Lorentz Center - Dispersion Forces and Nano-Electro-Mechanical Systems from 11 Dec 2006 through 15 Dec 2006
  Current Workshop  |   Overview   Back  |   Home   |   Search   |     

    Dispersion Forces and Nano-Electro-Mechanical Systems
    from 11 Dec 2006 through 15 Dec 2006

 
Title: A novel experimental approach for the measure of the Casimir effect at large distances

keynote talks

 

 

Monday, December 11, 2006

 

New developments in the Casimir effect

C. Genet

The Casimir force is an attractive force arising between two mirrors in vacuum. It is named after the famous Dutch physicist Hendrik Brugt Gerhard Casimir who predicted its existence in 1948. Compared with the ideal situation Casimir considered at that time, a number of factors have to be taken into account when one wants to describe correctly real Casimir force measurements. I will review the influence of some main corrections coming from material properties, finite temperature and surface roughness of the plates. Furthermore, I will present some recent calculations on the Casimir interaction between corrugated plates, which are not relying on the Proximity Force Approximation. The plate corrugation leads to a lateral component of the Casimir force as well as to a Casimir torque.

 

 

A novel experimental approach for the measure of the Casimir effect at large distances

P. Antonini, G. Bressi, G. Carugno, G. Galeazzi, G. Messineo, G. Ruoso

 

We describe an experimental apparatus and the measurement technique aimed to measure the Casimir effect in the parallel plates configuration in the 3-6 micrometer range. The apparatus is based on a mechanical resonator and will use a homodyne detection technique to sense the Casimir force in the plane-parallel configuration. Calibrations show that a force of 5 x 10-11 N can be measured with this set-up. This corresponds to the Casimir force between the two 1 cm2 aluminium parallel plates at a distance of 5.5 micrometer. It is possible to measure the built-in voltage and to control it with an accuracy of 1 mV, that opens up the possibility to measure the Casimir force at large distances. The minimum distance between the surfaces so far obtained is 10 micrometer, limited by imperfect parallelization, at level of 10-3 rad. Improvement are in progress to increase the parallelization at a level of 10-4 rad, and to decrease other sources of systematic errors.

 

Detectability of dissipative motion in quantum vacuum through nanomechanical structures and atomic physics detection schemes

R. Onofrio

 

I will discuss a feasibility study to detect the dynamical Casimir effect using nanomechanical structures and cold atom systems. A high frequency mechanical resonator driven in resonance is expected to create Casimir photons. The photons are stored in a high quality electromagnetic cavity  and detected through their interaction with ultracold alkali atoms prepared in an inverted population of hyperfine states, or with ultrasensitive detection schemes based upon Rydberg states.

 

 

A possible route to quantum control of a nanomechanical resonator

A. Cleland

 

We are engaged in a project to investigate mechanical resonators in the single-phonon quantum regime. The key to achieving quantum control of a mechanical system is to use an extremely strong nonlinearity in either the resonator or in its measurement system; we have chosen to use the latter. We are coupling the highly nonlinear inductance of a Josephson phase qubit with a microwave frequency mechanical resonator, which we hope will enable us to demonstrate the coherent creation and manipulation of single phonons in the resonating element. The mechanical system is a novel type of high quality factor, GHz frequency piezoelectric resonator, which can have unprecedented quality factor in this frequency band. The quantum mechanical properties of the resonators, especially in the single-phonon regime, will be probed by the Josephson qubit, and we plan to measure the single phonon T1 decay and T2 coherence times. I will describe our progress to date in developing this unique system.

 

 

Tuesday, December 12, 2006

 

Acoustic Casimir effect and its applications in MEMS

R. Esquivel-Sirvent

 

The most representative manifestation of the Casimir effect is the attraction of two parallel neutral plates due to quantum vacuum fluctuations. A classical analog  of this effect was shown experimentally by Larraza et al. [1], who placed two parallel plates in the presence of acoustic white noise, showing that there was a force between the plates similar to the Casimir force.  The theory of the acoustic Casimir effect for arbitrary materials was later presented in [2].  Another important breakthrough in acoustics has been achieved with MEM technologies giving rise to what is known as nano phononics. Examples include excitonic transport using surface acoustic waves and more recently MEM resonators capable of generating  sound at a frequency close to tera Hertz [3].  These examples are indicative of the possibility of rescaling acoustic phenomena from the macroscopic to the micro and nano scale. In this talk, the fundamentals of the experiment and theory of the Acoustic Casimir effect are presented, showing the differences and similarities with the quantum Casimir effect. In particular, unlike the quantum effect, the acoustic case shows a force that changes sign in a periodic way due to the finite bandwidth of the noise.   This allows the acoustic Casimir effect to be use as an actuating force in mems and nems, as will be discussed in this talk.

