R. Esquivel-Sirvent

Scaling of micro- and nanodevices actuated by Casimir forces

The effect of the Casimir force in micro- and nanoelectromechanical systems is examined taking fully into account the dielectric properties of the materials, as well as the finite thickness of movable elements in micro- and nanosystems. The resulting equations are exact, and from the bifurcation diagrams the critical separation before jump-to-contact is determined. It is shown how the critical separation changes, for example, with the dielectric properties of the materials and how these systems can be rescaled based on the information from the bifurcation diagrams.

Scattering of shear horizontal piezoelectric waves in piezocomposite media

The theory of shear horizontal wave scattering processes in layered piezoelectric composites is discussed in terms of a recursive system of equations involving the piezoelectric impedance. Piezoelectric materials of hexagonal 6 mm symmetry are considered. The behavior of an incident shear horizontal piezoelectric wave is analyzed as a function of the material properties, layer thicknesses, and frequency (ω). By an appropriate choice of the materials and layer thicknesses, frequencies at which almost all energy is transmitted can be found, optimizing the properties of the system for ultrasound transducers. This behavior is also dependent on the incident angle. Furthermore, most laminated materials are bonded using polymers. We show that adding these polymers hampers the response of the piezoelectric laminated system, localizing the transmission at particular incident angles. Thus, sharp spikes of ultrasonic pulses could be generated with these laminated structures.

Casimir Forces in Nanostructures

We present a theoretical calculation of Casimir forces in structures made of parallel slabs that can be made of dispersive and absorptive materials. The materials are characterized by the reflectivity amplitude coefficients of the vacuum modes between the slabs. In particular, we present results for an antisymmetric configuration in which one plate is a metal and the other one a dielectric material. As a reference, we also calculate the force for the symmetric case (two metallic or two dielectric slabs). Our results show that the Casimir force could have a relevant contribution to the interaction between the tip and sample in atomic force microscopy experiments.

Pull-in control due to Casimir forces using external magnetic fields

We present a theoretical calculation of the pull-in control in capacitive microswitches actuated by Casimir forces using external magnetic fields. The external magnetic fields induce an optical anisotropy due to the excitation of magnetoplasmons that reduces the Casimir force. The calculations are performed in the Voigt configuration and the results show that as the magnetic field increases the system becomes more stable. The detachment length for a cantilever is also calculated for a cantilever, showing that it increases with increasing magnetic field. At the pull-in separation, the stiffness of the system decreases with the increasing magnetic field.

Stability and the proximity theorem in Casimir actuated nano devices

A brief description of the stability problem in micro and nano electromechanical devices (MEMS/NEMS) actuated by Casimir forces is given. To enhance the stability, we propose the use of curved surfaces and recalculate the stability conditions by means of the proximity force approximation. The use of curved surfaces changes the bifurcation point, and the radius of curvature becomes a control parameter, allowing a rescaling of the elastic restitution constant and/or of the typical dimensions of the device.

High-multipolar effects on the Casimir force: The non-retarded limit

We show that the dispersive force between a spherical nanoparticle (with a radius

Spatial dispersion in Casimir forces: a brief review

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Ultrasonic velocity and absorption measurements for poly(acrylic acid) and water solutions

100 nm) and a substrate is enhanced by several orders of magnitude when the sphere is near to the substrate. We calculate exactly the dispersive force in the non-retarded limit by incorporating the contributions to the interaction from of all the multipolar electromagnetic modes. We show that as the sphere approaches the substrate, the fluctuations of the electromagnetic field, induced by the vacuum and the presence of the substrate, the dispersive force is enhanced by orders of magnitude. We discuss this effect as a function of the size of the sphere.We implement a spectral representation formalism to calculate exactly the non-retarded Casimir force or dispersive van der Waals force, between a spherical nanoparticle and a planar substrate beyond the dipolar approximation. For a sphere of radius R separated a distance z from the substrate, we find that high-order multipole interactions induce extra factors R/z in the force, as compared to the dipole power law of ~ R3/z3. As a consequence, at small separations the non-retarded Casimir force increases by several orders of magnitude with respect to that estimated in the usual dipole approximation.

Ultrasonic absorption and velocity measurements for poly (vinyl alcohol) and water solutions

We present the basic principles of non-local optics in connection with the calculation of the Casimir force between half-spaces and thin films. At currently accessible distances L, non-local corrections amount to about half a per cent, but they increase roughly as 1/L at smaller separations. Self-consistent models lead to corrections with the opposite sign as models with abrupt surfaces.

Mixing rules and the Casimir force between composite systems

The ultrasonic velocity and absorption at 21 MHz and the shear viscosity were measured for poly(acrylic acid) and water solutions, as a function of concentration and temperature. The concentrations used were 1, 3, 5, 9% by weight and the temperatures were 25 °, 30 °, 35 °, 40 °, 45 °C. It was found that the velocity increases with the concentration and temperature. The absorption α/f2 decreased with increasing temperature and increased with increasing concentration. The shear viscosity decreased with temperature and increased with concentration.