Stability under shear

In what follows, we will explore the first of the two suggested scenarios. Starting with a multilamellar cylinder configuration, we will study its stability under shear flow (Fig. 17). Our aim is to check whether we can find an instability of the cylinder-a secondary instability-that would be responsible for the break-up into onions. 

The temperatures for maximum stability under shear in air for eight different polymers are listed in Table 3.2. There are only a few well-documented cases such as polystyrene and natural rubber, that fully reveal the general characteristics of mechanochemistry as a function of temperature. The temperature for maximum stability depends on polymer composition and its associated transitions and stability conditions. The temperature for maximum stability in shear must be above the melting and glass temperatures and below the temperature for extensive thermal and oxidative degradation. The temperatures in Table 3.2 are reduced slightly at ever higher imposed stresses. Correlations of this type for maximum stability in shear are likely to hold for the rubbery melt, and solution states. 

Australia, is the doyen of materials scientists who study the elastic and plastic properties of minerals under hydrostatic pressure and also phase stability under large shear stresses (Paterson 1973). J.-P. Poirier, in Paris, a professor of geophysics, was trained as a metallurgist one of his special skills is the use of analogue materials to help understand the behaviour of inaccessible high-pressure polymorphs, e.g., CaTi03 perovskite to stand in for (Mg, FelSiOi in the earth s mantle (Poirier 1988, Besson el al. 1996). 

Rheological observations of the UHMWPE pseudo-gels of different concentrations under oscillatory shear conditions at different temperatures showed that these systems exhibit considerable drawability at temperatures above ambient. The deformation of the crystalline phase of the gel-like system is not reversible and, as shown in the sequence of photographs Figure 2, for a pseudo-gel of 4% concentration, it was greater when the sample was sheared under the same oscillatory conditions at higher temperatures. The displaced crystals of the UHMWPE pseudo-gel showed remarkable dimensional stability after shear cessation and removal of any compression load in the optical rotary stage. 

The addition of water-soluble polymers such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA) into the synthetic mixture of the C TMAX-HN03-TE0S-H20 system (n = 16 or 18 X = Br or Cl) under shear flow is found to promote uniformity and elongation of rope-like mesoporous silica. The millimeter-scaled mesoporous silica ropes are found to possess a three-level hierarchical structure. The addition of water-soluble polymer does not affect the physical properties of the silica ropes. Moreover, further hydrothermal treatment of the acid-made material under basic ammonia conditions effectively promotes reconstruction of the silica nanochannels while maintaining the rope-like morphology. As a result, a notable enhancement in both thermal and hydrothermal stability is found. 

Throughout this chapter we focus on the extended hydrodynamic description for smectic A-type systems presented in [42,43], We discuss the possibility of an undulation instability of the layers under shear flow keeping the layer thickness and the total number of layers constant. In contrast to previous approaches, Auernhammer et al. derived the set of macroscopic dynamic equations within the framework of irreversible thermodynamics (which allows the inclusion of dissipative as well as reversible effects) and performed a linear stability analysis of these equations. The key point in this model is to take into account both the layer displacement u and the director field ft. The director ft is coupled elastically to the layer normal p = in such a way that ft and p are parallel in equilibrium z is the coordinate perpendicular to the plates. 

Some food O/W emulsions, including milk, cream, ice cream, and coffee whiteners and toppings, are stabilized by proteins such as casein that form a coating around the fat globules. These products also need to have some of the fat (oil) be partly crystalline to link the droplets in a network structure and to prevent complete coalescence of the oil droplets, especially under shear [78,825]. 

M. Renardy and Y. Renardy, Stability of shear flows of viscoelastic fluids under perturbations perpendicular to the plane of flow, J. Non-Newtonian Fluid Mech., 32 (1989) 145-155. 

Understanding how to control the colloidal stability of polymer colloids is critical, whatever route is used for their preparation. Predictive knowledge of simple systems is well established, but more complex stabilisation systems that combine several approaches are less well understood. Additionally, research into colloidal stability under extremes of temperature, pFl, ionic strength and shear rate is important and will require detailed studies of the particle interactions and dynamics in well-characterised polymer colloids. The results from such studies will also be important for achieving process control. 

In a metered dose inhaler, where fine drug particles are suspended in a propellent, stability and aggregation of the suspension are crucial for the performance of the inhaler. In an investigation into the aerosol formulation stability. Span 80 was added to a suspension of lactose to study the effect on deaggregation under shear.f An increase in shear stress was found to decrease aggregate size and boundary fractal dimension (Fig. 9), which was interpreted as a more compact aggregate. 

The influence of a shear field on emulsion crystallization is of great interest as it relates to behavior during product processing and distribution. Emulsions can be destabilized under shear in a controlled manner to deliver desirable properties uncontrolled or unintentional destabilization may lead to poor product performance. Comparisons of emulsions under perikinetic (at rest) and orthokinetic (under shear) conditions were made in an effort to understand the role of shear on the stability of the systems studied. Davies et al. (22) found stability of triglyceride emulsions containing crystals to be sensitive to both shear and crystal concentration. Crystal morphology also plays an important role in the destabilization of emulsions under shear. Boode and Walstra (4) reported the presence of needle-like... 

Another aspect of cement slurry stability is the stability under dynamic conditions. Dynamic conditions are usually more severe than static ones because cement slurries are shear-thinning. This is a problem in the laboratory as the solid particles may settle while the fluid is being sheared (thickening time, rheology), and also in the field especially if the well is deviated from vertical. But there is currently no standard test in the industry to evaluate the stability of cement slurries under dynamic conditions. 

Very high mechanical stability under severe actions, especially high shear in mortar or concrete mixing and in metering and transfer pumps. 

For use with high-speed applicators, high shear cone and plate viscometer results may be quoted as secondary information. Many poly(vinyl alcohol)-stabilized products are comparatively insensitive to shear and give broadly similar results with different types of viscometer. This behavior under shear is known as Newtonian and is a feature, inter alia, of large particles with a narrow particle size distribution. High shear viscosity testing also indicates if there is sufficient mechanical stability to allow application by knife or roller, although this is not usually a problem with colloid-stabilized emulsion polymers and adhesives. 

The softening point of the modified asphalt was higher than 60°C at concentrations above 5% of LDPE/GRT at a ratio of 30/70 and POE-g-MA content of 8% under shearing speed of 3000 rpm for 20 min. In addition, these blends showed good storage stability at high temperatures. 

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