Characterization of the dynamics of magnetizable coloidal systems
by means of optical techniques


  We use optical techniques: Small-Angle-Light-Scattering (SALS), Scattering dichroism, Videomicroscopy.
  • Structure formation and orientation dynamics of magnetorheological suspensions subject to rotating magnetic fields 
  • Aggregation and relaxation processes of magnetic colloids under unidirectional magnetic fields
  • Generation of transient diffraction gratings 
Sacttering pattern
Professor Miguel A. Rubio and I co-adviced the Thesis work by P. Dominguez-Garcia defended on July, 2007 and entitled:
             Experimetnal study of MR fluids using video-microscopy and image analysis:
             dynamics, structure and microfluidics applications.


Magnetorheological (MR) suspensions are colloidal suspensions which consist of micron-sized magnetizable particles suspended in a non-magnetic fluid. These particles contain nanometric iron oxide grains dispersed and uniformly distributed in a polymer matrix. In absence of a magnetic field each iron oxide grain has a permanent magnetic moment. However the micron-sized particle they are embedded in has not because the grains are randomly oriented. When a magnetic field is applied to the suspension, the permanent magnetic moment of each grain orients in the field direction causing each MR particle to acquire a net dipole moment.

Optical techniques, which are not invasive, are probably the best choice in order to characterize the microstructural change that takes place in MR suspensions when external field are applied. We are using scattering dichroism, SALS and videomicroscopy.
Detailed information on the scattering dichroism theory and experimental setup can be found here.

particles  
   aggregation

UNIAXIAL FIELDS

When an uniaxial magnetic field is applied on a colloidal suspension of magnetizable particles, the suspension experiences a dramatic change in its mechanical (rapid and drastic increase of the fluid viscosity) and optical properties. Such a change is due to the aggregation of the suspended particles which form clusters of macroscopic size usually in the form of chains oriented in the field direction.

We investigated the different aggregation and relaxation processes that govern the kinetics behavior of magnetizable colloidal systems by using scattering dichroism, small-angle-light-scattering (SALS) and videomicroscopy tecniques.
uniaxial_aggregation aggregation


ROTATING FIELDS

  When a rotating field is applied these chain-like induced structures rotate synchronously with the magnetic field but lag behind with a constant retarded phase angle, which increases when increasing rotating field frequency. For low rotational frequencies continuous fragmentation and aggregation of chains have been observed during the rotation process. The average length of the chains decreases with the inverse of the square root of the field frequency. The Mason number (ratio of viscous to magnetic forces) governs the rotational dynamics for suspensions where the magnetic energy dominates the thermal one. As rotational frequency increases, the viscous forces start to dominate over the magnetic ones and the aggregation process is inhibited and the formation of more isotropic structures occur.
See videos below

rotating chains 0.001Hz
rotating chains 0.05Hz rotating simulations
rotating chains 0.1Hz

rotating chains 1Hz

When we increase further the frequency of the applied field (100-1500 Hz), sheet-like structures are form in the plane of the field. In this way we generate transient diffraction gratings
whose grating spacing can be controlled by the amplitude of the applied field. This absorption gratings are characterized by absoption an light scattering measurements.

grating_diffraction_side_view grating_diffraction_top_view scattering lobes


To learn more about our work you can check my Thesis entitled: Study of the dynamics in MR suspensions subject to external fields by means of optical techniques: aggregation processes, structure formation and temporal evolution [English version  PDF (7,8 kB)].

Collaborators

Gerald G. Fuller, Chemical Engineering Department, Stanford University, CA

Miguel Angel Rubio
, Departamento Física Fundamental, UNED
Oscar G. Calderón, Departamento Optica, UCM
James E. Martin, Sandia National Laboratories, Albuquerque, NM

Publications 
Proceedings: International Meetings
Proceedings: National Meetings

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