Shear properties manipulation

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

Another method used to dampen the catalyst activity in the fluidized-bed process is to deliberately add poisons to the reactor, such as 02 in small amounts. It was discovered that these poisons sometimes cause a broadening or narrowing of the MW distribution of the polymer, because they also affect the active site distribution on the catalyst. 02, for example, tends to increase polymer MI and broaden the MW distribution, which makes the polymer more shear-thinning. Consequently, poisons are sometimes intentionally added to manipulate polymer properties in this process. 

The hardest of the links shown in Figure 1.1 to make is the one between microstructure and texture. It requires consideration of the microstructure, the physical properties, the manner in which ice cream is eaten and how the microstructure breaks down as it warms up and is manipulated in the mouth. A further complication is that while the deformations applied to samples in physical property measurements are designed to be simple, the deformation applied in the mouth is complex. For example, when ice cream is squashed between the tongue and the roof of the mouth as it melts, the deformation is a combination of stretching (Figure 6.10a) and simple shear (Figure 6.10b). Thus the sensory experience relates to more than one physical property. 

Method and the Mori-Tanaka Method (Mori and Tanaka, 1973). In the former technique, the composite is viewed as a sequence of dilute suspensions and, thus, one can use the exact solutions for these cases to determine the effective composite properties. For example, the solution by Eshelby (1957) for ellipsoidal inclusions can be used. The increments of added inclusions are taken to an infinitesimal limit and one obtains differential forms for the bulk and shear moduli, which are then solved. The Mori-Tanaka method involves complex manipulations of the field variables. This approach also builds on the dilute suspension solutions (low F,) and then forces the correct solution as F, -> 1. 

The thixotrqiic behavior of suspensions and emulsions is a rather difficult property to measure, and this is further cmnplicated by the contribution of viscoelastic behavior (see next section). The main reason for this difflculty lies in the nature of the phenomenon involved. Thixotropy arises because of the existence of interparticle forces that produce three-dimensional microsmictures called flocculates or aggregates (see Sec. III). Depending on the magnitude of these forces, these microstructures are more or less prone to destruction by shearing, in consequence, any manipulation of a thixotropic sample may induce structural breakdown, thereby changing, most often irreversibly, the viscosity and yield stress of the sample. 

A technique based on injection molding, as it is a versatile, efficient and highly reproducible process, and capable of fast production of complex geometric shapes with tight dimensional tolerances has been studied. The mechanical property of PLLA can be optimized up to a certain level through manipulation of thermomechanical environment of conventional injection molding. Non-conventional injection molding processes like shear controlled orientation in... 

From a materials engineering perspective, what is needed in order to completely characterize these capsular structures, is a tool with which to probe their mechanical properties - an ability to manipulate individual giant lipid vesicles capsules and cells, that can not only apply well defined stresses for each of the three basic modes of deformation, (dilational, shear, and bending), but that can also measure the strain resulting from the applied stress, and therefore characterize the material behavior in terms of elastic moduli and viscous coefficients. The micropipet technique, initiated by Rand and Burton [92] and later perfected by Evans and Hochmuth [16], provides such an ability. It has been used extensively since the late 1970s to measure and characterize the material properties of red cells, white cells, and giant vesicles as reviewed in several recent publications [30,69,82]. 

In Section 3.1 we define two basic flows used in the characterization of polymeric fluids along with the appropriate material functions. These basic flows are also found in polymer processes. In Section 3.2 several constitutive equations capable of describing the viscoelastic behavior of polymer melts are presented. The emphasis in this section is on manipulating these equations for flows in which the deformation history is known. In this section we have added discussion of fiber suspensions as they are commonly processed to yield materials with increased stiffness and strength. In Section 3.3 an introduction into the methods for measuring rheological properties is presented. In Section 3.4 several useful relationships between material functions are presented. These relationships (or correlations) are important as they allow one to obtain estimates, for example, of steady shear material functions from linear viscoelastic data. Because... 

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