Project funded by Marie Curie Actions IEF. Project 235673 "Study of magnetic responsive biopolymer based materials"

Magnetic cell

The development of a specific device allowed the measurements of mechanical properties under continuous magnetic field. This device consists of a mechanical part and a magnetic one. The former is a cone and plate geometry (D = 59.9 mm, angle = 1 degree, distance between the cone and the plan in the measurement position: 0.052 mm) made of non-magnetic material. The later is composed of two coils which are placed on both sides of the cone-plan geometry mounted on the torque controlled rheometer (Mars II from Thermo Fisher Scientific company) with the bottom plate placed in the upper part of the lower coil (Figure 3).

Magnetic cell

Figure 3. Magnetic cell

This Helmholtz configuration allows creating a homogeneous magnetic field perpendicular to the shear direction. An electric current of variable intensity between 0 and 25 A induces between the coils a magnetic field strength between 0 and 40.2 mT.

The temperature of the sample introduced on the bottom plate was controled during rheological measurements by a water circulation from a thermostatic bath. The high intensity flowing through the coils causes a large increase in temperature due to the Joule effect. In order to overcome this problem, the cooling of coils was achieved by a second water circulation with a set of pipes and flowmeters.

Every part of the magnetic cell (except the upper cone cell with long axis) was developed in collaboration between MSC and PECSA laboratories.

Firstly, the calibration curve between the applied current intensity and the magnetic field strength created by the coils was determined. Secondly, steady shear viscosity measurements of non magnetic standard oils was carried out in the absence and presence of applied magnetic field, in order to verify that the viscosity does not change with the application of the magnetic field (Figure 4).


Figure 4. Viscosity as a function of the shear rate for the viscosity standard at different magnetic field strengths

Microscopic observations

To study the structures that can be formed in the solutions by the applied magnetic field, microscopic observations were carried out, using a microscopy slide which allows the application of a magnetic field during the observations. Two moving magnets were placed in slots, creating a magnetic field from 4 mT to 30 mT in the same range as for the rheological measurements (Figure 5).


Figure 5. Microscopy slide

Rheological measurements

Steady-state shear flow measurements

  • Protocol for ferrofluids: The samples were subjected to a shear rate ramp between 20 s-1 and 500 s-1 and the corresponding shear stress (τ) and viscosity (η) were recorded as a function of the shear rate (dγ/dt).
  • Protocol for alginate and alginate and ferrofluid solutions: In this case, the shear rate ramp was varied between 0.01 s-1 and 2000 s-1.

Oscillatory measurements

In this kind of measurements, an oscillatory strain either at constant frequency (f) and variable strain amplitude (γ), or at variable frequency and constant strain amplitude was applied, and the storage modulus (G') and the loss modulus (G") were measured.
G' is the real part of the complex rigidity modulus (G*) that relates the sinusoidal stress and the sinusoidal strain, both in their complex form. G' is proportional to the storage power per volume unity in the suspensions during a quarter of cycle. The complex part, G", of the rigidity modulus is called loss modulus and it is proportional to the dissipated power by viscous friction.

All rheological measurements were performed at constant temperature (25±1)°C