We theoretically study the adhesion and membrane-mediated interaction of cylindrical colloids to a flat fluid membrane. There are two ways to approach this problem. The first way, based on energy, requires finding the equilibrium shape of the membrane given the placement of the particle(s). In order to do so, we need to know how the energy of the surface depends on its shape (i.e. the surface Hamiltonian), as well as how the adhered colloid deforms the membrane. The second way to approach this class of problems is “geometrical”, where forces between the membrane-adhered particles are related directly to the geometry of the deformed membrane via the surface stress tensor. The surface Hamiltonian allows finding the stress at any point on the membrane in terms of local geometry. The force acting on the colloid can then be found by integrating this surface stress tensor along any contour enclosing the colloid. In this thesis, using the approach based on free energy calculations, we look into the problem of cylindrical colloids adhering to a membrane with fixed constant adhesion energy between the membrane and the colloids. Angle-arclength parameterization is used in order to treat the problem beyond small gradient approximation. We present three different cases here: single cylinder adhering on a membrane, two cylinders adhering on the same side of the membrane, and two cylinders adhering on different sides of the membrane. For the single cylinder case we present a structural phase diagram to separate no wrapping, partial wrapping and closure states and we compare it to the phase diagram obtained for a related system of spherical colloids. For two cylinders adhered on the same side of the membrane we obtain repulsive interaction and transition from shallow to deep wrapping as the cylinders move apart from each other. We also look into a phase where two cylinders are vertically stacked and discuss its energetics. For two cylinders adhering to the opposite sides of the membrane, attractive interaction is obtained in accordance with previous results and we further show that in that case two cylinders are generally in contact and a first-order transition from shallow to full wrapping is possible. In the last section, we put a framework for the class of problems where the particle is between the membrane and the supporting interface, where adhesion is assumed between the interface and the membrane.

No description available.

