AFM tip effect on a thin liquid film.

We study the interaction between an AFM probe and a liquid film deposited over a flat substrate. We investigate the effects of the physical and geometrical parameters, with a special focus on the film thickness E, the probe radius R, and the distance D between the probe and the free surface. Deformation profiles have been calculated from the numerical simulations of the Young-Laplace equation by taking into account the probe/liquid and the liquid/substrate interactions, characterized by the Hamaker constants, Hpl and Hls. We demonstrate that the deformation of a shallow film is determined by a particular characteristic length λF = (2πγE(4)/Hls)(1/2), resulting from the balance between the capillary force (γ is the surface tension) and the van der Waals liquid/substrate attraction. For the case of a bulk liquid, the extent of the interface deformation is simply controlled by the capillary length λC = (γ/Δρg)(1/2). These trends point out two asymptotic regimes, which in turn are bounded by two characteristic film thicknesses Eg = (Hls/2πΔρg)(1/4) and Eγ = (R(2)Hls/2πγ)(1/4). For E > Eg, the bulk behavior is recovered, and for E < Eγ, we show the existence of a particular shallow film regime in which a localized tip effect is observed. This tip effect is characterized by the small magnitude of the deformation and an important restriction of its radial extent λF localized below the probe. In addition, we have found that the film thickness has a significant effect on the threshold separation distance Dmin below which the irreversible jump-to-contact process occurs: Dmin is probe radius-dependent for the bulk whereas it is film-thickness-dependent for shallow films. These results have an important impact on the optimal AFM scanning conditions.

Near-field deformation of a liquid interface by atomic force microscopy.

We experiment the interaction between a liquid puddle and a spherical probe by Atomic Force Microscopy (AFM) for a probe radius R ranging from 10 nm to 30 µm. We have developed a new experimental setup by coupling an AFM with a high-speed camera and an inverted optical microscope. Interaction force-distance curves (in contact mode) and frequency shift-distance curves (in frequency modulation mode) are measured for different bulk model liquids for which the probe-liquid Hamaker constant H_{pl} is known. The experimental results, analyzed in the frame of the theoretical model developed in Phys. Rev. Lett. 108, 106104 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.106104 and Phys. Rev. E 85, 061602 (2012)PLEEE81539-375510.1103/PhysRevE.85.061602, allow to determine the "jump-to-contact" critical distance d_{min} below which the liquid jumps and wets the probe. Comparison between theory and experiments shows that the probe-liquid interaction at nanoscale is controlled by the liquid interface deformation. This work shows a very good agreement between the theoretical model and the experiments and paves the way to experimental studies of liquids at the nanoscale.

Note: Void effects on eddy current distortion in two-phase liquid metal.

A model based on the first order perturbation expansion of magnetic flux in a two-phase liquid metal flow has been developed for low magnetic Reynolds number Rem. This model takes into account the distortion of the induced eddy currents due to the presence of void in the conducting medium. Specific experiments with an eddy current flow meter have been realized for two periodic void distributions. The results have shown, in agreement with the model, that the effects of velocity and void on the emf modulation are decoupled. The magnitude of the void fraction and the void spatial frequency can be determined from the spectral density of the demodulated emf.

Scaling properties of contact area of pressure-sensitive adhesives.

Two new devices were developed to provide accurate measurements of both the contact area and the tack strength of pressure-sensitive adhesives (PSAs). The first one is the "mechano-optical tack tester" (MOTT), which was designed to apply controlled contact pressure by means of a quartz prism probe, for determined contact times, onto the surfaces of PSA samples. The contact area is measured by the reflection of light at the quartz probe surface, which is in contact with the adhesive. The second device is an "acoustic contact tester" (ACT) that measures the tack strength and the contact area between a silicate glass and an adhesive by the reflection of an acoustic wave. Two ultrasonic sensors of different acoustic wavelengths have been built in order to study the scaling effects of the contact kinetics between an adhesive and the probe. MOTT and ACT experiments on a commercial PSA show that the contact area is the main parameter that governs the tack properties of the PSA. The experiments and the modeling point out that the contact area depends on the compression stress, the roughness, the thickness, and the Young's modulus of the PSA. However, comparison of contact kinetics from MOTT and ACT experiments show that the contact area is a subjective parameter that depends on the wavelength of the reflected beam used for its measurement.

Gas flow in plant microfluidic networks controlled by capillary valves.

The xylem vessels of trees constitute a model natural microfluidic system. In this work, we have studied the mechanism of air flow in the Populus xylem. The vessel microstructure was characterized by optical microscopy, transmission electronic microscopy (TEM), and atomic force microscopy (AFM) at different length scales. The xylem vessels have length ≈15 cm and diameter ≈20µm. Flow from one vessel to the next occurs through ∼102 pits, which are grouped together at the ends of the vessels. The pits contain a thin, porous pit membrane with a thickness of 310 nm. We have measured the Young's moduli of the vessel wall and of the pits (both water-saturated and after drying) by specific nanoindentation and nanoflexion experiments with AFM. We found that both the dried and water-saturated pit membranes have Young's modulus around 0.4 MPa, in agreement with values obtained by micromolding of pits deformed by an applied pressure difference. Air injection experiments reveal that air flows through the xylem vessels when the differential pressure across a sample is larger than a critical value ΔPc=1.8 MPa. In order to model the air flow rate for ΔP⩾ΔPc, we assumed the pit membrane to be a porous medium that is strained by the applied pressure difference. Water menisci in the pit pores play the role of capillary valves, which open at ΔP=ΔPc. From the point of view of the plant physiology, this work presents a basic understanding of the physics of bordered pits.

Breakup of a pendant magnetic drop.

We report experiments on a millimeter-sized pendant drop of ferrofluid in a horizontal magnetic field. The initial drop size is chosen just below the breakup threshold under gravity. As the magnetic field is increased, the drop tilts in order to align with the direction of the total volume force that is exerted on it: weight plus magnetic force. The breakup is controlled by a generalized Bond number based on this total force and on the radius of the neck of the drop. The evolution of drop shape turns out to be a complex process governed by many parameters such as the angle between the total force and the needle, the drop size relative to the needle radius, and the wettability of the liquid on the needle material. This suggests a certain universality, that a single value of the critical Bond number is found regardless of magnetic fluid properties and whether the force is inclined or not.

Broadband nanodielectric spectroscopy by means of amplitude modulation electrostatic force microscopy (AM-EFM).

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip-sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2-3 × 10(4)Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip-sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip-sample distances.

Nanodielectric mapping of a model polystyrene-poly(vinyl acetate) blend by electrostatic force microscopy.

We present a simple method to quantitatively image the dielectric permittivity of soft materials at nanoscale using electrostatic force microscopy (EFM) by means of the double pass method. The EFM experiments are based on the measurement of the frequency shifts of the oscillating tip biased at two different voltages. A numerical treatment based on the equivalent charge method allows extracting the values of the dielectric permittivity at each image point. This method can be applied with no restrictions of film thickness and tip radius. This method has been applied to image the morphology and the nanodielectric properties of a model polymer blend of polystyrene and poly(vinyl acetate).