Bulk Properties of Suspensions

This process of incipient flocculation can be followed by measuring the turbidity of the suspension as a function of temperature. Above the CFT, the turbidity of the suspension rises very sharply. 

For the above purpose, the cell in the spectrophotometer that is used to measure the turbidity is placed in a metal block that is connected to a temperatureprogramming unit (which allows one to increase the temperature rise at a controlled rate). 

As discussed in Chapter 5, Ostwald ripening is the result of the difference in solubility S between small and large particles. The smaller particles are more soluble than the larger ones. 

To obtain a measure of the rate of crystal growth, the particle size distribution of the suspension is followed as a function of time, using either a Coulter counter, a Master sizer or an optical disc centrifuge. One usually plots the cube of the average radius versus time, which gives a straight line, from which the rate of crystal growth can be determined (the slope of the linear curve). 

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Bulk Properties of Suspensions Equilibrium Sediment Volume (or Height) and Redispersion... 

PARTICLES AND suspensions of many kinds play an important role in our interaction with the physical environment. They abound in earth, soil, water, and air. They are also present in chemicals and many other industrial products. If particles were spherical or cubical, characterization would be easy. Unfortunately, most of the particles present in the environment are of irregular size and shape. In many industrial processes, size characterization is a critical aspect of understanding particle behavior in suspension, the bulk properties of suspensions, and their bulk behavior. Therefore, it is important to understand the strengths and limitations of techniques used to characterize particles and suspensions. 

Rheological measurements are used to investigate the bulk properties of suspension concentrates (see Chapter 7 for details). Three types of measurements can be applied (1) Steady-state shear stress-shear rate measurements that allow one to obtain the viscosity of the suspensions and its yield value. (2) Constant stress or creep measurements, which allow one to determine the residual or zero shear viscosity (which can predict sedimentation) and the critical stress above which the structure starts to break-down (the true yield stress). (3) Dynamic or oscillatory measurements that allow one to obtain the complex modulus, the storage modulus (the elastic component) and the loss modulus (the viscous component) as a function of applied strain amplitude and frequency. From a knowledge of the storage modulus and the critical strain above which the structure starts to break-down , one can obtain the cohesive energy density of the structure. 

Three different rheological measurements may be applied to study the bulk properties of suspension concentrates [118-120] (i) Steady state shear stress-shear rate measurements (using a controlled shear rate instrument), (ii) Constant stress (creep) measurements (carried out using a constant stress instrument), (iii) Dynamic (oscillatory) measurements (preferably carried out using a constant strain instrument). These... 

Bulk properties of the suspension. This is particularly important for concentrated systems, and requires measurement of the rate of sedimentation and equilibrium sediment height. More quantitative techniques are based on assessing the rheological properties of the suspension (without disturbing the system that is, without its dilution and measurement under conditions of low deformation) and how these are affected by long-term storage. This subject is discussed in detail in Chapter 20. 

To discuss the macroscopic (or bulk ) properties of a suspension, it is necessary to specify the connection between local variables at the particle scale andmacroscopic variables at the scale L. One plausible choice, in view of the relationship between continuum and molecular variables in Chap. 2, is to assume that the macroscopic variables are just volume averages of the local variables. In particular, we assume in the discussion that follows that the macroscopic (or bulk) stress can be defined as a volume average of the local stress in the suspension, namely,... 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

For the full assessment of the properties of suspension concentrates, three main types of investigations are needed (i) the fundamental investigation of the system at a molecular level (ii) investigations into the state of the suspension on standing (iii) bulk properties of the suspension. All these investigations require a number of sophisticated techniques such as zeta potential measurements, surfactant and polymer adsorption and their conformation at the solid/liquid interface, measurement of the rate of flocculation and crystal growth, and several rheological measurements. Apart... 

When 20% of the phosphoms was polyphosphate, the compounds ia the product were ia the ratio of 3.5 mole MAP per mole of the pyrophosphate. The principal use of the material was ia the production of suspension fertilizers. In this appHcation the polyphosphate content imparted improved storage properties to the suspensions. The granular soHd APP, however, also had excellent storage properties and was a good material for use ia bulk blends and for direct appHcation. 

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