The extraction of geothermal heat can cause precipitation of the minerals dissolved in geothermal fluid. Their deposition on the walls of wells and above-ground plant and in pores near reinjection wells, also known as mineral scaling, is one of the main obstacles to increasing the effectiveness of utilization of the limited geothermal resources. If not controlled properly it can result in accumulation of a significant amount of scale which obstructs pipes and reinjection wells and reduces the efficacy of heat exchangers. The most abundant mineral in geothermal fluid is silica and thus its precipitation can cause the highest scaling rate. While this dissertation is devoted to the study of silica scaling the results obtained may be applicable to other minerals with similar deposition mechanism. Oversaturated silica is known to precipitate from aqueous solution either by the direct chemisorption of single silicic acid molecules (monomers) or by forming colloidal particles suspended in the solution. These particles can subsequently be transported to, and attach onto, a wall. This process of colloidal silica deposition was previously recognised to cause much faster scaling than the direct deposition of silica monomers under typical geothermal plant conditions. While the chemical kinetics of silica polymerization and colloid formation are relatively well understood, transport of these colloids and their stability, which control their aggregation and attachment rates, on the other hand are not. Previous studies of the silica scaling process have identified prominent effects of geothermal brine hydrodynamics on the scaling rate. It was found to increase with the flow rate and particle size, thus suggesting the dominance of the advective (inertial) deposition of colloidal silica. However, this conclusion contradicted the present theory of particle transport in turbulent flows which argues the dominance of the diffusive transport for the relevant range of particle sizes (<1 μm). The development and continuing improvement of the anti-scaling measures required deeper understanding of the complex combination of the phenomena involved in the process of silica scaling. This was pursued in the present study using theoretical and experimental methods. First, the rate of colloidal silica transport from a turbulent flow onto the internal surface of a circular pipe, a cylinder and a flat plate were calculated using available analytical and numerical methods. The obtained theoretical transport rate was found to be about four orders of magnitude higher than the corresponding experimental scaling rate. The latter was determined in the previous studies to be 4.2·10-8 kg/s/m2 for silica colloids of 125 nm in diameter which corresponded to the dimensionless deposition velocity (the dimensionless deposition velocity is the scaling rate normalised by the particle mass concentration and friction velocity) of 1.2·10-6 for the dimensionless particle relaxation time of 2·10-4. Next, based on the standard DLVO theory of particle interactions and in the framework of the Smoluchowski approach the probability of colloidal silica particle attachment to a wall was found to be 10-6. Therefore, the theoretical scaling rate, calculated as a product of this probability and the above-mentioned transport rate was two orders of magnitude lower than the experimental scaling rate. This suggested that the implemented theoretical approach either underestimated particle transport rate or overestimated particle stability. Both possibilities are explored in this dissertation. In addition, the silica scaling rate was measured for a range of conditions: particle size from 20 to 60 nm, particle concentration 1600-10000 ppm, friction velocity from 0.09 to 0.18 m/s (Re = 9-50·103) and ionic strength from 30 to 80 mM, pH 8.1-9.5 and temperature from 25 to 44 °C. For this, laboratory experiments were designed and progressively modified in order to improve the repeatability of the results and to study the scaling process. In these experiments colloidal silica deposition onto the walls of mild steel pipe sections was studied with a recirculating flow rig with variable (but controllable) particle size, concentration, flow rate, pH and ionic strength of the solution. In addition, a parallel plate flow test section was designed and built which will provide better capabilities for the control over the hydrodynamic and test surface conditions in future experiments. The control over the chemical conditions was achieved by the use of the synthetic colloidal solutions. Two methods of their production – hydrolysis of either sodium metasilicate or active silicic acid – were employed. The influence of the synthesis conditions, ion content and pH on the long term behaviour of these colloidal solutions was investigated. The particle size data, obtained using dynamic light scattering (DLS) and verified by electron microscopy, was analysed and compared against the predictions of the current models of nanoparticle growth and stability. The kinetic aggregation was identified to be the dominant particle growth mechanism. Experimental data collected during the long-term observations of the particle growth allowed relationships between the aggregative stability and such parameters as the particle size, ion concentration and pH of the solution to be elucidated. In particular, the aggregative stability of 10-20 nm particles was found to be 108-1010 which is 7-9 orders of magnitude higher than the corresponding DLVO stability. It was also found to decrease with the increase of the particle size. This agreed with the theory of the colloid stabilization by steric interactions. Moreover, the model of the “gel” layer was used to explain the observed “anomalies” of the colloidal silica behaviour. The deposition experiments conducted with these synthetic colloidal solutions showed that the scaling rate increased with the particle size, flow rate and ionic strength (IS) of the solution. Thus, it was measured to be 9.7·10-9 kg/s/m2 for the 45 nm particles in a solution with IS = 0.05 M, which corresponded to the dimensionless deposition velocity of 6.6·10-8 for a dimensionless particle relaxation time of 2.2·10-6. The scaling rate was calculated for these conditions by multiplying the corresponding transport rate and the actual attachment probability determined as an inverse of the experimental stability. It was found to agree with the experimental value within an order of magnitude. In addition, the observed increase of the scaling rate with the increase of particle size was explained by the compensation of the decreased rate of the particle transport by faster decrease of actual particle stability (increase in attachment probability). Therefore the contradiction between the theory and the experiment was resolved for the particles of 20 to 60 nm in diameter. Moreover, the observations of the dimensions and distribution of the scale elements formed in some of the present experiments strongly suggested the significance of the advective (inertial) mechanism of particle deposition. This and comparative analysis of other experimental and theoretical data suggested that the present theory may underestimate the convective transport of the particles onto a rough wall. Therefore, the hypothesis of the parallel-to-wall advective deposition of the nanoparticles onto the roughness/scale elements (not accounted in the current theory) was proposed. The corresponding mass transfer problem was solved analytically using experimentally found dimensions of the scale elements. The additional transport was found to decrease the above-stated discrepancy between the theoretical and experimental scaling rate for large (125 nm) particles by one order of magnitude. The remaining difference of one order of magnitude was speculated to be due to the underestimation of these particles attachment probability derived with the standard DLVO theory. The actual aggregative stability of the silica colloids larger than 60 nm in diameter and for a wider range of IS values is of interest for future experimental studies. An improved understanding of the interrelation between the chemical and hydrodynamic phenomena in the process of silica scaling and its dominant mechanisms was achieved in this dissertation. This allowed optimization of the present anti-scaling practices aimed to minimize the negative effects of mineral scaling on the operation of geothermal power stations. Besides the practical recommendations, which may ultimately help to increase the efficiency of geothermal power stations, the results of the present study may be of value in the fields of mass transfer and colloid science.

