Biblioteca Digital

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Processo FAPESP: 02/06838-1; Processo FAPESP: 03/05802-6; Processo FAPESP: 04/12620-4; Suspensions formed by saturated sucrose solutions were analyzed at different temperatures (0 <= T <= 65C) and with different added fractions (0.005 <= phi <= 0.40) of known size sucrose crystals (0.163 <= d(p) <= 1.26 mm). Saturated sucrose solutions, taken as the suspension dispersant mean, were rheologically characterized using a concentric-cylinder rheometer and showed Newtonian behavior. The crystal suspensions were rheologically evaluated by using a mixer-type rheometer constituting an anchor impeller coupled to a rotational rheometer, also known as a mixer-type rheometer. The studied crystal suspensions also showed Newtonian behavior. Theoretical and semiempirical models were used to describe the correlation between suspension relative viscosity and crystal volumetric fraction. The best results were obtained using the Krieger and Dougherty, and the Mooney model, resulting in values of intrinsic viscosity, (eta), and maximum packing fraction, phi(m), compatible with data reported for coarse concentrated suspensions of nonspherical particles.

O principal objectivo da presente dissertação é desenvolver microagulhas cerâmicas para administração transdérmica de fármacos à base de alumina-zircónia consolidadas por uma nova técnica denominada epoxy gel casting a partir de suspensões aquosas concentradas. Para o efeito estudaram-se as várias fases do processo, incluindo (i) preparação e caracterização de suspensões aquosas concentradas na presença de agentes de gelificação (resina e endurecedor), (ii) consolidação directa de suspensões por epoxy gel casting; (iii) caracterização das amostras em verde e sinterizadas; (iv) produção de micromoldes à base de silicone a partir de uma madre polimérica de microagulhas e, finalmente, (v) enchimento dos micromoldes e caracterização microestrutural das microagulhas cerâmicas em verde e sinterizadas. Os métodos de consolidação directa, como o epoxy gel casting, requerem o uso de suspensões concentradas de forma a maximizar a densidade dos corpos em verde e minimizar a retracção durante os processos de secagem e sinterização. Assim, numa primeira fase do trabalho prepararam-se suspensões concentradas em meio aquoso à base de alumina, zircónia e mistura dos dois materiais. Foram estudadas diferentes quantidades de resina (10...

We discuss in this study the advantages and limitations of the osmotic stress method that enables to set the osmotic pressure to a given system. By investigating aqueous suspension of monodisperse silica nanoparticles of radius 78 at an ionic strength of 10-2 mol/L, we show that the method is very accurate to probe the phase behavior of colloidal suspensions because it allows to prepare samples all along the equation of state of the system at constant ionic strength without any aggregation and with a well defined structure, as shown by Small-Angle Neutron Scattering (SANS) experiments. However the method fails to yield crystalline structures, since solid samples obtained are always glassy, even when the fluid-solid transition is crossed with small successive jumps of 1000 Pa. This phenomenon comes from the kinetics of the process which exhibits in our experimental conditions an exponential decay time with a characteristic time of ≈ 3 hours that induces a very strong change of the volume fraction of the suspension in the early stages of the stress. When the jump of pressure is very important, the system is frozen in the vicinity of the dialysis bag and forms a dense shell that eventually prevents some spatial regions of the sample to reach equilibrium. In this case...

Produtos cer?micos provenientes de suspens?es concentradas de Al2O3necessitam que as mesmas sejam estabilizadas para que possam atender ?s crescentes exig?ncias de performance. As suspens?es estudadas neste trabalho s?o, usualmente, empregadas na obten??o de folhas cer?micas pelo processo de colagem de folhas cer?micas (Tape Casting). Part?culas, quando em suspens?o, desenvolvem cargas superficiais. Quando estas part?culas entram em choque umas com as outras pode acontecer o fen?meno da agrega??o, gerando estruturas floculadas que aprisionam ?gua em seu interior, o que resultaria num produto final poroso e conseq?entemente de baixa resist?ncia mec?nica e at? mesmo qu?mica. Estabilizar uma suspens?o concentrada ? fazer com que a resultante entre as for?as atrativas de van der Waals e as for?as repulsivas, entre duplas camadas el?tricas igualmente carregadas ou decorrentes da adi??ode material polim?rico na superf?cie das part?culas, seja repulsiva. Os par?metros de avalia??o da estabilidade das suspens?es concentradas de Al2O3, aqui utilizados, foram o potencial zeta e a viscosidade. Foram, tamb?m, propostos novos par?metros, a saber: 1) estruturas milim?tricas, 2) microestruturas e 3) histerese nas curvas de viscosidades. Neste trabalho foram feitas suspens?es concentradas de Al2O3que variavam de 40% a 70%...