[1] A. Larraza, C.D. Holmes, R. T. Susbilla and B. Denardo, “An acoustic Casimir Effect” , J. Acoust. Soc. Am. vol.  103 (1998) 276.   

[2]  J. Barcenas, L. Reyes and R. Esquivel-Sirvent, “ Acoustic Casimir Pressure for Arbitrary Materials”, J. Acoust. Soc. Am.  vol.  116 (2004) 720.

[3]  A.  Huynh, N. D. Lanzellotti-Kimura et al. “Subterahertz Phonon Dynamics in Acoustic Nanocavities”, Phys. Rev. Lett. vol. 97, (2006) 115502.

 

 

Nanomechanical sensing and metrology: Recent progress

K. L. Ekinci

 

Nanoelectromechanical systems (NEMS) have been at the center of recent applied and fundamental research. This presentation will start with a brief description of our recent work on nanomechanical mass sensing. It will then outline some of the challenges involved in realizing a practical NEMS mass sensor and focus on our efforts in addressing these challenges. One of the challenges, namely the operation of a nanomechanical resonator in a rarefied gas atmosphere, has led us to re-investigate a well-known fluid dynamics problem: Stokes’ second problem of the oscillating plate in a fluid.  At the frequencies of NEMS motion, Stokes’ second problem needs to be reformulated in order to accurately describe NEMS motion. On the other hand, our efforts to develop tunneling displacement transducers have resulted in progress towards a functional radiofrequency scanning tunneling microscope (STM).

 

 

Dynamic Mode AFM Measurements of the Casimir Force

G. Torricelli and C. Binns

 

We have used dynamic mode AFM to measure the Casimir force between different geometric gold-coated shapes, including a 10µm diameter Au-coated sphere, and Au surfaces. Due to the small size of the sphere we have been able to demonstrate good agreement with the Lifshitz model down to a separation of 55nm. For closer distances an increase in the power-law of the force is observed due to surface roughness. Preliminary data on the interaction between patterned surfaces will also be presented.

 

 

Wednesday, December 13, 2006

 

Van der Waals forces in dynamic fluctuation phenomena in double membrane films

E. Kats

 

We investigate eigen-modes of double membrane films. The excitation spectrum includes overdamped squeezing mode and elastic modes. We demonstrate that thermal fluctuations and Van der Waals forces essentially modify the modes due to non-linear coupling to the transverse shear viscous hydrodynamic mode.

 

 

Casimir forces and the thinning of superfluid Helium films

J. Rudnick

 

Recent experiments by Garcia, Chan and co-workers provide striking evidence for Casimir forces mediated by both critical fluctuations and Goldstone bosons. I will comment on three aspects of their observations and the system they investigate: a) thinning of the film well inside the superfluid phase, b) the relatively substantial thinning of the film in the critical regime and c) the possible influence of the Kosterlitz-Thouless transition on the thinning of the Helium film

 

 

 

Cluster-Formation in DNA-Coated Colloidal Systems

T. Schmatko, B. Borzogui, N. Geerts, E. Eiser, W.C.K. Poon, and D. Frenkel

 

Over the past ten years, researchers have focused on building new types of nanomaterials based on DNA assemblies, involving the specific recognition of two complementary strands. If at the beginning the interest was essentially for the determination of   mismatch between base pairs within the structure of the genetic code1, the attention moves to a more material oriented goal, to promote for instance the creation of photonic crystals. Indeed, such specific assemblies have the ability to form reversible networks, (via the melting of the DNA double helix) with different degrees of order, which can be tuned by the strength of the interactions involved in the system. As far as we know, all groups have directed their efforts on short single stranded (ss) DNA. However a recent theoretical work predicts that very long flexible DNA, with a polymer like behavior, would allow the formation of various phases (from an interesting diamond crystal like to a 2D membrane like phase) that are forbidden for short DNA-colloids systems2. This paper directly inspires our work. We present the first experimental results of aggregates of DNA coated colloids made with l-phage DNA - a long double stranded DNA molecule of 16mm length. Confocal imaging shows that the formation of a cluster phase is strongly temperature dependent, with a very interesting reversible behavior around room temperature. We find that the inter-colloidal distance within the cluster is much smaller than the gyration radius of the DNA molecule. In order to understand better these observations, we compare our results with molecular dynamic simulations: we first focus on the attraction induced by bridging of polymers between colloids and then we look at the temperature effect on flexible polyelectrolyte in varying mono-valent salt concentrations.

