No abstract

Ferrofluids can be used to make deformable mirrors having highly interesting characteristics (e.g., extremely large strokes and low costs). Until recently, such mirrors were thought to be restricted to corrections of frequencies lower than 10 Hz, thus limiting their usefulness. We present counterintuitive results that demonstrate that the limiting operational frequency can be increased by increasing the viscosity of the ferrofluid. We tested the response of ferrofluids having viscosities as high as 494 cP, finding that they could allow an adaptive optics correction frequency as high as 900 Hz. We also demonstrate that we can counter the amplitude loss due to the high viscosity by overdriving the actuators. The overdriving technique combines high current, short duration pulses with ordinary driving step functions to deform the mirror. The integration of a FDM in a complete closed-loop adaptive optics system running at about 500 Hz thus appears to be a realistic goal in the near future.

Stable ferrofluids composed of positively charged magnetic iron oxide nanoparticles coated with 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MOEEAA) are prepared in ethylene glycol. These new ferrofluids exhibit a magnetic response that is equivalent to that found for corresponding citrate stabilized particles. Unlike the uncoated positively charged particles, nanoparticles coated with MOEEAA and dispersed in ethylene glycol remained stable in the presence of a magnetic field. Infrared spectra indicate that surface grafting occurs through the terminal carboxylate group which is bound to the iron oxide nanoparticles both through bridging and unidentate structures. A surface grafting density of 1.2 molecules/nm 2 is determined from thermogravimetry measurements. Although MOEEAA functionalization increases the stability of nanoparticle suspensions in ethylene glycol, surface charge is also essential for the prevention of particle agglomeration. Importantly, the MOEEAA stabilized ferrofluid is compatible with the deposition of surface films of silver nanoparticles and thus allows for the preparation of magnetically deformable liquid mirrors.

We present the research status of a deformable mirror made of a magnetic liquid whose surface is actuated by a triangular array of small current carrying coils. We demonstrate that the mirror can correct a 11 µm low order aberrated wavefront to a residual RMS wavefront error 0.05 µm. Recent developments show that these deformable mirrors can reach a frequency response of several hundred hertz. A new method for linearizing the response of these mirrors is also presented.

A procedure for the fabrication of aluminum‐coated reflective PDMS thin films having a low surface roughness and high reflectance is presented. Elastomer films are prepared by spin‐coating on a sacrificial layer that permits subsequent release from the substrate. Appropriate surface treatments are established for the deposition of a thin, highly reflective metal coating. The composite membranes are deposited on an ethylene glycol based ferrofluid to fabricate a new ferrofluidic deformable mirror. Surface deformations of a few micrometers can be produced with low strength magnetic fields. The reflective elastomer membranes do not alter the rheological behavior of the ferrofluid in magnetic fields and thus offer new perspectives for the use of ferrofluidic mirrors in adaptive optics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44542.