Electroacoustic techniques are promising tools for the size determination and electrokinetic characterization of concentrated colloidal suspensions. When particles are not homogeneous in size and/or density, the dynamic mobility obtained is a kind of average of the mobilities of every particle. In this paper, we try to discern which averaging procedure provides a better description of the dynamic mobility of bidisperse suspensions consisting of a mixture of two very different types of particles. The results show that the amplitude of the sound wave induced by an applied ac field (electrokinetic sonic amplitude) is not just the sum of the amplitudes of the waves generated by every particle but has a larger contribution from the larger particles, although the small size entities considerably influence the behaviour of the latter because of their interference in the fluxes of the fluid and ions around them.

The aggregation of interacting Brownian particles in sheared concentrated suspensions is an important issue in colloid and soft matter science per se. Also, it serves as a model to understand biochemical reactions occurring in vivo where both crowding and shear play an important role. We present an effective medium approach within the Smoluchowski equation with shear which allows one to calculate the encounter kinetics through a potential barrier under shear at arbitrary colloid concentrations. Experiments on a model colloidal system in simple shear flow support the validity of the model in the concentration range considered. By generalizing Kramers' rate theory to the presence of shear and collective hydrodynamics, our model explains the significant increase in the shear-induced reaction-limited aggregation kinetics upon increasing the colloid concentration.; CNPq; INCT-FCx

The general problem of microrheology is to predict the macroscopic flow properties of a material from a detailed description of the behavior of its constituent elements. This approach has been used to study suspensions of human red cells in plasma or Ringer's solution flowing steadily in rigid tubes 8–25 times the red cell diameter by observing individual cell motions under the microscope. The results have been compared with those previously obtained with model particles under similar conditions. In very dilute suspensions single red cells rotated in orbits similar to those of rigid discs at low flow rates, but, in common with model deformable particles, were observed to migrate away from the tube wall. Linear rouleaux of red cells rotated as rodlike particles and were flexible, bending during their rotational orbits in a manner similar to that of filaments of nylon or Dacron. Transparent concentrated suspensions were produced by preparing ghost cells reconstituted in biconcave form in plasma. In these, the motions of some unhemolyzed red cells were followed. The erythrocyte velocity profiles were blunted at concentrations above 20%; the cell paths were erratic because of frequent radial displacements, especially at the tube periphery...

Highly concentrated suspensions of graphene stabilized with surfactant were prepared using ultrasonic exfoliation. Concentrations of up to 1.5% w/w (15 mg/mL) were achieved through the continuous addition of the surfactant during the exfoliation process.

Concentrated suspensions may shear-thin when the suspended particles form planar sheets that slide over one another with less friction than if the particles are randomly distributed. In a na\"ive model the suspension is described by a mean effective viscosity, and particles that collide with each other redistribute the mean density in the shearing direction. This leads to a diffusion equation for the particle density. If the viscosity in the unthinned state is a steeply increasing function of particle density the effective diffusion coefficient is negative and the diffusion equation, meaningful only on scales larger than the particle separation, is ill-posed. This singularity corresponds to the formation of planar sheets of particles and defines a critical particle density for the onset of shear thinning.; Comment: 7 pp, 1 fig As published; revised in response to referees' comments

We present a comprehensive rheological study of a suspension of thermosensitive particles dispersed in water. The volume fraction of these particles can be adjusted by the temperature of the system in a continuous fashion. Due to the finite polydispersity of the particles (standard deviation: 17%), crystallization is suppressed and no fluid-crystal transition intervenes. Hence, the moduli $G'$ and $G"$ in the linear viscoelastic regime as well as the flow curves (shear stress $\sigma(\dot{\gamma})$ as the function of the shear rate $\dot{\gamma}$) could be measured in the fluid region up to the vicinity of the glass transition. Moreover, flow curves could be obtained over a range of shear rates of 8 orders of magnitude while $G'$ and $G"$ could be measured spanning over 9 orders of magnitude. Special emphasis has been laid on precise measurements down to the smallest shear rates/frequencies. It is demonstrated that mode-coupling theory generalized in the integration through transients framework provides a full description of the flow curves as well as the viscoelastic behavior of concentrated suspensions with a single set of well-defined parameters.

