摘要:

A model is presented that predicts the critical shear rate of shear thickening of soft sphere colloidal suspensions. It is based on the idea that shear in a colloidal crystal leads to a periodic variation of the elastic modulus with time. At a specific shear rate an acoustic resonance occurs which leads to an increase of the viscosity. Good agreement with experimental results could be obtained by fitting the single parameter of the model.

ISSN:0148-6055    

DOI:10.1122/1.550891

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Startup of steady shear experiments is used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching.

ISSN:0148-6055    

DOI:10.1122/1.550892

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The shear-induced particle migration in a polydisperse concentrated suspension is described using migration potentials for the various particle size fractions. The model is applied to solve the flow patterns and the particle concentration distributions in bidisperse suspensions in various viscometric flows. For the case of a continuous particle size distribution a formulation of this model in terms of concentration distribution moments is presented. The model predicts the total migration of the particles in the various flow devices and the size segregation of the various fractions. The calculations capture well the few experimental observations reported so far. It is found that in all calculated cases the total energy dissipation rate and, therefore, the power required to drive the flow were diminished considerably at steady state.

ISSN:0148-6055    

DOI:10.1122/1.550963

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The thixotropic behavior of a colloidal dispersion of clay consisting of disk-shaped particles was studied by means of a combination of rheometric measurements, static light scattering, and small-angle neutron scattering. At rest, the structure of the gel consists of dense micrometer-sized aggregates assembled into a fractal mass of dimensionD. Under shear, in the case of volume fractions with(1⩽D⩽1.2)at rest, a butterfly-type light scattering pattern is observed. This is attributed to the formation of rollers within the dispersion, which align themselves on average perpendicular to the direction of shearing. This produces a fall in resistance to flow and in viscosity. The influence of shear rate on this disaggregation process was studied and linked to the rheometric measurements. Under shear flow conditions, the fall in viscosity is due to orientation and disaggregation processes occurring at length scales on the order of 1 μm. During recovery, the two time scales identified correspond, respectively, to a rapid relaxation of the particle orientations and a slow aggregation process. Thixotropic behavior is identified as being mainly a reversible aggregation process, for which large length scales on the order of a micrometer associated with a fractal arrangement play a decisive role.

ISSN:0148-6055    

DOI:10.1122/1.550964

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

This paper deals with the influence of composition [sucrose ester SE and oil O concentrations] and temperature on the linear viscoelasticity properties of highly concentrated oil-in-water emulsions, using a hydrophilic sucrose palmitate as emulsifier. Different oil-in-water emulsions were prepared using a sucrose palmitate(SE=1–5 wt %)with a hydrophilic–lipophilic balance of 15, sunflower oil(O=60–80 wt %)and water. Oscillatory measurements were carried out in the linear viscoelasticity region. The relaxation time spectra of the emulsions were calculated using regularization techniques and were fitted to an empirical model proposed by Reference 1996. The results obtained are explained on the basis of the relationship among linear viscoelasticity properties, droplet size distribution, and characteristics of the continuous phase. Thus, an increase in sucrose palmitate concentration produces higher values of the linear viscoelasticity functions and a broader plateau region in the relaxation time spectrum, which suggests a certain enhancement of the elastic network.

ISSN:0148-6055    

DOI:10.1122/1.550965

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Upstream instability and wall slip of a high molecular weight polydimethylsiloxane (PDMS) have been studied in a rough stainless-steel slit die. The velocity field at a micrometric scale close to a wall is determined with a fluorescence technique. A film of a mixture of PDMS and small fluorescent-labeled chains (PDMS–NBD) is deposited on the steel surface prior to the high-pressure flow. During the flow, the fluorescence of a small area in the middle of the surface is excited with an argon laser line and measured with a phototube. The signal decays towards an asymptotic low value, due to stray light and thermal current. The slip velocity is inferred from a comparison between the experimental decrease and a theoretical model, which takes diffusion effects into account. It is shown that the upstream instability induces a transverse oscillating velocity in the channel. A precise determination of the residual fluorescence after the wall slip, with a known diffusion coefficient and surface roughness, shows that the wall slip occurs in a plane localized at less than 1 μm from the ridges of the surface roughness.

