This paper analyzes various techniques to use viscometers equipped with vane spindles to characte... more This paper analyzes various techniques to use viscometers equipped with vane spindles to characterize rheological properties of yield stress fluids. Specifically, application of Brookfield viscometers to this end is discussed. A wide selection of toothpastes and lotions were tested. It is shown that a simple method based on apparent shear rate and stress, commonly referred to as a representative viscosity method, works well for moderately non-Newtonian samples but may significantly underestimate viscosity for samples with a more pronounced yield stress behavior. To get more accurate data an integral equation relating torque to angular velocity needs to be solved which can be easily done numerically to get a good agreement between the data collected on an inexpensive viscometer and the data from high-end rheometers.
ABSTRACT Rheological measurements and cryogenic transmission electron microscopy are used to prob... more ABSTRACT Rheological measurements and cryogenic transmission electron microscopy are used to probe the structure of model magnetic paints which contain ferrimagnetic pigment particles of barium ferrite, a wetting resin, and a solvent. As the resin concentration c increases, the storage modulus goes through a minimum. At low c, a flocculated network is developed in the paint. Milling of the paint at insufficient resin concentration does not improve its dispersion quality, but instead results in the formation of a stronger network. Milling at higher c effectively disrupts the network into small aggregates. The resin concentration required to provide effective milling is lower for paints containing the larger iron oxide particles.
We calculate the lubrication force that acts in the normal direction between spherical surfaces b... more We calculate the lubrication force that acts in the normal direction between spherical surfaces bearing grafted polymer brushes and immersed in a viscous fluid. Brinkman's equation is employed to describe the flow in the brushes. For noncompressed brushes we include a slip approximation that applies to poorly permeable brushes of arbitrary density profile. For the step-function profile we present an
ABSTRACT Steady and oscillatory shear rheological, magnetic susceptibility, and particle size mea... more ABSTRACT Steady and oscillatory shear rheological, magnetic susceptibility, and particle size measurements are used to characterize the microstructure of metal particles dispersions in organic solvents with respect to changes induced by milled-in polymer only and polymer+ dispersant. A hierarchical structure built of inseparable clusters, aggregates with their size fixed during mix preparation, and a volume-filling network of these aggregates is consistent with our observations. As polymer-to-particles ratio and/or number of active groups in the polymer increase, the aggregates decrease in size and the network becomes weaker and less brittle. For each polymer-to-particles ratio an optimum level of dispersant is established. Below the optimum dispersant level aggregates are too large, while above it aggregates appear to interact too strongly. A plateau at intermediate shear rates occurs unless an optimum dispersant level is maintained along with sufficient polymer. This plateau is attributed to wall-slip and may be used to discriminate dispersion quality. (C) 2003 The Society of Rheology.
This paper analyzes various techniques to use viscometers equipped with vane spindles to characte... more This paper analyzes various techniques to use viscometers equipped with vane spindles to characterize rheological properties of yield stress fluids. Specifically, application of Brookfield viscometers to this end is discussed. A wide selection of toothpastes and lotions were tested. It is shown that a simple method based on apparent shear rate and stress, commonly referred to as a representative viscosity method, works well for moderately non-Newtonian samples but may significantly underestimate viscosity for samples with a more pronounced yield stress behavior. To get more accurate data an integral equation relating torque to angular velocity needs to be solved which can be easily done numerically to get a good agreement between the data collected on an inexpensive viscometer and the data from high-end rheometers.
ABSTRACT Rheological measurements and cryogenic transmission electron microscopy are used to prob... more ABSTRACT Rheological measurements and cryogenic transmission electron microscopy are used to probe the structure of model magnetic paints which contain ferrimagnetic pigment particles of barium ferrite, a wetting resin, and a solvent. As the resin concentration c increases, the storage modulus goes through a minimum. At low c, a flocculated network is developed in the paint. Milling of the paint at insufficient resin concentration does not improve its dispersion quality, but instead results in the formation of a stronger network. Milling at higher c effectively disrupts the network into small aggregates. The resin concentration required to provide effective milling is lower for paints containing the larger iron oxide particles.
We calculate the lubrication force that acts in the normal direction between spherical surfaces b... more We calculate the lubrication force that acts in the normal direction between spherical surfaces bearing grafted polymer brushes and immersed in a viscous fluid. Brinkman's equation is employed to describe the flow in the brushes. For noncompressed brushes we include a slip approximation that applies to poorly permeable brushes of arbitrary density profile. For the step-function profile we present an
ABSTRACT Steady and oscillatory shear rheological, magnetic susceptibility, and particle size mea... more ABSTRACT Steady and oscillatory shear rheological, magnetic susceptibility, and particle size measurements are used to characterize the microstructure of metal particles dispersions in organic solvents with respect to changes induced by milled-in polymer only and polymer+ dispersant. A hierarchical structure built of inseparable clusters, aggregates with their size fixed during mix preparation, and a volume-filling network of these aggregates is consistent with our observations. As polymer-to-particles ratio and/or number of active groups in the polymer increase, the aggregates decrease in size and the network becomes weaker and less brittle. For each polymer-to-particles ratio an optimum level of dispersant is established. Below the optimum dispersant level aggregates are too large, while above it aggregates appear to interact too strongly. A plateau at intermediate shear rates occurs unless an optimum dispersant level is maintained along with sufficient polymer. This plateau is attributed to wall-slip and may be used to discriminate dispersion quality. (C) 2003 The Society of Rheology.
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Papers by Andrei Potanin