Phase switching of ordered arrays of liquid crystal emulsions

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D. Rudhardt, A. Fernandez-Nieves, D.R. Link, and D. A. Weitz Applied Physics Letters 82, 2610 (2003).


Soft Matter Keywords

Liquid Crystals, Emulsion, Viscous Drag, Surface Tension

Summary

The authors report a unique fabrication method for producing electro-optic phase gratings from liquid crystal emulsions. They use monodisperse liquid crystal doplets that spontaneously form into an ordered two dimensional hexagonal close packed monolayer, and show that the intensity and direction of incident light can be modulated by applying an electric field to the liquid crystal. The use of monodisperse droplets allows for the switching between diffracting and transparent states to occur in short times, and at low electric fields (0.1 V/um).

Experiment Details

Monodisperse droplets of nematic crystal are generated by extrusion through a thin capillary and put into an emulsion with water and polyvinyl alcohol. The emulsion is then placed on an indium tin oxide (ITO) coated glass cover slip and the water is slowly evaporated. As the concentration increases, at some point these droplets spontaneously order into a hexagonal close packed monolayer (Fig 1a). As more of the water is evaporated, the droplets change from being spherical to being hexagonal in shape (Fig 1b). Another layer of ITO coated glass is then placed on top of the liquid crystal monolayer, which allows for electric fields to be applied to the array.

Figure 1 - The liquid crystal monolayer before (a), and after (b) drying.

Results

When an electric field is applied between the two plates, the intensity of the transmitted light, and first order diffraction peak changes dramatically (Figure 2 a and b). This can be explained as follows. When no voltage is applied, the suspended particles are arranged in random orientations and scatter the light, so that the glass panel looks opaque. When voltage is applied, the suspended particles align and let light pass. The degree of transparency can be controlled by the magnitude of the applied voltage.


Figure 2 Transmitted intensity (a) and first order diffraction peak (b) as a function of applied electric field.

Conclusions and Soft Matter Discussion

Polymer-dispersed liquid crystals (PDLCs) are a relatively new class of materials that hold promise for many applications ranging from switchable windows to projection displays. These materials, which are simply a combined application of polymers and liquid crystals, are the focus of extensive research in the display industry.

The ability to make monodisperse droplets of the liquid crystal, and have them arrange in a hexagonal structure is crucial to the successful operation of this device. The authors use different sized capillary tubes in order to make different size drops. When a droplet at the tip of their capillary grows to reach a critical size, viscous drag exceeds the surface tension and the droplet breaks off.