We analyze the influence of finite ion size effects in the response of a salt-free concentrated suspension of spherical particles to an oscillating electric field. Salt-free suspensions are just composed of charged colloidal particles and the added counterions released by the particles to the solution, that counterbalance their surface charge. In the frequency domain, we study the dynamic electrophoretic mobility of the particles and the dielectric response of the suspension. We find that the Maxwell-Wagner-O'Konski process associated with the counterions condensation layer, is enhanced for moderate to high particle charges, yielding an increment of the mobility for such frequencies. We also find that the increment of the mobility grows with ion size and particle charge. All these facts show the importance of including ion size effects in any extension attempting to improve standard electrokinetic models.; Comment: J. Colloid Interface Sci., in press, 13 pages, 9 figures

We propose an improved effective-medium theory to obtain the concentration dependence of the viscosity of particle suspensions at arbitrary volume fractions. Our methodology can be applied, in principle, to any particle shape as long as the intrinsic viscosity is known in the dilute limit and the particles are not too elongated. The procedure allows to construct a continuum-medium model in which correlations between the particles are introduced through an effective volume fraction. We have tested the procedure using spheres, ellipsoids, cylinders, dumbells, and other complex shapes. In the case of hard spherical particles, our expression improves considerably previous models like the widely used Krieger-Dougherty relation. The final expressions obtained for the viscosity scale with the effective volume fraction and show remarkable agreement with experiments and numerical simulations at a large variety of situations.; Comment: 28 pages, 7 figures

We consider numerical algorithms for the simulation of the rheology of two-dimensional vesicles suspended in a viscous Stokesian fluid. The vesicle evolution dynamics is governed by hydrodynamic and elastic forces. The elastic forces are due to local inextensibility of the vesicle membrane and resistance to bending. Numerically resolving vesicle flows poses several challenges. For example, we need to resolve moving interfaces, address stiffness due to bending, enforce the inextensibility constraint, and efficiently compute the (non-negligible) long-range hydrodynamic interactions. Our method is based on the work of {\em Rahimian, Veerapaneni, and Biros, "Dynamic simulation of locally inextensible vesicles suspended in an arbitrary two-dimensional domain, a boundary integral method", Journal of Computational Physics, 229 (18), 2010}. It is a boundary integral formulation of the Stokes equations coupled to the interface mass continuity and force balance. We extend the algorithms presented in that paper to increase the robustness of the method and enable simulations with concentrated suspensions. In particular, we propose a scheme in which both intra-vesicle and inter-vesicle interactions are treated semi-implicitly. In addition we use special integration for near-singular integrals and we introduce a spectrally accurate collision detection scheme. We test the proposed methodologies on both unconfined and confined flows for vesicles whose internal fluid may have a viscosity contrast with the bulk medium. Our experiments demonstrate the importance of treating both intra-vesicle and inter-vesicle interactions accurately.

We investigate the stability of the pressure-driven, low-Reynolds flow of Brownian suspensions with spherical particles in microchannels. We find two general families of stable/unstable modes: (i) degenerate modes with symmetric and anti-symmetric patterns; (ii) single modes that are either symmetric or anti-symmetric. The concentration profiles of degenerate modes have strong peaks near the channel walls, while single modes diminish there. Once excited, both families would be detectable through high-speed imaging. We find that unstable modes occur in concentrated suspensions whose velocity profiles are sufficiently flattened near the channel centreline. The patterns of growing unstable modes suggest that they are triggered due to Brownian migration of particles between the central bulk that moves with an almost constant velocity, and highly-sheared low-velocity region near the wall. Modes are amplified because shear-induced diffusion cannot efficiently disperse particles from the cavities of the perturbed velocity field.; Comment: 11 pages, accepted for publication in Journal of Fluid Mechanics

We present an easy-to-use analytic toolbox for the calculation of short-time transport properties of concentrated suspensions of spherical colloidal particles with internal hydrodynamic structure, and direct interactions described by a hard-core or soft Hertz pair potential. The considered dynamic properties include self-diffusion and sedimentation coefficients, the wavenumber-dependent diffusion function determined in dynamic scattering experiments, and the high-frequency shear viscosity. The toolbox is based on the hydrodynamic radius model (HRM) wherein the internal particle structure is mapped on a hydrodynamic radius parameter for unchanged direct interactions, and on an existing simulation data base for solvent-permeable and spherical annulus particles. Useful scaling relations for the diffusion function and self-diffusion coefficient, known to be valid for hard-core interaction, are shown to apply also for soft pair potentials. We further discuss extensions of the toolbox to long-time transport properties including the low-shear zero-frequency viscosity and the long-time self-diffusion coefficient. The versatility of the toolbox is demonstrated by the analysis of a previous light scattering study of suspensions of non-ionic PNiPAM microgels [Eckert et al....

