C. Blom

A counter-rotating Couette apparatus to study deformation of a sub-millimeter sized particle in shear flow

We describe a new counter-rotating Couette apparatus that has been developed for deformation studies of single sub-millimeter sized particles in shear flow. New features are the adaption to the low viscosities of water-based systems and temperature control of the device. The inner to outer radius ratio of the cylinders used is 0.9785 and the height to width aspect ratio of the gap is 4.0, while the inner radius is 100 mm. Because of the limited particle size a high mechanical accuracy of the Couette geometry is necessary. The swing of the inner cylinder is less than 2 μm and that of the outer cylinder less than 4 μm. We achieved this by carefully choosing the design parameters of the aerostatic bearing and the coupling between cylinder and motor unit. Furthermore, special drive units give a shear rate resolution of 0.018 s−1, while the maximum shear rate is 100 s−1. For a liquid viscosity on the order of 1 mPas the effective maximum shear rate is 30 s−1. We have shown that deformations as small as (L−B)/(L+B) ≈ 0.01 of giant bilayer vesicles (typical radius 10 μm) with length L and width B can be observed with our device.

Preparation and magnetisation of a silica-magnetite inverse ferrofluid

We introduce an ‘inverse ferrofluid’ comprising sterically stabilized, colloidal silica spheres and oleic acid stabilized magnetite particles. The preparation is described as well as magnetisation measurements which turns out to be a linear function of the silica volume fraction.

Driven torsion pendulum for measuring the complex shear modulus in a steady shear flow

To investigate the viscoelastic behavior of fluid dispersions under steady shear flow conditions, an apparatus for parallel superimposed oscillations has been constructed which consists of a rotating cup containing the liquid under investigation in which a torsional pendulum is immersed. By measuring the resonance frequency and bandwidth of the resonator in both liquid and in air, the frequency and steady-shear-rate-dependent complex shear modulus can be obtained. By exchange of the resonator lumps it is possible to use the instrument at four different frequencies: 85, 284, 740, and 2440 Hz while the steady shear rate can be varied from 1 to 55 s−1. After treatment of the theoretical background, design, and measuring procedure, the calibration with a number of Newtonian liquids is described and the accuracy of the instrument is discussed.

Linear Viscoelasticity of a Model Magnetorheological Fluid

Magnetorheological fluids are suspensions of magnetisable particles. Their rheology can be tuned externally: In the absence of a magnetic field they show liquid like behavior, whereas in the presence of a magnetic field solid like behavior is observed. The physical mechanism behind this transition is the formation of chains of dipolarly interacting magnetised particles.

Torsion Pendulum for Measuring the Dynamic Moduli in a Steady Shear Flow

To investigate the viscoelastic behaviour of fluid dispersions under steady shear flow conditions, an apparatus for parallel superimposed oscillations has been constructed which consist of a rotating cup containing the liquid under investigation in which a torsional pendulum is immersed. By measuring the resonance frequency and bandwidth of the resonator both in the liquid and in air, the frequency and steady shear rate dependent dynamic moduli G’ and G” can be obtained. By exchange of the resonator lumps it is possible to use the instrument at 4 different frequencies: 85, 284, 740 and 2440 Hz while the steady shear rate can be varied from 1 to 55 s-1. After discussing the design and calibration procedure, measurements on a 0.4% aqueous solution of polyacrylamide are presented. These measurements clearly show the steady shear rate of G’.