Shearing forces, aqueous fluids

It was previously shown that the formation of a stable emulsion of methylene chloride in water was vital for the successful formation of individual microspheres [4,9]. Two main factors played an important role in the emulsification of methylene chloride in water and influenced the microsphere size the interfacial tension of the methylene chloride droplets in the surrounding aqueous phase and the forces of shear within the fluid mass. The former tends to resist the distortion of droplet shape necessary for fragmentation into smaller droplets whereas the latter forces act to distort and ultimately to disrupt the droplets. The relationship between these forces largely determines the final size distribution of the methylene chloride in water emulsion which in turn controls the final size distribution of the solid microspheres formed. 

An emulsion is a biphasic solution including dispersed small oil droplets in an aqueous phase. Because of the small channel size and high fluid velocity in micromixers, high shear force is applied to fluids, resulting in the generation of an emulsion in addition to enhancement of the mixing performance. The emulsion produced using micromixers has the characteristics of stability without surfadant. 

Albumen is a 10% aqueous solution of various proteins. Other components are present in very low amounts. The thick, gel-like albumen differs from thin albumen (cf. Fig. 11.1) only in its approx, four-fold content of ovomucin. Albumen is a pseudoplastic fluid. Its viscosity depends on shearing force (Fig. 11.2). The surface tension (12.5% solution, pH 7.8, 24 °C) is 49.9 dynes cm The pH of albumen of freshly laid egg is 7.6-7.9 and rises to 9.7 during storage due to diffusion of solubilized CO2 through the shell. The rise is time and temperature dependent. For example, a pH of 9.4 was recorded after 21 days of storage at 3-35 °C. 

Friction and lubrication involve liqirids imder normal and shear stress. Those fluids are highly confined, at least in some parts of the interface. The behavior of those confined flirids has important consequences on shear and lubrication forces. Before we proceed to address the behavior of potymer-bearmg surfaces immersed in solutions, we discuss briefly, therefore, the behavior of confined monomeric, particularly aqueous, fluids. The properties of these fluids determine the dissipation mechanism of frictional forces also when potymer-bearmg surface are considered. This is particularly important at high pressures, often found in hioluhricated systems such as joints or between rocks in geological context or in fabricated devices. 

The physical properties of nonnewtonian fluids necessary for the study of forced convection heat transfer are the thermal conductivity, density, specific heat, viscosity, and elasticity. In general these properties must be measured as a function of temperature and, in some instances, of shear rate. In the special case of aqueous polymer solutions it is recommended that all properties except the viscous and elastic properties be taken to be the same as those of water. 

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