(480g) Magnetic Displays - Magnetically Controlled Particle Orientation Enables Tunable Reflectivity

Controlled orientation of anisotropic micro- and nanostructures in fluids attracts considerable interest in biology and nanotechnology. Electric field induced orientation is common, mostly used for assembling and ordering particles. Magnetic induced orientation was thought to restrict the anisotropic structures to magnetic or magnetically labeled particles. However, recent studies with ferrofluids as forcing agent have shown the possibility of even orienting nonmagnetic microwires by magnetic fields [1]. Here, we present a system with tunable optical properties which is based on the magnetically controlled orientation of aluminum microflakes incorporated in a stabilized, highly superparamagnetic ferrofluid dispersion. By the application of an external magnetic field the orientation of the highly reflecting aluminum flakes could be switched from parallel to vertical, allowing a change in reflectivity. Theoretical investigations have shown that the mechanism of the orientation of the aluminum flakes can be related to a continuum approach of the magnetic fluid. The highly field sensitive change of reflectivity allowed us to demonstrate a possible area of application. A display without the need of background light and a switching time of less than 100 milliseconds was constructed, in which the optical features are visible even under a small angle observation. Besides large screen applications also stationary display (e-paper) or tunable heat isolating windows could be application areas, with the benefit of low energy use [2]. Figure: Switchable Reflectivity. Magnetic fluid film (0.2 mm) between two capton foils (left) reflecting due to a magnetic field parallel to the plane and (right) transparent, due to a perpendicular field. [1] C. Ooi, R.M. Erb, B.B. Yellen, J. Appl. Phys. 103 (2008) 07E910. [2] S.B. Bubenhofer, E.K. Athanassiou, R.N. Grass, F.M. Köhler, M. Rossier, W.J. Stark, in preparation (2009).