Transparent superhydrophobic surfaces for applications of controlled reflectance.

This work involves a new optical application for transparent superhydrophobic materials, which enables low-energy optical contact between a liquid and solid surface. The new technique described here uses this surface property to control the reflectance of a surface using frustration of total internal reflection. Surface chemistry and appropriate micro-scale and nano-scale geometries are combined to produce interfaces with low adhesion to water and the degree to which incident light is reflected at this interface is controlled by the movement of water, thereby modifying the optical characteristics at the interface. The low adhesion of water to superhydrophobic surfaces is particularly advantageous in imaging applications where power use must be minimized. This paper describes the general approach, as well as a proof-of-principle experiment in which the reflectance was controlled by moving a water drop into and out of contact with a superhydrophobic surface by variation of applied electrostatic pressure.

Control of reflection at an optical interface in the absence of total internal reflection for a retroreflective display application.

Reflection at an interface between two materials can be modulated by means of varying the optical properties at the interface. We have studied this modulation of the reflected light with an aim to develop a flashing retroreflector for roadside conspicuity applications. Reflectance modulation has previously been studied under the conditions of total internal reflection (TIR), where a light-absorbing material placed in the associated evanescent wave region can be used to attenuate the intensity of the reflected light. If instead the light rays strike the interface at an angle that is slightly smaller than the critical angle required for TIR, they instead undergo a substantial, but partial, reflection. We have demonstrated that an analogous attenuation effect to the TIR situation is observed, even though there is no evanescent wave present under these circumstances. We have studied this behavior and have developed a model to describe the motion of the absorbing material and the related interference effects that occur.

Controlled frustration of total internal reflection by electrophoresis of pigment particles.

A model based on geometrical optics has been developed to describe the photometric observations associated with a novel method to control the reflectance of a surface. In this new reflectance modulation approach, electrophoresis of pigment particles is used to absorb light reflected by total internal reflection (TIR). The pigment particles are sufficiently small that they substantially do not scatter light, but rather they modify the effective refractive index at the reflection interface. An incident light ray interacting with this modified effective index is attenuated in a spectrally selective manner. Although frustrated TIR has been understood and used in various applications for some time, in this case it is used to substantially modify the color of the reflected light, which to our knowledge has not been previously reported. A numerical model of the pigment particle distribution has been developed to describe the observations.

Control of reflectance of liquid droplets by means of electrowetting.

It has recently been noted that hemispherical structures have useful reflection characteristics. We describe a new application that makes use of these characteristics by controlling the reflectance of a surface composed of an array of hemispherical liquid droplets. In this system the reflectance state is spatially controlled through the use of electrowetting to alter the shape of an array of droplets. This may have an application in the field of electronic image displays.

Electrical modulation of diffractive structures.

We present an experimental demonstration of two new methods for electronically modulating diffractive structures. The first involves the creation of periodic displacement of a liquid-air interface by application of a spatially modulated electric field produced by an array of electrodes. The second method also uses an electrode array but creates a diffraction grating by selectively attracting and repelling electrophoretic particles in a dielectric fluid. Potential areas of application of these techniques include controllable holography and wavelength division multiplexing.