin, was present in very low concentrations of 1 - 4 nM that correlated with ―but systematically fell short of― total copper concentrations. Its conditional binding constant tended to increase with salinity, with most of the increase taking place in the near-field portion of the river plume. The weaker, more abundant ligand (L2, log KCu2+L2’ = 11.8 - 12.8) was present in total concentrations of 60 – 170 nM and had a controlling influence on the value of the labile (i.e. inorganic) copper concentration which ranged from 0.001 to 0.0001 nM. Both organic ligands were fairly evenly partitioned between soluble and colloidal phases but their sources appeared to differ significantly. Ligand L1 appeared to be of riverine origin (although one cannot exclude active microbial production, as opposed to passive release from peat) while ligand L2 distributions suggested in situ production within Thurso Bay.