The work presented in this dissertation provides a novel technique of manipulation of colloidal entities during membrane filtration based on an AC electrokinetic phenomenon called dielectrophoresis. First, the influence of dielectrophoretic (DEP) forces created on a membrane surface to levitate colloidal particles is studied both theoretically and experimentally. A numerical model based on the convection-diffusion-migration equation is presented to calculate the concentration distribution of colloidal particles in shear flow under the influence of a repulsive DEP force field. The simulation results indicate that particle accumulation on the membrane (or membrane fouling) during filtration can be averted by creating a repulsive DEP force field on the membrane surface. Corresponding experimental study employs a microelectrode array on a glass surface in a tangential flow cell, to apply repulsive DEP forces on polystyrene particles suspended in an aqueous medium. Applying a non-uniform AC electric field on the microelectrodes generates the DEP force field that levitates the polystyrene particles above the surface. This study indicates that the repulsive dielectrophoretic forces imparted on the particles suspended in the feed can be employed to effectively mitigate membrane fouling in a crossflow membrane filtration process. The second phase of the study is aimed at controlling colloid transport through a microporous membrane using DEP forces acting across the pores. A theoretical analysis of colloid transport through straight cylindrical capillaries in the presence of a non-uniform AC electric field is developed. Numerical simulations demonstrate that the interaction of the particles with the electric field generates strong repulsive DEP forces, acting selectively on the colloidal particles to control particle transport through the pore. A combination of DEP forces and size exclusion in porous material is proposed to develop an energy efficient technique for colloid filtration. Experimental results on this steric-dielectrophoretic filtration are also obtained using novel ``sandwich membranes" and colloidal suspensions in a dead-end filtration system. The primary advantage of this steric-dielectrophoretic mechanism is that the filtration can be achieved by filter media (such as membranes) that have considerably larger pore sizes than the retained colloids. The technique can also result in tunable filtration mechanisms, where particles with same size but different electrical properties can be separated using suitably designed membranes. / Fluid mechanics, Electrokinetic filtration

The focus of this thesis is fundamental surface force and friction studies of silica and cellulose surfaces, performed mainly with the atomic force microscope (AFM). The normal interactions between model cellulose surfaces have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion. Studies of friction on the same surfaces reveal that surface roughness is the determining factor for the friction coefficient, with which it increases monotonically. The absolute value, however, is determined by the surface chemistry. The above is illustrated by studies of the effect of adsorbed xyloglucan, a prospective paper additive, which is found in the cell wall of all plants. Xyloglucan is like cellulose a poly- saccharide but the effect of its adsorption was to reduce the friction significantly, while following the identical trend with surface roughness. Xyloglucan also increases the adhesion between cellulose surfaces in a time dependent manner, interpreted in terms of a diffusive bridging interaction. These facts combined provide a mechanistic explanation to contemporaneous findings about xyloglucans benefit in paper strength and formation. In air, the adhesion between e.g. particles or fibres, must be at least partially determined by the formation of capillary condensates. The dependence of capillary condensation on relative humidity is however not yet fully understood so studies have been performed to cast light on this phenomenon. Above about 60 % relative humidity the adhesion and friction increase dramatically due to the formation of large capillary condensates. The extent of the condensates depends both on the time the surfaces equilibrate, but also on the surface roughness. Harvesting of the condensate during shearing is also observed through hysteresis of the friction-load relationship. Measurements of surface forces and friction in surfactant systems show a clear relation between the adsorbed surfactant layer and the barrier force and adhesion, which in turn determine the friction. All of these interactions are critically dependent on the composition of the surfactant solution. A mixed surfactant system has been studied consisting of a trimethylammonium cationic surfactant and a polyoxyethylene nonionic surfactant. The results are interpreted in terms of current theories of adsorption and synergistic interactions. Finally, a novel technique for the in situ calibration and measurement of friction with the AFM is proposed. Comparison with lateral measurements show that the approach is successful. / QC 20100920

- / -

Application of conventional theory of transport and deposition to small particles or large colloids, on the order of 1 micron in diameter, has received surprisingly little attention in colloid science. While the favorable deposition of colloidal particles ( < 0.5 micron diameter) has repeatedly been shown to agree with the Smoluchowski-Levich approximation for a convective-diffusion process, larger particles are known to deviate from this solute-like mass transfer behavior. The rotating disk, used in the experiments performed in this work, is a model experimental system that has been employed in the past to de-convolute and quantify the mechanisms of particle transport. Experimental evidence shows that particle transport to the rotating disk deviates from the predictions of the complete three-dimensional convective-diffusion equation, including hydrodynamic and surface-surface interaction forces, in that non-uniform deposition is observed over the surface of the disk. Fluid inertial effects, observed to be significant in capillary flow, have been suggested in the literature as an explanation of non-uniform deposition on the rotating disk. Calculations performed in this work show that while inertial lift forces are significant, they are not the dominant cause of non-uniform deposition. Instead, hydrodynamic blocking of available deposition surface area is shown to accurately describe experimental deposition profiles. The effect of particle size on surface area exclusion and hydrodynamic scattering are separately assessed to demonstrate that the blocking model is not only phenomenologically accurate, but also an important part of the mechanistic description of transport in the rotating disk system.

