A rheological in vitro system has been developed to study and quantify cellular adhesion under precisely defined external shear forces. The system is similar to a cone-and-plate viscosimeter. A rotating transparent cone produces both steady and pulsatile flow profiles on cultured cells. Direct visualization of cells by phase-contrast or fluorescence optics and connection of the optical system to a computer-controlled x/y-linear stage allows automatic recording of any point of the cell cultures. With the use of up to 12 individual rheological units, this setup allows the quantitative analysis of cell substrate adhesion by determination of cell detachment kinetics. Two examples of application of this rheological system have been studied. First, we show that the extracellular matrix protein laminin strongly increases endothelial cell adhesion under fluid shear stress. In a second approach, we obtained further support for the concept that shear stress-induced formation of actin filament stress fibers is important for endothelial cells to resist the fluid shear stress; inhibition of stress fiber formation by doxorubicin resulted in significant detachment of endothelial cells exposed to medium levels of fluid shear stress (5 dyn/cm2). No detachment was seen under resting conditions.

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Zusammenfassung: Der Cauchy-Green-Tensor einer beliebig vorgegebenen Grundstr6mung wird ffir kleine St6rungen linearisiert und das Ergebnis in geschlossener Form angegeben. AnschlieBend werden das Stabilit~itsverhalten einer viskoelastischen Fltissigkeit hinsichtlich spezieller St6rungen untersucht und verschiedene Grenzfalle (kurze und lange Wellen) diskutiert. Zum SchluB wird ein durch die elastischen Eigenschaften der Fltissigkeit bestimmter Instabilit~itsmechanismus ftir Maxwell-und Doi-Edwards-Fluide aufgezeigt.Abstract: The Cauchy-Green tensor of a given basic flow is linearized for small perturbations and the result is given in explicit form. The stability behaviour of an elastic fluid is then investigated for a special type of perturbation, and several limiting cases (short and long waves) are discussed. Finally, an instability mechanism of Maxwell and Doi-Edwards fluids caused by the elastic properties of these fluids is demonstrated.

Zusammenfassung: Die Stabilit~it der ebenen Couette-Str6mung zweier nichtnewtonscher Fluidmodelle mit Relaxation wird ffir kleine St6rungen in der viskosimetrischen Ebene untersucht. Um die Darstellung der Ergebnisse zu vereinfachen, wird sie auf einFluidmodell mit konstanten Stoffdaten beschrgnkt. Abstract:The stability of plane Couette flow for two models of non-Newtonian liquid having relaxation properties was investigated for small perturbations in the viscometric plane. To simplify the presentation of the results only models with constant material properties are considered.

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Zusammenfassung: Die Stabilit~it der ebenen Scherstr6mung eines einfachen Fluids wird im Rahmen der Kurzwellenapproximation ftir St6rungen in der viskosimetrischen Ebene untersucht. Ffir kurze Wellen ist eine Stabilit~itsanalyse unabhiingig von der speziellen Form der Stoffgleichung m6glich. Die vorliegende Analyse stellt einen ersten Schritt in diese Richtung dar und ftihrt zu einem hinreichenden Stabilit~itskriterium. Ffir kurze Wellen sind die Maxwell-Flfissigkeiten A und B bezifglich ebener St6rungen stabil. Abstract:The stability of a plane shear flow of simple fluids is investigated for perturbations in the viscometric plane within the framework of the short wave approximation. For short waves it is possible to carry out a stability analysis, which is independent of the type of constitutive equation. The analysis presented is the first step in this direction and leads to a sufficient stability criterion. For short waves Maxwell fluids A and B are stable for perturbations in the viscometric plane.

In the above-mentioned paper [1], the aim of which was "to work out some consequences of the 'short memory approximation' and to give a critical assessment of it", a number of questions were posed. In the present short communication a few comments, which may be useful for further scientific discussion in this area, are addressed to some of these questions.The first critical remark (or problem) was formulated as follows: "It thus appears that the short memory hypothesis is inconsistent unless the fluid has a different memory for perturbations than it has for the shear flow itself. We do not know if and how this could be made precise". The idea of different relaxation times is one possible way of understanding the short memory approximation. Another way is to construct a constitutive model with the following properties,N~ 2 without distinguishing between the relaxation time of the basic flow and its perturbations by assuming that the relaxation time 2 is a decreasing function of the shear rate × in some range of (probably high) ~. This idea could be demonstrated by a modification of the upper convected Maxwell fluid:(S and D are the stress and the strain rate tensors) with deformation-dependent viscosity i/ and relaxation time 4. 38 It is assumed in a simple shear flow that 2 and ~/are functions of ~ (in general, functions of the invariants of D) and they can be approximated for high shear rates by the equations 2 = 20 , (3 a) ~/= ~/0 , (3 b)with constant exponents m, n ~ R+ (n < 1) and material constants 20, ~/0, n0. Making use of eq. (2) leads to the following expressions:We 2 = 4 (1 -m -n) 2 i----n "For any given small value of 2 z, We 2 can be made larger than 16 by suitable choice of the exponents m and n.The second problem was formulated as follows: "The upper and lower convected Maxwell models satisfy, at high enough shear rates, the condition but it was recently shown [2, 3] that a change of type does not occur in those models. Thus any existing