Controllable Propulsion by Light: Steering a Solar Sail via Tunable Radiation Pressure

(1 of 6) 1600668 polymer dispersed liquid crystal (PDLC) films stand out due to their ability to be switched from an opaque to a transparent state with the applied external electric field without the need for polarizers. Since its discovery,[13] researchers have extensively explored the potential applications of PDLCs in the areas of flat panel displays,[14] smart windows,[15,16] microlens,[17] etcetera. PDLC films consist of liquid crystal microdroplets dispersed in a polymer matrix. The liquid crystal droplets are optically birefringent with ordinary refractive index no and extraordinary refractive index ne, while the polymer is an optically isotropic material with refractive index np. In the absence of an applied electric field (off state), the optical axes of individual bipolar droplets align randomly, resulting in spatial variations of refractive indices across the film. Liquid crystal microdroplets whose refractive indices differ from polymer matrix strongly scatter light and the PDLC film appears hazy. When enough voltage is applied across the film (on state), the liquid crystal molecules, which have positive dielectric anisotropy, align their optical axes with the electric field so that light incident normal to the film experiences a refractive index no in the liquid crystal droplets. By choosing the appropriate liquid crystal and polymer so that no and np are equal, the film becomes highly transparent as a result of reduced light scattering in the on state.[18] Unlike traditional liquid crystal displays, which need polarizers and waste half of the light in the bright state, PDLCs do not require polarizers, offering much higher optical/energy efficiency. PDLCs also have significant optical contrast between on and off states over a broad range of the solar spectrum and the ability to be fabricated on a flexible substrate, both of which make it an ideal material for solar sail attitude control. Further, researchers have recently shown that polyimide-based PDLC films have potential for aerospace applications due to radiation tolerance.[19–21] In this Communication, we present a steerable solar sail concept based on a PDLC that switches between transparent and scattering states, enabling attitude control without mechanically moving parts or chemical propellant. Devices are fabricated and characterized (transmission, reflection, absorption and scattering) over the visible and near infrared range of solar spectrum (400–1100 nm) and are found to outperform previous designs by more than a factor of four in terms of overall weighted momentum switchablility between on and off states. Devices require no power in the diffusely reflective state and dissipate <0.5 mW cm−2 while in the on state, showing great potential as a low-power switching mechanism for solar sail attitude control. Figure 1 shows an exemplary design of a solar sail incorporating PDLC devices. The main part of the solar sail is coated with highly reflective aluminum to provide maximum thrust from solar radiation pressure. PDLC devices are attached to the sail edges to maximize the torque generated from the radiation Controllable Propulsion by Light: Steering a Solar Sail via Tunable Radiation Pressure