Smart electro- and magneto-responsive colloids of pigment particles
Colloidal suspensions of anisotropic particles can show liquid crystalline properties at sufficiently high concentration of the particles. Nematic order have been found and extensively studied in suspensions of tobacco mosaic viruses (TMV) and fd-viruses, clay particles, gibbsite and magnetic goethite. In our research, conducted together with Susanne Klein from HPLabs (Bristol, UK), we study properties of colloidal suspensions formed by pigment rod-like and plate-like particles in a non-polar solvent. Already at low concentrations (<15 wt/%) those suspensions show liquid crystalline order manifested by the schlieren texture in polarizing microscopy.
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Electro-optical switching of the pigment dispersion observed by polarising microscopy.
Schlieren texture of a thin layer of the suspension under polarising microscope. This manifests orientational order of the particles.
A number of fascinating effects have been observed in electric field such as homeotropic-to-planar transition and several kind of electroconvective patterns. Perhaps, one of the most interesting feature of this suspension is the ability to reversibly switch in electric field. The switching is accompanied by alignment of the particles along the electric field which is usually applied to thin layer of the suspension confined between two glass plates (cell). If the electric field is applied in the cell plane, birefringent is induced which is manifested by increased transmission of the cell observed in a polarizing microscope. The switching character is complex: it is accompanied by a current transient. Absorption of light leads to a reversible enhancement of the current transient and affects the character of the electro-optical switching.
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