Rheology of polymer suspensions: I. Local flow effects

A systematic study is made of the average local velocity field acting at a selected particle in a fluid suspension. The flow disturbance due to a single particle is analyzed in terms of force multipoles. The theory is developed in close analogy to that for the corresponding problem of the local electric field at a molecule in a polarizable medium. Closed expressions are derived in continuum approximation for the average local velocity, vorticity and strain in terms of the macroscopic average velocity field and force multipole densities. The effect of correlations is discussed briefly.     

Rheology of active-particle suspensions

We study the interplay of activity, order, and flow through a set of coarse-grained equations governing the hydrodynamic velocity, concentration, and stress fields in a suspension of active, energy-dissipating particles. We make several predictions for the rheology of such systems, which can be tested on bacterial suspensions, cell extracts with motors and filaments, or artificial machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis, and active mechanosensing find natural explanations within our model.     

Rheology of nanosized hairy grain suspensions

Suspensions of nanosized hairy grains have been prepared by grafting long polydimethylsiloxane chains (molecular weight ) onto silica particles (radius ), dispersed into a good solvent of PDMS. Depending on the particle volume fraction, different rheological behaviors are observed. In the very dilute regime, the suspensions are perfectly stable and the particles behave almost as hard spheres: flow penetration inside the corona is then very weak. When the particle volume fraction goes to the close packing volume fraction, the suspension viscosity does not diverge as for hard spheres due to the increase of flow penetration inside the corona and to corona entanglements. The particles have then the same behavior as polymer stars having an intermediate number of arms (). Finally, in the concentrated regime (), the suspensions form irreversible gels. We shown that this unexpected gelation phenomenon is related to the presence of the silica cores: grafted PDMS chains can adsorb onto different particles and form irreversible bonds between the cores. The viscosity and elastic modulus evolutions during gelation are well described by the scalar percolation model of sol-gel transition. Received 23 March 1998     

Rheology of suspensions of rodlike particles

Suspensions of rigid rodlike particles in Newtonian suspending fluids are considered. We discuss the dependence of the relative viscosity _r upon the volume fraction of particles, their aspect ratioa _r, and the particle orientation distribution when the particles are sufficiently large that hydrodynamic forces are dominant. Theoretical results are reviewed for a variety of long slender particles. Experimental results obtained using classical rheometrical techniques are discussed. It is shown that whena _r25, data from several laboratories agree and they indicate that _r depends more strongly upon thana _r. Previous experimental results using falling ball rheometry are discussed as well as some more recent findings. These are shown to provide insights heretofore unavailable into the macroscopic rheology of suspensions of randomly oriented and oriented rods.     

Rheology of sheared monodisperse granular suspensions

Sheared granular suspensions can either flow or be jammed. They show as well a ‘melting’ transition: partially ordered flowing states are found which can be melted into fully disordered arrangements of grains by sufficient shear. While these are well documented phenomena, the underlying mechanisms and their control parameters are still far from clear. Via Molecular Dynamics simulations, we study the rheology of a model system of sheared frictional monodisperse granular materials [7, 8]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram. We outline as well connections and differences with thermal glass formers and colloidal suspensions.     

Modeling effective viscosity reduction behaviour of solid suspensions

Under a simple shearing flow, the effective viscosity of solid suspensions can be reduced by controlling the inclusion particle size or the number of inclusion particles in a unit volume. Based on the Stokes equation, the transformation field method is used to model the reduction behaviour of effective viscosity of solid suspensions theoretically by enlarging the particle size at a given high concentration of particles. With a lot of samples of random cubic particles in a unit cell, our statistical results show that at the same higher concentration, the effective viscosity of solid suspensions can be reduced by increasing the particle size or reducing the number of inclusion particles in a unit volume. This work discloses the viscosity reduction mechanism of increasing particle size, which is observed experimentally.     

