1.4Time-dependent fluid behaviour
The flow behaviour of many industrially important materials cannot be
described by a simple rheological equation like (1.12) or (1.13). In practice,
apparent viscosities may depend not only on the rate of shear but also on the
time for which the fluid has been subjected to shearing. For instance, when
materials such as bentonite-water suspensions, red mud suspensions (waste
stream from aluminium industry), crude oils and certain foodstuffs are sheared
at a constant rate following a long period of rest, their apparent viscosities
gradually become less as the ‘internal’ structure of the material is progressively
broken down. As the number of structural ‘linkages’ capable of being broken
down decreases, the rate of change of apparent viscosity with time drops
progressively to zero. Conversely, as the structure breaks down, the rate at
which linkages can re-form increases, so that eventually a state of
dynamic equilibrium is reached when the rates of build-up and of break-down
are balanced. Time-dependent fluid behaviour may be further sub-divided into two categories:
thixotropy and rheopexy or negative thixotropy
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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