Microscopic shape and contact angle measurement at a superhydrophobic surface.

We have studied the microscopic shape, contact angle and Laplace law behavior of the liquid-gas interfaces at a superhydrophobic surface. A superhydrophobic surface is immersed in water, and the radius of liquid gas menicsi that span between adjacent ridges of the surface texture is measured. The surface pattern consists of rectangular grooves, such that the sample is simultaneously an optical grating. The diffraction properties encode the shape of the menisci. The shape of the menisci is determined by measuring the intensity of several diffraction orders as a function of the incident angle, and fitting the data to numerical calculations of the diffraction. The uncertainty of the determined menisci deflections is a few nanometres. Observing the deflection as a function of externally controlled hydrostatic pressure, Laplace's law is probed for the menisci on the micrometre scale. The microscopic contact angle is determined by measuring the radius of the menisci prior to collapse. Close agreement with the macroscopic Young angle is found. A stability limit for the superhydrophobic-to-impregnated transition is given. The measurement is a microscopic analogue of 'bubble' and 'sessile drop' type methods.

Large bandwidth, highly efficient optical gratings through high index materials.

We analyze the diffraction characteristics of dielectric gratings that feature a high index grating layer, and devise, through rigorous numerical calculations, large bandwidth, highly efficient, high dispersion dielectric gratings in reflection, transmission, and immersed transmission geometry. A dielectric TIR grating is suggested, whose -1dB spectral bandwidth is doubled as compared to its fused silica equivalent. The short wavelength diffraction efficiency is additionally improved by allowing for slanted lamella. The grating surpasses a blazed gold grating over the full octave. An immersed transmission grating is devised, whose -1dB bandwidth is tripled as compared to its fused silica equivalent, and that surpasses an equivalent classical transmission grating over nearly the full octave. A transmission grating in the classical scattering geometry is suggested, that features a buried high index layer. This grating provides effectively 100% diffraction efficiency at its design wavelength, and surpasses an equivalent fused silica grating over the full octave.

Micromachined Fabry-Pérot interferometer with embedded nanochannels for nanoscale fluid dynamics.

We describe a microfabricated Fabry-Pérot interferometer with nanochannels of various heights between 6 and 20 nm embedded in its cavity. By multiple beam interferometry, the device enables the study of liquid behavior in the nanochannels without using fluorescent substances. During filling studies of ethanol and water, an intriguing filling mode for partially wetting water was observed, tentatively attributed to the entrapment of a large amount of gas inside the channels.

Nanometer-resolved collective micromeniscus oscillations through optical diffraction.

We study the dynamics of periodic arrays of micrometer-sized liquid-gas menisci formed at superhydrophobic surfaces immersed into water. By measuring the intensity of optical diffraction peaks in real time, we are able to resolve nanometer-scale oscillations of the menisci with submicrosecond time resolution. Upon driving the system with an ultrasound field at variable frequency, we observe a pronounced resonance at a few hundred kilohertz, depending on the exact geometry. By modeling the system using the unsteady Stokes equation, we find that this low resonance frequency is caused by a collective mode of the acoustically coupled oscillating menisci.