Shear viscosity of polymer and surfactant monolayers

The surface shear viscosity of monolayers formed at the surface of water by adsorbed polyethyl- eneoxyde and by stearic acid is measured as a function of the surface pressure of the monolayer using a new surface viscometer. The principle of the viscometer is the measurement of the drag force on a circular disk undergoing a uniform translation at the water surface: a hydrodynamic model based on the lubrication approximation allows a calculation of the surface viscosities from the absolute measurement of the drag forces. Received: 26 August 1999     

The Newtonian viscosity of polymer solutions: Scaling relationships

Scaling in terms of temperature, composition, and molecular weight variables has practical and fundamental significance. The viscosity of polymer solutions deviates from that predicted by the Huggins equation when the concentration is higher than a characteristic concentration c_ch. The value of c_ch depends on the molecular weight of the polymer and the thermodynamic conditions of the system. It is also a known fact that the deviations are due to the entanglements and interactions of polymer molecules. Therefore, we believe cch can be used as a concentration-reducing parameter to get the superposition curves. It can be shown that the concentration corresponding to a minimum value of η_sp/ch² (in the case of η_sp/c² vs concentration curves) is the value of c_ch of that system. Moreover, this c_ch is related to the intrinsic viscosity and molecular weight through the Huggins and Mark-Hauwink-Sakurada equations (c_ch = k′M^?a′). Using c_ch values for different systems and plotting log η_r versus C/C_ch, the superposition curves are obtained. In each case these curves are found to be linear, at least when concentrations approach zero. Master curves may be plotted by making use of the initial slopes of the curve (log η_r vs Bc/c_ch) and it is found that the data obtained at different thermodynamic conditions fit these (log η_r vs Bc/c_ch, B being the initial slope of log η vs c/c_ch) curves very well. The slopes are also compared to k′, a′, and the expansion coefficient of the system and the relationships are found to be linear. It is concluded that c_ch is a better parameter for the superposition of viscosity data, as well as being easy to obtain experimentally.     

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.     

Modeling the viscosity and aggregation of suspensions of highly anisotropic nanoparticles

The rheology of nanofiber suspensions is studied solving numerically the Population Balance Equations (PBE). To account for the anisotropic nature of nanofibers, a relation is proposed for their hydrodynamic volume. The suspension viscosity is calculated using the computed aggregate size distributions together with the Krieger-Dougherty constitutive equation. The model is fitted to experimental flow curves for Carbon NanoFibers (CNF) and for NanoFibrillated Cellulose (NFC), giving a first estimation of the microscopic anisotropy parameter, and yielding information on the structural properties and rheology of each system.     

Frictional properties of dilute polymer solutions IV. Intrinsic viscosity

In the framework of our generalization of the Debye-Bueche theory for the frictional properties of dilute polymer solutions we study the intrinsic viscosity. A careful definition of the intrinsic viscosity for polymers with spherically symmetric density distribution is given. The equations are solved explicity for uniform spheres and spherical shells. For the general case a perturbation method is applied and a variational principle of minimum energy dissipation is formulated which is suitable for numerical work.     

Rheology of PVC compounds. II. Effects of lubricants on fusion

The effect of calcium stearate and paraffinic wax on the progression of fusion of PVC compounds was investigated by a rheological method based on the capillary entrance pressure loss. It was found that calcium stearate can increase or delay fusion. The way in which fusion is affected changes with temperature and the presence or absence of paraffinic wax. Increased calcium stearate levels in compounds with wax enhance the fusion process. Increased calcium stearate levels in compounds without wax decrease fusion at relatively low temperatures but accelerate fusion at higher temperatures. Paraffinic wax delays the resin particle breakdown and the fusion process. At the same level of calcium stearate, simple compounds without wax fuse faster than compounds with wax.     