No description available.

The aim of this work is to look into the applicability of Discrete Element Modelling (DEM) coupled to Computational Fluid Dynamics (CFD) to simulate micro-scale colloidal particles immersed in fluid. Numerical methods were implemented through the commercial framework of EDEM2.3. As opposed to dissolved matter, which behaves as a continuum within the fluid medium, particulate matter is made of discrete entities that interact amongst themselves, and with the fluid and any physical boundaries. Particulate matter is ubiquitous in many purification processes that would beneficiate from having an easy way to model particle dynamics immersed in water. In an effort to understand better the dynamics of particle deposition under surface forces and hydraulic forces, a micro-scale numerical model was built adopting both a mechanistic and a statistical approach to represent the forces involved in colloidal suspension. The primary aim of the model was to simulate particle aggregation, deposition and cluster re-suspension in real world micro-systems. Case studies include colloidal flocculation in a constricted tube, and colloidal fouling around membrane filtration feed spacers. This work used a DEM-CFD coupling method that combined the DEM particle flow simulation with hydrodynamics forces from a velocity field computed through CFD. It also implemented boundary-particle and particle-particle interactions by enabling the modelling of surface and interfacial forces. Two kinds of coupling method were considered: two-way and one-way coupling. Two-way coupling is suitable for high particle concentration flow where particle loading affects the hydrodynamics. One-way coupling is suitable for dispersed particle configuration where the flow field is assumed to be undisturbed by the particles. The advantages and drawbacks of both techniques for micron-size particles were investigated. EDEM 2.3 was customised with plug-ins to implement Van der Waals forces and Brownian forces and its post-processing features offered the ability to investigate easily the microparticles behaviour under the influence of fluid forces. In this context, DEM-CFD modelling using EDEM 2.3 represents an improvement on previously published works as it enables higher visibility and reproducibility along with increasing the number of potential users of such modelling. Emphasis was given in presenting original findings and validation results that illustrate DEMCFD applicability, with respect to modelling of hydraulically mediated colloidal surface interaction; while highlighting factors that limit the ability of the technique. For instance, the effect of particle disturbance on the surrounding medium currently proves difficult to model.

Brillouin spectroscopy probes the thermally generated pressure fluctuations (sound waves) which propagate in a material. The resulting information on sound velocity and absorption provides a fast and efficient method of monitoring high frequency (GHz) dynamics in the system being studied. In certain cases, structural information may also be inferred from changes in the Brillouin spectrum as a function of temperature, pressure or composition (in the case of multi-component systems). The aim of the work presented in this thesis was to integrate Brillouin spectroscopy into current soft condensed matter research projects at Edinburgh, namely (i) hydration in methanol-water mixtures and (ii) the behaviour of hard-sphere colloidal dispersions. A Brillouin spectrometer based on a Fabry-Perot interferometer was developed and tested, resulting in a high-resolution instrument operating at variable scattering vector (exchanged momentum), temperature and pressure. The technical aspects of this work were carried out in collaboration with a colleague. Data analysis routines were designed and implemented, enabling calibrated Brillouin spectra to be produced automatically from raw experimental data. Excellent agreement with results on several materials studied in the literature confirmed the accuracy and sensitivity of the spectrometer. The molecular details of hydration in methanol-water mixtures are of great interest due to the prototypical amphiphilic nature of the methanol molecule. The effect of deep cooling on the Brillouin spectrum across a wide range of methanol concentrations was studied in detail, resulting in the first observation of an anomalous increase in sound velocity and maximum in sound absorption at intermediate compositions. A similar effect was then found at higher temperature in aqueous tertiary butanol, and was identified in a brief survey of several other aqueous solutions. High pressure Brillouin spectra indicate that this anomalous behaviour may also be present in pure water. It is suggested that these novel effects may be due to the presence of a relatively unperturbed water structure in the aqueous solutions studied, even at quite high solute concentration. Preliminary results from a neutron diffraction experiment performed on a 40% by mass methanol-water mixture were consistent with this hypothesis. Brillouin spectroscopy was also used to study the propagation of high frequency sound in monodisperse colloidal suspensions of sub-micron hard spheres. A second longitudinal sound mode was observed for scattering vectors of magnitude greater than pi/d where d is the diameter of the spheres. These results are the first reproduction and extension of the pioneering work in the field, which identified the additional mode with a surface acoustic excitation, propagating between adjacent spheres via an evanescent wave in the solvent. The new results show that the second mode is extinguished at a particular scattering vector - an effect not reported previously. It is suggested that this extinction is due to the minimum in the form factor for elastic scattering from a single sphere.