We describe experimental studies of the deformation of giant lipid bilayer vesicles in shear flow. The experiments are carried out with a counterrotating Couette apparatus. The deformation depends on the mechanical properties of the lipid bilayer, the vesicle radius, and the viscosity of the surrounding Newtonian liquid. We show that the relevant mechanical parameter is the bending rigidity. A simple model has been developed that describes the deformation of a vesicle. This model takes thermal undulations of the bilayer into account. We have obtained a value for the bending rigidity of dimyristoyl-phosphatidylcholine bilayers and its value has been compared with literature data and with results from micropipette aspiration experiments. From the measurements we are able to discriminate between unilamellar and multilamellar vesicles.
A magnetorheological fluid consisting of colloidal silica spheres suspended in an organic ferrofluid is described. Its linear viscoelastic behavior as a function of frequency, magnetic field strength, and silica volume fraction was investigated with a specially designed magnetorheometer. The storage modulus GЈ is at least an order of magnitude larger than the loss modulus GЉ at all magnetic field strengths investigated. GЈ does depend only weakly on frequency, and linearly on volume fraction. A model is presented for the high frequency limit of the storage modulus G ϱ Ј . In the model our system is treated as a collection of single noninteracting chains of particles. Assuming a dipolar magnetic interaction, theory and experiment show reasonable agreement at high frequencies.
Rheology of cohesive sediments: comparison between a natural and an artificial mud Rheologie des sediments cohesifs: comparaison entre une vase naturelle et une vase artificielle THUS VAN KESSEL, ABSTRACT Concentrated cohesive sediment suspensions exhibit strongly non-Newtonian and time-dependent behaviour caused by particle interactions, which complicates the prediction of their strain or shear rate response to applied forces. In this paper the rheological properties of both China clay, an artificial mud, and Caland chan nel mud, a natural mud, are investigated. Four types of experiments are involved: equilibrium flow curves, stepwise changes in shear rate to investigate the time-scale of structural changes, experiments in which the increase in yield stress with recovery time after structural destruction was measured, and harmonic oscillation experiments to investigate the visco-elastic behaviour of mud at low strains. For these measurements both shear rate and shear stress controlled instruments were used. Equilibrium flow curves revealed strong shearthinning behaviour for China clay, which was much less observed for Caland channel mud. Experimental results are described in terms of a thixotropic model including two structural parameters to account for struc tural changes within the material, one to describe short-term changes when the material is flowing, another to describe long-term changes when the material is at rest. From experiments with stepwise changes in shear rate it followed that the time-scale of structural changes is in the order of seconds. Both materials showed a yield stress, Caland channel mud being the more cohesive mud. Yield stress measurements as a function of rest time after pre-shearing showed that the time-scale for the build-up of an interconnected aggregate structure is in the order of I0 4 -10 5 s. From harmonic oscillation experiments it can be concluded that the range within which muds exhibit linear visco-elastic behaviour is limited to strains smaller than approximately 1%. RESUME Les suspensions concentrees de sediments cohesifs presentent clairement un comportement non-Newtonien et variable en temps cause par des interactions de particules. Ceci complique la prediction de leur taux de defor mation en reponse ä des forces appliquees. Dans cette etude les proprietes rheologiques de la vase chinoise, une vase artificielle, et de la vase du Canal du Caland, une vase naturelle, sont etudiees. Quatre types d'experiences ont ete utilises : des courbes d'equilibre de flux; des reponses du taux de deformation ä un creneau afin d'etudier l'echelle de temps des changements structuraux; des experiences oil l'augmentation de la rigidite ini tiale, avec un temps de recuperation apres la destruction structurelle a ete mesuree, et des essais sous oscilla tion harmonique afin d'etudier le comportement viscoelastique de la vase ä basse deformation. Pour ces mesures des instruments controlant ä la fois le taux de deformation et les contraintes de cisaillement ont ete utilises. Les courbes d'equil...
Summary-From measurements with a Couette-type viscometer provided with a guard ring it was shown that at the saliva-air interface a protein layer is adsorbed. Measurements of the surface shear modulus of this layer on saliva of 7 healthy subjects were performed at a frequency of about 70Hz and a temperature of 25°C. For a surface age of about 1.5 h the surface shear modulus and the surface viscosity were in the order of 1 Nm-' and IO-' Nm-' s, respectively. From ellipsometric measurements it was found that the thickness of the protein layer was approx. 100 nm and, using this value, it could be concluded that the shear modulus and the dynamic viscosity were in the order of 10' Pa and lo4 Pa s, respectively. The layer appeared to be fragile. Even shear deformation amplitudes of 4 x IO-' are too high to assure linearity. The c.omplex viscosity (q = '1' -iv") of the bulk liquid of human submandibular saliva below the absorbed layer was measured in the frequency range 70 Hz-200 kHz with 3 torsional resonators, each for a different frequency, and a Ni-tube resonator. It was concluded, that the real part of the complex viscosity (n') decreases from 1.1 mPa s at 70 Hz to a value of 0.95 mPa s at high frequencies. Except at the lowest frequency (70 Hz), the value of n" was too small to be detected.
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