MAGBIOMAT

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

Citrated ferrofluids

The citrated ferrofluids were characterized to obtain the volume fraction, the iron, sodium and citrate content, the particles isotropy, the ionic strength, the charge surface density, and the particle size and shape.

Magnetization curve

magnetization_curve













Figure 6. Ferrofluid (maghemite particles stabilized by citrated ions in aqueous solution) magnetization curve. Volume fraction Φ = 9.5%, magnetic permeability χ = 0.925, saturation magnetization ms = 325342 A/m



The magnetic characterization was also carried out, and the magnetic properties were determined by means of the magnetization curves. Un example is given in Figure 6.

In particular, the magnetic susceptibility and the saturation magnetization were computed from the linear part of the curve in the low magnetic field range and the constant value at high magnetic field values respectively.

Rheological behaviour

Un example of shear rate dependence of apparent viscosity for a ferrofluid with Φ = 3.47 is given in Figure 7.

ferrofluid_viscosity















Figure 7. Viscosity as a function of shear rate at different values of the magnetic field strength for a ferrofluid with Φ = 3.47



In the absence of the applied magnetic field, the behaviour is the typical one corresponding to a newtonian fluid and the viscosity is constant within the studied range of shear rate.
However, when the magnetic field is applied, a weak increase of the viscosity at low shear rate is observed. This behaviour could be explained by the under field deformation of microscopic droplets of citrated ferrofluids.

Alginate and ferrofluids solutions

Steady shear flow measurements

The behaviour of the solution in the absence of an applied magnetic field is the typical one corresponding to a solution of entangled polymers. The first part of the curve shows newtonian behaviour: the shear stress is too weak to modify the chain conformation and the viscosity remains constant. The second one corresponds to a decrease of the viscosity: the shear stress is more important and it can induce the disentanglement of the polymer chains.

alginate_ferrofluid_viscosity














Figure 8. Viscosity as a function of shear rate at different values of the external magnetic field for an alginate and ferrofluid solution. Calg = 18g/L; ΦFF = 1%.


When the magnetic field is applied, the viscosity at low shear rate increases as the magnetic field strength increases. As in case of ferrofluids, it means that magnetic field induced structures are formed as observed by optical microscopy (see below) with the increase of magnetic field.

The analysis of these curves together with the magnetic parameters obtained from the magnetization curves provides the ratio between the viscous and magnetic forces, giving information about the structures formed in the solutions by the magnetic field and the breaking of these structures by the shear.

Oscillatory measurements

alginate_ferrofluid_deformation_sweep














Figure 9. G' et G" as a function of the strain amplitude at different external magnetic fieldd strengths for an alginate and ferrofluid solution. f = 1 Hz; Calg = 18g/L; ΦFF = 1%.

As it can be seen on Figure 9, G' and G" increase with the increase of the magnetic field in the linear viscoelastic region (LVR), which is the part of the curve where G' et G" are constant with the variation of the amplitude of the strain.

alginate_ferrofluid_lvr_moduli















Figure 10. G' and G" in the linear viscoelastic region (obtained from Figure 9) as a function of the external magnetic field.


Figure 10 shows the values of G' and G" in the LVR as a funtion of the external magnetic field. In the absence of applied magnetic field, G' is lower than G", but as the magnetic field strength increases, the increase of G' is more important than the increase of G" and G' becomes higher than G". There is a change from a viscous state to an elastic state with the increase of the magnetic field strength.
A similar behaviour was observed in the oscillatory measurements at constant strain and variable frequency.

From the obtained results both in flow and oscillatory measurements, a magneto-viscous effect in the magnetic field-responsive nanocomposite biopolymer solutions was highlighted by an increase of both the shear viscosity at low shear rate and the linear viscoelastic modules when the external magnetic field was increased. This effect suggests the existence of magnetic field induced structures.

Microscopic observations

In the absence of applied magnetic field, spherical droplets of demixion (regions with higher concentration of magnetic nanoparticles) were observed. The droplets were reversibly deformed under weak applied magnetic field. As the magnetic field increased, the deformed droplets become to interact, inducing the formation of chain-like structures.

microscopic_observations
































Figure 11. Microscopic observations of an alginate and ferrofluid solution


Ferrogels

The gelation process was studied by shear oscillatory measurements at constant strain and constant frequency. The gelation time, tg was estimated in a first stage by the crossover of both moduli, G' and G", as function of time.

alginate_ff_gel_time















Figure 12. Determination of the gelation time for a ferrogel. Time dependence of G' et G". Calg = 18 g/L; ΦFF = 0.5%; f = 1 Hz; γ = 0.01; [Ca2+]/[Na+] = 0.5.


In the absence of the external magnetic field, it was shown that the gelation time does not change significantly with the addition of the ferrofluid to the biopolymer network. However, when the magnetic field is applied, the increase of G' is faster, and the gelation time is lower (Figure 12).

This behaviour could be explained by enhancing of the alginate network formation by the orientation of the demixion droplets.

If the volume fraction of the magnetic nanoparticles in the ferrogel is increased, the observed behaviour is similar. In the absence of the external magnetic field, the addition of the ferrofluid does not change significantly the gelation time. Nevertheless, the application of the applied magnetic field speeds up the gelation process and, for example, in the case of Calg = 18 g/L and volume fraction of ferrofluid 1%, the gelation is almost inmediate.

Conclusions

This project was concerned with a first study of rheological properties under magnetic field of new nanocomposite biopolymers based on concentrated aqueous solutions of sodium alginate and functionalized magnetic nanoparticles of maghemite.

This study required the development of a magneto-rheological cell which allowed to perform temperature controlled flow and oscillatory shear measurements with the application of an external continuous magnetic field over the sample.

A magneto-viscous effect in the magnetic field-responsive nanocomposite biopolymer network was highlighted by an increase of both the viscosity at low shear rate and the linear viscoelastic moduli when the external magnetic field is increased.

Our results offered new perspectives of applications for these nanocomposites bio-based polymer networks.