ISSN:0148-6055    

DOI:10.1122/1.550893

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The behavior of single drops of isotropic hydroxypropylcellulose solutions immersed in a polydimethylsiloxane matrix subjected to a constant shear rate was investigated using a rheo-optical technique. Our main results concern the nonstationary deformation and breakup mechanisms of drops characterized by large capillary numbers. The deformation of threads follows a pseudoaffine deformation forCa/Cacritlarger than 2.5 and above a certain strain. The end pinching mechanism occurs at a specific scaled strain, which includes the initial drop size and the applied shear rate. Capillary instabilities develop when the thread diameter reaches a critical value,dr,inversely proportional to the applied shear rate and independent of the initial drop diameter. The time necessary for the total rupture of a thread depends on the initial drop diameter, the applied shear rate, and the critical thread diameterdr.The droplet size resulting from the final rupture of a thread is found to be half that of the critical size, below which a drop deforms into a stable ellipsoid.

ISSN:0148-6055    

DOI:10.1122/1.550894

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

Nuclear magnetic resonance imaging is used to study rates of sedimentation in several non-Newtonian fluids. It is shown that either shear thinning or elasticity in a fluid can result in a time-dependent sedimentation rate, as measured by the motion of the suspension–supernatant interface. Sedimentation is most rapid at the beginning of the process, and grows steadily slower in time. Two-dimensional images of vertical sections of suspension show clearly the development of a nonhomogeneous microstructure during sedimentation in a viscoelastic fluid. Elongated columns of particles form in the direction of gravity, and these columns are separated by comparably sized regions of pure fluid. Such structures are not present in sedimentation in Newtonian fluids, where the suspension microstructure is homogeneous for the duration of the process.

ISSN:0148-6055    

DOI:10.1122/1.550895

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

This paper reports measurements of yield stress of aqueous foams approaching the dry foam limit using a pendulum device. Traditionally, the vane rheometer has been used to measure the yield stress in liquids that exhibit wall slip. However, using the simple and inexpensive pendulum technique, shear rates many orders of magnitudes lower can be achieved. The pendulum was used to observe the change in yield stress for the foam as the gas fraction and bubble size increased. The local gas fraction in the foam was found by measuring the sonic velocity, and the bubble size was determined photographically. Strong support is found for the existence of a true yield stress in aqueous foams at the dry foam limit. Yield stress results, once scaled byσ/〈R〉,agree well with data from previous studies.

ISSN:0148-6055    

DOI:10.1122/1.550896

出版商:The Society of Rheology    

年代:1998

数据来源: AIP

摘要:

The flow properties of suspensions of weakly attractive particles are investigated as a function of volume fraction and strength of interparticle attraction. The suspensions are composed of uniform silica spheres with covalently bound octadecyl chains with diameters 100–125 nm suspended in decalin and tetradecane. At elevated temperatures, the particles interact like hard spheres. As the temperature is lowered, the suspensions gel. At the gel point, storage and loss moduli are power law functions of strain frequency providing evidence that cluster relaxation rates are much slower than the lowest strain frequency used. At volume fractions below the gel point (temperatures above the gel point), suspensions shear thin in much the same way as hard spheres. These studies suggest that the effect of volume fraction and attractive strength can be correlated when the normalized zero shear rate viscosity,η0/η0HSis written as a function ofφ/φG,whereη0HSis the high temperature (hard sphere), zero shear rate viscosity of a suspension at volume fraction φ, andφGis the volume fraction where, at the same strength of attraction, the suspensions gel. Atφ/φG⩽0.95,η0/η0HS⩽5suggesting suspension viscosities are weak function of attractions up to the gel point where relaxation times rapidly increase.

ISSN:0148-6055    

DOI:10.1122/1.550966

出版商:The Society of Rheology    

年代:1998

数据来源: AIP