Rheology of ring polymer melts: from linear contaminants to ring-linear blends

Ring polymers remain a challenge to our understanding of polymer dynamics. Experiments are difficult to interpret because of the uncertainty in the purity and dispersity of the sample. Using both equilibrium and nonequilibrium molecular dynamics simulations we have investigated the structure, dynamics, and rheology of perfectly controlled ring-linear polymer blends of chains of up to about 14 entanglements per chain, comparable to experimental systems. Linear contaminants increase the zero-shear viscosity of a ring polymer melt by about 10% around one-fifth of their overlap concentration. For equal concentrations of linear and ring polymers, the blend viscosity is about twice that of the pure linear melt. The diffusion coefficient of the rings decreases dramatically, while the linear polymers are mostly unaffected. Our results are supported by a primitive path analysis.     

Fibrillar crystals of isotactic polystyrene. II. Aggregation in stirred suspensions

Macroscopic particles of various morphological forms are created by the crystallization of isotactic polystyrene from rapidly stirred solutions. Crystallization from trimethylbenzene over a 150° C temperature range produced spherical particles, globules, fibers, and rings depending upon the growth temperature. Crystallization from cyclohexanol (θ = 83.5°C) produced fibers in the temperature range of 85° to 140° C. Globular structures, similar to those formed by stirrer crystallization in trimethylbenzene developed during the Ziegler-Natta polymerization of styrene in trimethylbenzene between 25° and 75°C. Highly birefringent conical appendages appeared on many of the globules and fibers. The fibers often exhibited a skin-core effect with an average longitudinal and transverse orientation of the chain molecules in the skin and core, respectively. The skin frequently displayed periodic banded extinctions. All of these macrostructures consisted of a highly porous assembly of constituent microfibrillar units. It is believed that the morphogenesis of these structures involves the gradual aggregation of microfibrils under the influence of the flow patterns of the system.     

Linear response theory of the viscosity of suspensions of spherical brownian particles

We study the frequency-dependent viscosity of a suspension of spherical particles on the basis of linear response theory applied to the generalized Smoluchowski equation. Hydrodynamic interactions are fully taken into account. We derive expressions for the frequency-dependent viscosity based on a cluster expansion of the linear response.     

Translation, rotation, and scale invariant image registration technique using angular and radial difference functions

An algorithm is proposed for registering images related by translation, rotation, and scale based on angular and radial difference functions. In frequency domain, the spatial translation parameters are computed via phase correlation method. The magnitudes of images are represented in log-polar grid, and the angular and radial difference functions are given and applied to measure shifts in both angular and radial dimensions for rotation and scale estimation. Experimental results show that this method achieves the same accurate level as classic fast Fourier transform （FFT） based method with invariance to illumination change and lower computation costs.     

Rheology of dense suspensions of shape anisotropic particles designed to show pH-sensitive anisotropic pair potentials

Here we investigate the flow properties of suspensions of dicolloidal particles composed of interpenetrating spheres where one sphere is rich in polystyrene and the second is rich in poly 2-vinyl pyridine. The synthesis method is designed to create both anisotropic shape and anisotropic interaction potentials that should lead to head to tail clustering. These particles are referred to as copolymer dicolloids (CDCs). The viscoelastic properties of stable and gelled suspensions of CDC particles are compared with analogs composed of homopolymer dicolloids (HDCs), having the same shape but not displaying the anisotropic attractions. After coating the particles with a nonionic surfactant to minimize van der Waals attractions, the flow properties of glassy and gelled suspensions of CDCs and HDCs are studied as a function of volume fraction, ionic strength and pH. Suspensions of HDC particles display a high kinetic arrest volume fraction (?(g)?>?0.5) over a wide range of pH and ionic strength up to [I]=0.5?M, demonstrating that the particles experience repulsive or weakly attractive pair potentials. Suspensions of CDC particles behave in a similar manner at high or low pH when [I]=0.001?M, but gel at a volume fraction of ?(g)?相似文献   

The contribution of Brownian motion to the viscosity of suspensions of spherical particles

We study the contribution of Brownian motion to the viscosity of a suspension of spherical particles immersed in an incompressible fluid. We evaluate expressions derived from linear response theory applied to the generalized Smoluchowski equation and from a cluster expansion of the response. This leads to results obtained earlier by Batchelor for hard spheres and by Russel for more general pair interactions.