1              C. A. Mirkin, R. L. Letsinger, R. C. Mucic, et al., Nature 382, 607 (1996).

2              A. V. Tkachenko, Phys. Rev. Lett 89, 148303 (2002).

 

 

 

Pull-in and stability analyses of NEMS switches due to dispersion forces

Ya-Pu Zhao

It has been recognized recently that vacuum induced forces played considerable roles in micro-, nano- and quantum-electromechanical systems (MEMS, NEMS and QEMS) with typical sizes in the micro- and nano-meter ranges. Actuation, pull-in and stiction are the three major concerns for the application of Casimir and van der Waals forces in MEMS, NEMS and QEMS.

My presentation will be in two major parts: First, a brief survey will be given about the research in my group on MEMS, NEMS, surface-stress based biosensors and molecular motors [1-7]. Second, a brief review will be presented about the research of pull-in and stability analyses of MEMS/NEMS switches by Casimir and van der Waals forces [8-13]. The dimensionless equilibrium equations of electrostatic torsional actuators are presented with the consideration of the vdW and Casimir torque. With the vdW and Casimir effects, the inherent instability of the actuators is dependent on the scales of structures. The critical tilting angle keeps constant approximately only in micron or larger scales, but it is not constant when the gap between two plates is in nano-scales. The pull-in voltage is also lower than that without consideration of vdW and Casimir torques. Without the electrostatic torque, the pull-in instability can still occur with small angle perturbation, and a critical gap is derived. Furthermore, the qualitative analyses of the equations of motion show that the equilibrium points of the corresponding autonomous system include stable focus point and center points, as well as unstable saddle points. The Hopf bifurcation points and fork bifurcation points also exist in the system. The phase portraits show the periodic, heteroclinic and homoclinic orbits.

Keywords: Pull-in instability; NEMS switches; Casimir and van der Waals forces; Hopf bifurcation; periodic, heteroclinic and homoclinic orbits.

References

1.        Ren Q, Zhao YP, Han L and Zhao HB. A nanomechanical device based on light-driven proton pumps. Nanotechnology, 17: 1778–1785 (2006).

2.        Ren Q, Zhao YP, Yue JC and Cui YB. Biological application of multi-component nanowires in hybrid devices powered by F1-ATPase motors. Biomedical Microdevices, 8(3): 201-208 (2006).

3.        Zhang Y and Zhao YP. Applicability range of Stoney's formula and modified formulae for a film/substrate bilayer. Journal of Applied Physics, 99: 053513 (2006).

4.        He FQ and Zhao YP. Growth of ZnO nanotetrapods with hexagonal crown. Applied Physics Letters, 88: 193113 (2006).

5.        Guo JG and Zhao YP. The size-dependent elastic properties of nanocrystals with surface effects. Journal of Applied Physics, 98: 074306 (2005).

6.        Yang CY and Zhao YP. Influences of hydration force and elastic strain energy on stability of solid film in very thin solid-on-liquid structure. Journal of Chemical Physics, 120(11): 5366-5376 (2004).

7.        Zhao YP, Wang LS and Yu TX. Mechanics of adhesion in MEMS-a review. Journal of Adhesion Science and Technology, 17(4): 519-546 (2003).

8.        Lin WH and Zhao YP. Dynamics behavior of nanoscale electrostatic actuators. Chinese Physics Letters, 20(11): 2070-2073 (2003).

9.        Wang GW, Zhang Y, Zhao YP and Yang GT. Pull-in stability study of nanotubes under van der Waals forces influence. Journal of Micromechanics and Microengineering, 14:1119-1125(2004).

10.   Lin WH and Zhao YP. Nonlinear behavior for nanoscales electrostatic actuators with Casimir force. Chaos, Solitons & Fractals, 23: 1777–1785 (2005).

11.   Lin WH and Zhao YP. Casimir effect on the pull-in parameters of nanometer switches. Microsystem Technologies, 11: 80-85 (2005).

