Casimir-Lifshitz Force Out of Thermal Equilibrium and Asymptotic Nonadditivity

We investigate the force acting between two parallel plates held at different temperatures. The force reproduces, as limiting cases, the well-known Casimir-Lifshitz surface-surface force at thermal equilibrium and the surface-atom force out of thermal equilibrium recently derived by M. Antezza et al., Phys. Rev. Lett. 95, 113202 (2005). The asymptotic behavior of the force at large distances is explicitly discussed. In particular when one of the two bodies is a rarefied gas the force is not additive, being proportional to the square root of the density. Nontrivial crossover regions at large distances are also identified

Optimization of a Thermal Flow Sensor for Acoustic Particle Velocity Measurements

In this paper, a thermal flow sensor consisting of two or three heated wires, the Microflown, is treated for application to acoustic measurements. It is sensitive to flow ("particle velocity"), contrary to conventional microphones that measure acoustic pressures. A numerical analysis, allowing for detailed parametric studies, is presented. The results are experimentally verified. Consequently, improved devices were fabricated, and also sensors with a new geometry consisting of three wires, instead of the usual two, of which the central wire is relatively most heated. These devices are the best performing Microflowns to date with a frequency range extending from 0 to over 5 kHz and a minimum detectable particle velocity level of about 70 nm/s at 2 to 5 kHz (i.e., 3 dB PVL or SPL, corresponding to a pressure of 3.1/spl middot/10/sup -5/ Pa at a free field specific acoustic impedance)

Surface Assisted Combustion of Hydrogen-Oxygen Mixture in Nanobubbles Produced by Electrolysis

The spontaneous combustion of hydrogen–oxygen mixture observed in nanobubbles at room temperature is a puzzling phenomenon that has no explanation in the standard combustion theory. We suggest that the hydrogen atoms needed to ignite the reaction could be generated on charged sites at the gas–liquid interface. Equations of chemical kinetics augmented by the surface dissociation of hydrogen molecules are solved, keeping the dissociation probability as a parameter. It is predicted that in contrast with the standard combustion, the surface-assisted process can proceed at room temperature, resulting not only in water, but also in a perceptible amount of hydrogen peroxide in the final state. The combustion time for the nanobubbles with a size of about 100 nm is in the range of 1–100 ns, depending on the dissociation probability

Applications of Casimir forces:Nanoscale actuation and adhesion

Here, we discuss possible applications of the Casimir forces in micro- and nanosystems. The main part of this paper is devoted to actuation with quantum fluctuations and to the relative contribution of van der Waals and Casimir interactions to adhesion. Switching between the amorphous and crystalline states of phase change materials could generate force contrast sufficient for actuation, though for practical applications, the influence of protective capping layers and volume compression have to be better understood. Resilience against the pull-in instability is also a critical point defined by the material choice, dissipation in the system, and roughness of the surfaces. The adhesion induced by the Casimir forces is omnipresent, and it can play a pivotal role in unwanted stiction demanding deeper understanding. The open problems are the distance upon contact and the relative area of the real contact since both of them control the adhesion. An experiment designed to answer these questions is briefly discussed

Evanescent character of the repulsive thermal Casimir force

The physical origin of the negative thermal correction to the Casimir force between metals is clarified. For this purpose the asymptotic behavior of the thermal Casimir force is analyzed at large and small distances in the real frequency representation. Contributions from propagating and evanescent waves are considered separately. At large distances they cancel each other in substantial degree so that only the attractive Lifshitz limit survives. At smaller separations the repulsive evanescent contribution of s-polarization dominates in the case of two metals or a metal and a high-permittivity dielectric. Common origin and order of magnitude of the repulsion in these two cases demonstrate naturalness of the controversial large thermal correction between metals.Comment: to be published in Phys. Rev.

Roughness correction to the Casimir force beyond perturbation theory

Up to now there has been no reliable method to calculate the Casimir force when surface roughness becomes comparable with the separation between bodies. Statistical analysis of rough Au films demonstrates rare peaks with heights considerably larger than the root-mean-square (rms) roughness. These peaks define the minimal distance between rough surfaces and can be described with extreme value statistics. We show that the contributions of high peaks to the force can be calculated independently of each other while the contribution of normal roughness can be evaluated perturbatively beyond the proximity force approximation. The developed method allows a reliable force estimation for short separations. Our model explains the strong hitherto unexplained deviation from the normal Casimir scaling observed experimentally at short separations.Comment: 6 pages, 2 figures, to be published in EP

Optimisation of a two-wire thermal sensor for flow and sound measurements

The Microflown is an acoustic sensor measuring particle velocity instead of pressure, which is usually measured by conventional microphones. In this paper an analytical model is presented to describe the physical processes that govern the behaviour of the sensor and determine its sensitivity. The Microflown consists of two heaters that act simultaneously as sensors. Forced convection by an acoustic wave leads to a small perturbation of this temperature profile, resulting in a temperature difference between the two sensors. This temperature difference, to which the sensitivity is proportional, is calculated with perturbation theory. Consequently the frequency dependent behaviour of the sensitivity is analysed; it is found that there are two important corner frequencies, the first related to the time constant velocity of heat diffusion between the sensors, the second related to the heat capacity of the heaters. The developed model is verified by experiments. Previously a very good model has been given for the performance of the Microflown in a channel, i.e. with both heaters between fixed walls walls in the positive and negative z-direction. Here, a model is presented that describes the situation of the present used sensors: without walls under and above them. Model predictions are compared to experimental result

Collective behavior of bulk nanobubbles produced by alternating polarity electrolysis

Nanobubbles in liquids are mysterious gaseous objects having exceptional stability. They promise a wide range of applications but their production is not well controlled and localized. Alternating polarity electrolysis of water is a tool that can control production of bulk nanobubbles in space and time without generating larger bubbles. Using the schlieren technique a detailed three-dimensional structure of a dense cloud of nanobubbles above the electrodes is visualized. It is demonstrated that the thermal effects produce different schlieren pattern and have different dynamics. A localized volume enriched with nanobubbles can be separated from the parent cloud and exists on its own. This volume demonstrates buoyancy from which the concentration of nanobubbles is estimated as 2x10^18 m^-3. This concentration is smaller than that in the parent cloud. Dynamic light scattering shows that the average size of nanobubbles during the process is 60-80 nm. The bubbles are observed 15 minutes after switching off the electrical pulses but their size is shifted to larger values of about 250 nm. Thus, an efficient way to generate and control nanobubbles is proposed.Comment: 8 pages, 7 figures, Supplemental, 3 video file