Optical sensor for detecting colloidal phase transitions induced by magnetic fields

In this paper we present the application of a new optical sensor for monitoring colloidal phase transitions in magnetic fields. The sensor utilizes the fact that the absorption of light depends strongly on the colloidal structure, and we demonstrate how it can be used to monitor magnetically induced phase transitions in colloidal solutions. Both static and dynamic response curves are recorded, thus providing useful information about the structural rearrangement. We find that the optical transmittance versus the magnetic field is path dependent for larger particles, and therefore depends on the history of the colloidal system. Furthermore, it is revealed that the colloidal assemblies exhibit memory-dependent kinetics, where the response times may differ by more than a factor of 5. We apply the sensor to study the kinetics of colloidal structural arrangement in a polymer solution, and find that the optical transmittance modulation decays with time, thus suggesting an increase in the number of immobile aggregates forced by the applied magnetic field.