12.   Guo JG and Zhao YP. Dynamic stability of electrostatic torsional actuators with van der Waals effect. International Journal of Solids and Structures, 43: 675-685 (2006).

13.   Guo JG and Zhao YP. Influence of van der Waals and Casimir forces on electrostatic torsional actuators. Journal of Microelectromechanical Systems, 13(6): 1027-1035 (2004).

 

The Shape Dependence of Fluctuation-Induced Forces

M. Kardar

 

The Casimir force is an attraction between parallel conducting plates due to quantum fluctuations of the electromagnetic (EM) field. Thermal fluctuations of correlated fluids (such as critical mixtures or superfluids) are also modified by boundaries, resulting in similar interactions. A nice demonstration is provided by the thinning of a wetting film of helium at and below the superfluid transition. Quantitative understanding of the latter requires inclusion of surface undulations. The EM Casimir force is also modified for corrugated surfaces in non-trivial fashion. I shall also discuss other non-trivial geometries, in particular addressing the possibility of a repulsive force for a piston, and the force between a plate and a cylinder.

 

Thursday, December 14, 2006

 

Measurements of static and dynamic forces between nonpolar surfaces in water: is the hydrophobic force a type of dispersion force?

J. Israelachvili

Recent improvements of direct force measuring techniques using the Surface Forces Apparatus (SFA) have allowed for direct measurement of the hydrophobic interaction potential between two surfaces in the distance regime from 100 Ĺ down to contact. There appears to be a rapidly decaying short-range force extending out to 10-20 Ĺ followed by a more slowly decaying interaction to ~100 Ĺ. The unusual polarizability and dielectric properties of water could be responsible for this interaction as a modified van der Waals attraction with a short-range component analogous to the non-retarded interaction and a longer ranged component analogous to the retarded force.

 

 

Non-equilibrium thermal effects in the Van der Waals forces

L. Pitaevskii

 

Long distance behaviour of the thermal Van-der Waals-Lifshitz force between an atom and the surface of a substrate is investigated in the absence of thermal equilibrium. When temperatures of the substrate and the environment are different, the new decay law 1/z3 of the force on large distances is discovered, which is slower than at thermal equilibrium. The force is of a quantum nature and attractive or repulsive depending on whether the temperature of the substrate is higher or smaller than the one of the environment. An elementary derivation of this law is presented. It is based on a picture of evanescent waves, created in vacuum by the black body radiation impinging on the surface near the angle of total reflection. Experimental prove of the theory by E. Cornell group in JILA is discussed. New effects in interaction between macroscopic bodies are predicted.

 

 

Casimir energies, stresses, and forces: When are they well defined and how can they be estimated?

R. Jaffe

 

Casimir pointed out  that the zero point energy of a quantum field changes when boundary conditions are applied, and that the change gives rise to a force that can be calculated for simple geometries. Modern experimental techniques enable precision measurements of Casimir forces. However, attempts to compute the Casimir energy for various geometries and boundary conditions are both technically difficult and plagued with divergences. Unlike the divergences of ordinary quantum field theory, that can be rendered finite by a renormalization program, divergences in the Casimir energy indicate that it depends on the cutoffs that distinguish real materials from idealized boundary conditions. I discuss where the divergences come from and when finite Casimir effects can be defined. Then I describe a semiclassical approximation based on ray optics that enables one to estimate Casimir forces for a variety of experimentally interesting geometries.

 

Friday, December 15, 2006

 

Oscillating cantilever as probe of sphere/plane non contact interactions

J. Chevrier, G. Jourdan, M. Hrouzek, G. Torricelli, M. Rodrigučs,, O. Dhez, F. Comin

 

Measurements of non contact forces between interacting surfaces meet nowadays a large interest due to the capability of MEMS and NEMS preparation especially as resonant nano-oscillators. Surfaces separated by vacuum gap of 100nm can strongly interact through both Casimir force and electrostatic interactions.

As well known, the necessary quantitative measurements of non contact interactions between surfaces face some severe difficulties:

-         These forces can spatially vary over many orders of magnitude on hundreds of nanometers.  A consequence is that a reasonable detection level at large distance is hardly compatible with mechanical stability at short distances.

-         Non linear effects are hardly avoidable.

-         Important noises inherent to the coupling with the external world (1/f, temperature, detection...) linked with dissipation. Thermally induced Brownian motion is often the dominant source of noise at resonance and is a limit at large distances.  