We consider the shear rheology of concentrated suspensions of non-Brownian frictional particles. The key result of our study is the emergence of a pronounced shear-thickening regime, where frictionless particles would normally undergo shear-thinning. We clarify that shear thickening in our simulations is due to enhanced energy dissipation via frictional inter-particle forces. Moreover, we evidence the formation of dynamically correlated particle-clusters of size $\xi$, which contribute to shear thickening via an increase in \emph{viscous} dissipation. A scaling argument gives $\eta\sim \xi^2$, which is in very good agreement with the data.

We analyse structure and dynamics in simulated high-concentration hard sphere colloidal suspensions by means of calculations based on the void space. We show that remoteness, a quantity measuring the scale of spaces, is useful in studying crystallization, since ordering of the particles involves a change in the way empty space is distributed. Calculation of remoteness also allows breakdown of the system into mesoscopic neighbor sets: statistics of mean remoteness and local volume fraction in these neighbor sets reveal that nuclei are formed at locally higher concentration, i.e. nucleation involves increased heterogeneity of the system. Full crystallization results in the transformation of the neighbor set mean remoteness distribution to an exponential form. The temporal fluctuation of local volume fractions in neighbor sets reveals significant details of dynamics, including abrupt dilations and compressions of local regions: leading to a clearer picture of the physical components of 'cage' dynamics in the colloidal glass.; Comment: 16 pages, 8 figs

We present a two-dimensional (2D) mathematical model of a highly concentrated suspension or a thin film of the rigid inclusions in an incompressible Newtonian fluid. Our objectives are two-fold: (i) to obtain all singular terms in the asymptotics of the overall viscous dissipation rate as the interparticle distance parameter $\delta$ tends to zero, (ii) to obtain a qualitative description of a microflow between neighboring inclusions in the suspension.; Comment: 76 pages, submitted

The non-Newtonian rheology is calculated numerically to second order in the volume fraction in steady simple shear flows for Brownian hard spheres in the presence of hydrodynamic and excluded volume interactions. Previous analytical and numerical results for the low-shear structure and rheology are confirmed, demonstrating that the viscosity shear thins proportional to Pe2, where Pe is the dimensionless shear rate or Péclet number, owing to the decreasing contribution of Brownian forces to the viscosity. In the large Pe limit, remnants of Brownian diffusion balance convection in a boundary-layer in the compressive region of the flow. In consequence, the viscosity shear thickens when this boundary-layer coincides with the near-contact lubrication regime of the hydrodynamic interaction. Wakes are formed at large Pe in the extensional zone downstream from the reference particle, leading to broken symmetry in the pair correlation function. As a result of this asymmetry and that in the boundary-layer, finite normal stress differences are obtained as well as positive departures in the generalized osmotic pressure from its equilibrium value. The first normal stress difference changes from positive to negative values as Pe is increased when the hard-sphere limit is approached. This unusual effect is caused by the hydrodynamic lubrication forces that maintain particles in close proximity well into the extensional quadrant of the flow. The study demonstrates that many of the non-Newtonian effects observed in concentrated suspensions by experiments and by Stokesian dynamics simulations are present also in dilute suspensions.

Evanescent wave dynamic light scattering and Stokesian dynamics simulations were employed to study the dynamics of hard-sphere colloidal particles near a hard wall in concentrated suspensions. The evanescent wave averaged short-time diffusion coefficients were determined from experimental correlation functions over a range of scattering wave vectors and penetration depths. Stokesian dynamics simulations performed for similar conditions allow a direct comparison of both the short-time self- and collective diffusivity. As seen earlier [V. N. Michailidou, G. Petekidis, J. W. Swan, and J. F. Brady, Phys. Rev. Lett.102, 068302 (2009)] while the near wall dynamics in the dilute regime slow down compared to the free bulk diffusion, the reduction is negligible at higher volume fractions due to an interplay between the particle-wall and particle-particle hydrodynamic interactions. Here, we provide a comprehensive comparison between experiments and simulations and discuss the interplay of particle-wall and particle-particle hydrodynamics in the self- and cooperative dynamics determined at different scattering wave vectors and penetration depths.