P. J. van Zwol

Influence of roughness on capillary forces between hydrophilic surfaces

Capillary forces have been measured by atomic force microscopy in the plate-sphere setup between gold, borosilicate glass, GeSbTe, titanium, and UV-irradiated amorphous titanium-dioxide surfaces. The force measurements were performed as a function contact time and surface roughness in the range 0.2-15 nm rms and relative humidity ranging between 2% and 40%. It is found that even for the lowest attainable relative humidity ( approximately 2%+/-1%) very large capillary forces are still present. The latter suggests the persistence of a nanometers-thick adsorbed water layer that acts as a capillary bridge between contacting surfaces. Moreover, we found a significantly different scaling behavior of the force with rms roughness for materials with different hydrophilicity as compared to gold-gold surfaces.

Optical properties of gold films and the Casimir force

Precise optical properties of metals are very important for accurate prediction of the Casimir force acting between two metallic plates. Therefore we measured ellipsometrically the optical responses of Au films in a wide range of wavelengths from 0.14 to 33 mu m. The films at various thicknesses were deposited at different conditions on silicon or mica substrates. Considerable variation of the frequency dependent dielectric function from sample to sample was found. Detailed analysis of the dielectric functions was performed to check the Kramers-Kronig consistency, and extract the Drude parameters of the films. It was found that the plasma frequency varies in the range from 6.8 to 8.4 eV. It is suggested that this variation is related with the film density. X-ray reflectivity measurements support qualitatively this conclusion. The Casimir force is evaluated for the dielectric functions corresponding to our samples, and for that typically used in the precise prediction of the force. The force for our films was found to be 5%-14% smaller at a distance of 100 nm between the plates. Noise in the optical data is responsible for the force variation within 1%. It is concluded that prediction of the Casimir force between metals with a precision better than 10% must be based on the material optical response measured from visible to mid-infrared range.

Plasmon enhanced near-field radiative heat transfer for graphene covered dielectrics

It is shown that a graphene layer on top of a dielectric slab can dramatically influence the ability of this dielectric for radiative heat exchange turning a poor heat emitter/absorber into a good one and vice versa. The effect of graphene is related to thermally excited plasmons. The frequency of these resonances lies in the terahertz region and can be tuned by varying the Fermi level through doping or gating. It makes possible the fast modulation of the heat flux by electrical means, which opens up new possibilities for very fast manipulations with the heat flux. The heat transfer between two dielectrics covered with graphene can be larger than that between best known materials and becomes especially efficient below the room temperature.

Switching Casimir forces with phase-change materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of similar to 100 nm between Au and Ag-In-Sb-Te (AIST) surfaces were achieved on crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches, by stimulating the phase-change transition via localized heat sources.

Influence of random roughness on the Casimir force at small separations

The influence of random surface roughness of Au films on the Casimir force is explored with atomic force microscopy in the plate-sphere geometry. The experimental results are compared to theoretical predictions for separations ranging between 20 and 200 nm. The optical response and roughness of the Au films were measured and used as input in theoretical predictions. It is found that at separations below 100 nm, the roughness effect is manifested through a strong deviation from the normal scaling of the force with separation distance. Moreover, deviations from theoretical predictions based on perturbation theory can be larger than 100%.

Repulsive Casimir forces between solid materials with high-refractive-index intervening liquids

In order to explore repulsive Casimir or van der Waals forces between solid materials with liquid as the intervening medium, we analyze dielectric data for a wide range of materials as, for example, (p)olytetrafluoroethylene, polystyrene, silica, and more than 20 liquids. Although significant variation in the dielectric data from different sources exists, we provide a scheme based on measured static dielectric constants, refractive indices, and applying Kramers-Kronig consistency to dielectric data to create accurate dielectric functions at imaginary frequencies. The latter is necessary for more accurate force calculations via the Lifshitz theory, thereby allowing reliable predictions of repulsive Casimir forces.

Influence of random roughness on the adhesion between metal surfaces due to capillary condensation

The capillary force was measured by atomic force microscopy between a gold coated sphere mounted on a cantilever and gold surfaces with different roughnesses. For smooth surfaces the capillary adhesive force surpasses in magnitude any dispersion, e.g., van der Waals/Casimir and/or electrostatic forces. A substantial decrease in the capillary force was observed by increasing the roughness ampltitude a few nanometers in the range of 1–10nm. From these measurements two limits can be defined: a smooth limit where a closely macroscopic size contact surface interacts through the capillary force and a rough limit where only a few asperities give a capillary contribution.

Distance upon contact: Determination from roughness profile

The point at which two random rough surfaces make contact takes place at the contact of the highest asperities. The distance upon contact d0 in the limit of zero load has crucial importance for determination of dispersive forces. Using gold films as an example we demonstrate that for two parallel plates d0 is a function of the nominal size of the contact area L and give a simple expression for d0(L) via the surface roughness characteristics. In the case of a sphere of fixed radius R and a plate the scale dependence manifests itself as an additional uncertainty δd(L) in the separation, where the scale L is related with the separation d via the effective area of interaction L2∼πRd. This uncertainty depends on the roughness of interacting bodies and disappears in the limit L→∞.

Transition from Casimir to van der Waals force between macroscopic bodies

The transition of van der Waals to Casimir forces between macroscopic gold surfaces is investigated by atomic force microscopy in the plane-sphere geometry. It was found that the transition appears to take place at separations ∼10% the plasma wavelength λp for evaporated gold surfaces, which compares to theoretical predictions by incorporation of experimental optical data and roughness corrections. Moreover, the force data allow estimation of the Hamaker constant AH in the van der Waals regime, which is in good agreement with the Lifshitz theory predictions (even if roughness corrections are taken into account) and former surface force apparatus measurements.

Roughness of microspheres for force measurements

We have investigated the morphology and surface roughness of several commercially available microspheres to determine their suitability for force measurements using the atomic force microscope. The roughness varies considerably, depending on sphere size and material, ranging from nearly ideally flat up to micrometer-sized features. Because surface roughness significantly influences the magnitude and accuracy of measurement of surface forces, the results presented here should be helpful for colloid physicists and in particular for those performing force measurements.