Measurements on real micro/nanosystems at room temperature have led many groups to build dedicated force machines and to apply specific measurement strategies. Here in introduction we shall first describe based on a commercial Omicron AFM the measured interplay between Casimir force and Brownian motion, then the changes in the lever damping induced by the Johnson noise in the electromechanical coupling. Second, a “home made” force machine based on a Fabry Pérot cavity for detection of displacement will be presented. It is designed for investigation of light mechanical effects from visible to X ray: bolometric effect, Casimir force and radiation pressure especially on structured surfaces at sub-micrometer scale. With this machine, measurement of the Casimir force gradient from 50nm up to about 500nm will be demonstrated and detailed analysis of the lever Brownian vibrations with/without sphere will be considered for the two first modes of vibration. Finally we shall describe a measurement method that could be considered for spatially rapidly varying forces based on cold damping techniques. As usual, cold damping here refers to damping through an external feedback loop of the thermal fluctuations and it is illustrated in the figure below: the spectral density of thermal displacement is measured at a lever mechanical resonance. As the feedback loop gain is increased, the temperature associated with this particular resonance mode decreases from 300K down to 30K, the largest damping allowed by the noise of optical detection.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure caption: A fast capacitive actuation driven by a feedback loop based on the cantilever speed progressively cancels the thermal fluctuations as the loop gain is increased.

 

Corresponding author contact information: joel.chevrier@grenoble.cnrs.fr

 

 

A theorem about the sign of Casimir forces between dielectric bodies

I. Klich

 

It is well known that Casimir forces between two flat mirrors are attractive. However, due to non-trivial dependence on geometry, suggestions were raised in which such forces may be repulsive between bodies of carefully chosen shape, such as two conducting hemispheres. Such proposals might be of importance in nano-mechanical applications as a solution to the stiction problem, and raise fundamental questions regarding Casimir forces. Therefore, it is of fundamental interest to understand the validity of such proposals. In the talk I will discuss recent results dealing with this problem. I will show that under symmetry conditions the Casimir force between two dielectric bodies is always attractive, independent of the exact form of the bodies or dielectric properties.  Due to it's generality we expect the theorem to play a role in other physical systems where fluctuating fields interact with immersed bodies.

 


 

Contributed talks

 

Monday, December 11, 2006

 

Dispersion interaction between dielectrics with non-ideal geometries

R. Golestanian

In this talk, a formulation will be presented for the calculation of theelectromagnetic--fluctuation forces for dielectric objects of arbitrary geometry at small separations, as a perturbative expansion in the dielectric contrast. The resulting Lifshitz energy automatically takes on the form of a series expansion of the different many-body contributions. The formulation has the advantage that the divergent contributions can be readily determined and subtracted off, and thus makes a convenient scheme for realistic numerical calculations, which could be useful in designing nano-scale mechanical devices.

 

 

Exploring the interactions of gold nanoparticles with their near-field environment: plasmonic van der Waals-Casimir interactions

 

U. Hakanson, M. Agio, S. Kühn, L. Rogobete, V. Sandoghdar

 

We present an experiment in which the interaction between a single gold nanoparticles and another particle or a substrate can be tuned by control of their separation using scanning probe technology. By recording the plasmon resonance of the coupled system as a function of the polarization of the incident field and the inter-particle distance, we explore the Casimir-van der Waals type interaction between the two particles. The distinct spectral changes of the scattered light from the particle pair are in good agreement with the outcome of finite difference time-domain (FDTD) calculations. We discuss the implications of our measurements and experimental technique for precise measurements of the Casimir type interactions and for studies of field enhancement in optical processes.

 

 

Tuesday, December 12, 2006

 

Interacting surface plasmons and the role of geometry on dispersive forces

C. E. Román-Velázquez and Cecilia Noguez

The role of geometry on dispersive forces is investigated by studying the force between different spheroidal particles and planar surfaces, due to the interaction between surface plasmons. The force is obtained, in the non-retarded limit, using a spectral representation formalism and calculating the Coulomb interaction between the surface plasmons of the macroscopic bodies with arbitrary dielectric properties. When the particle is close to the substrate the multipolar interactions induced by the substrate modify the electromagnetic response of the system. It is found that the force is a power-law function of the minimum separation between bodies, where the exponent value depends on the geometrical parameters of the system, like the aspect ratio among minor and major axes of the spheroid, as well as on the separation itself. In this work, we study in detail the dispersive force for particles with the same curvature but different aspect ratios that shows that in this case the Derjaguin Approximation is not applicable. We analyze in detail the interaction of oblate and prolate particles with a substrate. The influence of the geometry on dispersive forces can be very important for experiments using Atomic Force Microscopy.

 

Casimir forces and geometry

T. Emig

According to quantum mechanics, all space is filled with electromagnetic vibrations, even at ultracold temperatures. Two parallel uncharged metal plates limit the number of vibrations between them, creating an effective inward pressure that pushes the plates together -- known as Casimir effect. But the most challenging aspect of this effect is its dependence on geometry. Due to its topological nature, Casimir forces can be controlled by tailoring the shapes of the interacting bodies. However, due to the diffraction of vibrations and the non-additivity of fluctuation induced interactions, there is no intuitive way to tell how the force will change with the object's shape.

In this talk, I shall present a brief introduction to Casimir forces, and a new approach to study the geometry dependence of this interaction. An exact trace formula is presented which yields the relevant information of the spectrum of the Helmholtz wave equation in arbitrary geometries. Perturbative and numerical implementations of this formula yield new and unexpected forms for the Casimir interaction in rather simple geometries. Implications on the non-linear dynamics of nanomechanical systems and actuation schemes will be presented.

Low-frequency response of metals and related problems with the Casimir force

 

V. B. Svetovoy

 

Dependence of the Casimir force on the dielectric function of materials is discussed. For metals the low-frequency response is important for separations as small as 50 nm. Known data on gold dielectric properties are reviewed and the methods to extract the Drude parameters from the data are discussed. Significant dependence of the material response on the conditions of sample preparation is reviled. Variation of the force due to this effect is estimated as large as 5-6%. Low-frequency behavior of metals gave rise to a principal problem with the thermal Casimir force: there is no continuous transition between real and ideal metals. Moreover, it seems like the Casimir entropy is not going to zero with the temperature. Both of these problems are discussed in simple physical terms. It is demonstrated that no contradictions with known physics exist. 

 

Wednesday, December 13, 2006

Thermal correction to the Casimir force and radiative heat transfer

G. Bimonte

 

We compare the theories of the thermal Casimir force and of the radiative heat transfer through a vacuum gap between real metal plates. It is shown that different models for the reflection coefficients of the surfaces lead to largely different predictions for the amount of heat transfer at submicron separations. A modification of the impedance of infrared optics is suggested taking into account relaxation processes. The power of radiative heat transfer predicted from this impedance is several times less than previous predictions due to different contributions from the transverse electric evanescent waves. New measurements of the radiative heat transfer are required to find out the adequate description of a metal in the theory of electromagnetic fluctuations.

 

Thermal effects of the Casimir force for surface-atom and surface-surface configurations

M. Antezza

 

The Casimir-Polder force characterizes the surface-atom force originating from the fluctuations of the electromagnetic field. Such a force and its cousin, the van der Waals force, are not only fascinating scientifically but also important technologically because of their relevance for instance to atomic force microscopy and to MEMS.

            Our work [1] focused on the theoretical study of the temperature dependence of the force both at equilibrium and out of thermal equilibrium.  In particular, when the temperature of the surface is different from the temperature of free space, the force is predicted to decay more slowly at large distances and to exhibit a stronger dependence on the temperature.

            By positioning a Rb-87 Bose-Einstein condensate a few microns from a dielectric surface and resonantly exciting it into a mechanical dipole oscillation [2], the JILA team has recently observed changes in the collective oscillation frequency that result from the spatial variations in the force [3,4]. Clear evidence of non-equilibrium effects have been found.  Measurements agree with the theoretical predictions, marking the first conclusive demonstration of the temperature dependence of the Casimir-Polder force. Future perspectives for accurate measurements of the surface-atom force using Bloch oscillations [5] will be also discussed.

            We will also show a recent investigation [6] of  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 derived in [1]. 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 cross-over regions at large distances are also identified

 

[1] M. Antezza, L.P. Pitaevskii, and S. Stringari, Phys. Rev. Lett. 95, 113202 (2005).

[2] M. Antezza, L.P. Pitaevskii, and S. Stringari, Phys. Rev. A 70, 053619 (2004).

[3] D.M. Harber, J.M. Obrecht, J.M. McGuirk and E.A. Cornell, Phys. Rev. A 72, 033610 (2005).

[4] J.M. Obrecht, R.J. Wild, M. Antezza, L.P. Pitaevskii, S. Stringari, and E.A. Cornell, submitted to Phys. Rev. Lett. (2006), arXiv: physics/0608074.

[5] I. Carusotto, L. Pitaevskii, S. Stringari, G. Modugno, and M. Inguscio, Phys. Rev. Lett. 95, 093202 (2005).

[6] M. Antezza, L.P. Pitaevskii, S. Stringari, and V.B. Svetovoy, accepted on Phys. Rev. Lett.  (2006), arXiv: cond-mat/0607205.

 

Casimir force between planar mirrors in the real world

F. Intravaia and C. Henkel

 

In the field of nanotechnology, there is a considerable interest in manipulating the Casimir force (both in magnitude and sign) playing with geometry and material structure. The benefit one can possibly achieve under realistic experimental conditions depends on properties like microscopic surface roughness, finite conductivity, material temperature, as shown by the comparison of accurate measurements with theory [1]. Regarding the sign of the force, we have recently identified a parameter range for Casimir repulsion within a certain class of artificial (or meta-) materials [2]. Our current activities aim at improving the  understanding of dispersion forces between non-local or dissipative media that pose intriguing theoretical challenges on their own. Both aspects play a role for the finite-temperature correction to the Casimir force, for example, on which a consensus is currently lacking. We investigate a particular non-local model that allows us to perform calculations from first principles and to assess the limits and scope of the widely used Lifshitz formula.

 

[1] S.K. Lamoreaux. The casimir force: background, experiments, and applications. Reports on Progress in Physics, 68(1):201–236, 2005.

[2] C.Henkel and K. Joulain. Casimir force between designed materials: What is possible and what not. Europhys. Lett., 72:929–935, 2005.

 

Thursday, December 14, 2006

Novel quantitative measurements on the Casimir-Polder force

M. F.M. DeKieviet

 

The Atomic Beam Spin Echo (ABSE) method, originally developed in Heidelberg combines the exclusive surface sensitivity of a (cold) beam of atoms with the high resolution of nuclear spin echo experiments. I will briefly introduce this atom interferometry technique and demonstrate its unparalleled resolving power in both space (0.01 Ĺ – 10 µm) and energy (meV – 10 peV) in some illustrative surface scattering experiments, involving the structure and dynamics of soft, quasi 2-D systems. Subsequently, I will discuss current extensions of our ABSE-method, with which we recently succeeded to address some fundamental questions in surface science and quantum optics. In particular, data are shown on the high energy (near-tothreshold) onset of quantum reflection, i.e. the very non-classical reflection of atoms from the attractive branch of the atom-surface interaction potential. One spectacular application of quantum reflection is the quantitative measurement of the Casimir-Polder force between a single atom and a surface. I will present quantitative measurements on this feeble force at the percent level for a variety of systems, involving a perfect metal, a semiconductor and an insulator surface. In addition, current experiments and first results on the temperature and geometry dependency of the Casimir force are presented. Finally, some actual questions and implications for surface science, equally relevant to both QED-theory and nanotechnology, will be discussed in the context of future studies in our lab.

 

 

Stiction and friction in MEMS

W. Merlijn van Spengen,

 

Micromachines have a much larger surface over volume ratio than macroscopic systems and hence are extremely sensitive to surface forces. Problems with stiction (the unintentional sticking of microscopic surfaces) and friction make that many MEMS (micro-electromechanical systems) designs cannot be realized or have a lifetime that is too short to be interesting for commercial applications. Although the detrimental effects caused by surface forces are a real showstopper for MEMS technology, relatively little research is devoted to investigating these effects on the scale of MEMS. Compared to AFM (atomic force microscope) and SFA (surface forces apparatus) measurements, MEMS have the added complication of a significant surface roughness on the scale of interest. This roughness gives rise to multi-asperity contacts and relatively large adhesive forces outside the real area of contact, with mixed plastic/elastic contact mechanics to complicate matters even further.

            To study stiction and friction on the MEMS scale, we have developed a MEMS device that can itself be used to act as a stiction or friction sensor. With a very sensitive electronic readout, it enables us to see normal (stiction) and friction forces with high accuracy in-situ, on-chip. We can make force-distance curves and friction loops on the sidewalls of a real MEMS device, with real MEMS surfaces, which gives us a unique opportunity to study the effect of different surfaces and environmental conditions on MEMS stiction and friction.

 

Surface tension effects in nanochannels: capillary filling and negative pressure

Niels Tas, Jeroen Haneveld, Maryana Escalante, Henri Jansen, Miko Elwenspoek

 

The capillary action in nanochannels is very pronounced due to the large surface to volume ratio. For water in hydrophilic channels with a hydrodynamic diameter of less than approx. 1 mm the absolute value of the Laplace pressure is larger than 1 bar and the pressure inside water plugs can become negative. We have found direct evidence for the existence of capillary negative pressure by measuring the elastic deformation of a rectangular nanochannel of 100 nm in height under influence of the pressure forces. Recently we have developed a more accurate method by inducing total collapse of the flexible channel capping due to the large negative pressures, which then could be quantified by measuring the critical channel width for total collapse and subsequent stiction. This way we have measured capillary negative pressures of several bars for water, aniline, ethanol, acetone and cyclohexane.

Dynamic measurements of the capillary filling process in nanochannels show that the classical Washburn law is still valid on the nanoscale, with small quantitative deviations indicating an enhanced (apparent) viscosity. Depending on the insulating properties of the channel wall this can be explained by the electroviscous effect or, for channels of around 10 nm in height, most likely by a real increase in the liquid viscosity. For water in hydrophilic silica channels the increased viscosity corresponds with a stationary layer of 4 ± 2 monolayers next to the channel walls.

 

 

Friday, December 15, 2006

 

Experiments for the detection of quantum electrodynamical torques and repulsive forces

Jeremy N. Munday, Davide Iannuzzi, Federico Capasso

 

During the past decade, there have been many experimental demonstrations of the attractive Casimir force between two metal surfaces in vacuum. While high precision experiments have been performed for this case, little work has been done between metalized objects in fluids, dielectric objects, or optically anisotropic materials. For materials with suitably chosen dielectric response functions, repulsive quantum electrodynamical (QED) forces can arise. In addition, optically anisotropic materials can lead to a QED toque resulting from the modification of the boundary conditions of the electromagnetic fluctuations between the materials. We will discuss experiments to detect these phenomena along with the technological implications of such experiments.

 

The Casimir force between the silicon slabs

A.Lambrecht, I.Pirozhenko, L.Duraffourg, Ph.Andeucci

We present calculations of the Casimir force between two Silicon slabs  of variable thickness. The force is found to depend strongly on this parameter and to decrease rapidly when the slab separation exceeds the slab thickness. We also present a simple algebraic approximation which allows to estimate qualitatively the Casimir force over 5 orders of magnitude of slab separation. The present results could be interesting for  nanotechnologies to eliminate an unwanted Casimir force.

 

Casimir curvature and edge effects
K. Klingmüller

The string-inspired worldline formalism is employed to study the Casimir effect. In this approach, the problem is mapped onto the path integral of a point particle, which can be evaluated with Monte Carlo techniques. In so doing, the general difficulty of finding and summing over the fluctuation spectrum of the quantum field is solved. As a consequence, Casimir energies for arbitrary geometries become straightforwardly computable. We present results for a fluctuating scalar field obeying Dirichlet boundary conditions in geometries with curvatures and edges.

 

The Aladin2 experiment: expected signal and present sensitivity

E. Calloni

 

Aladin2 is a cryogenic experiment aimed at the first detection of vacuum energy variation in a rigid body via the demonstration of the first phase transition influenced by vacuum fluctuations. The origin of the experiment stems from a possible future measurement of the "weight" of vacuum energy, whose experimental feasibility passes through the possibility of vacuum energy modulation in a rigid body. In Aladin2 experiment the rigid body is a rigid  Casimir cavity and the vacuum energy modulation is obtained and measured via its theoretically foreseen influence on the super-conducting phase transitions of a cavity mirror. The Casimir cavity is a three-layer system of thin films: the first layer is a normal metal, the second is a dielectric layer, the third is a metal which can undergo super-conducting transition. Following theoretical calculations (PRL, 94, 180402 (2005)) the change in Casimir free energy due to transition from super-conducting to normal metal is  found to be comparable with the condensation energy. Therefore we expect a measurable shift of critical magnetic field, with respect to a sample of the same material not  included in the cavity. In this talk, after a brief review of scientific motivations and theoretical calculations, the expected signal will be presented and the reached experimental sensitivity discussed in details.

 

 

 

 



   [Back]