Meniscus forces

As the tip is brought towards the surface, there are several forces acting on it. Firstly, there is the spring force due to die cantilever, F, which is given by = -Icz. Secondly, there are the sample forces, which, in the case of AFM, may comprise any number of interactions including (generally attractive) van der Waals forces, chemical bonding interactions, meniscus forces or Bom ( hard-sphere ) repulsion forces. The total force... 

Fig. 22 Patterning of OND or DNA solutions by means of an AFM tip. (i) Nanografting, (ii) Meniscus-Force Nanografting, (iii) Dip-pen lithography...

Quantifying Meniscus Forces with an Electronic Balance Direct Measurement of Liquid Surface Tension 210... 

An important consequence of capillary condensation is that liquids are strongly adsorbed into porous materials. Another important consequence is the existence of the capillary force, also called the meniscus force. Capillary condensation often strengthens the adhesion of fine particles and in many cases determines the behavior of powders. If, for instance, two particles come into contact, liquid (usually water) will condense into the gap of the contact zone [22], The meniscus is curved. As a consequence, the Laplace pressure in the liquid is negative, and the particles attract each other. 

One consequence of the curvature dependence of the vapor pressure is capillary condensation, that is the spontaneous condensation of liquids into pores and capillaries. Capillary condensation plays an important role for the adsorption of liquids into porous materials and powders. It also causes the adhesion of particles. The condensing liquid forms a meniscus around the contact area of two particles which causes the meniscus force. ... 

Force in atomic force microscopy. It is lower because the van der Waals attraction is reduced and the meniscus force is absent. 

Differences in the chemical nature of regions of the sample can be highlighted through measurements of the adhesion in water (see the review article by J. Vancso, contained within this volume). The adhesion in water, measured on lithographed squares like the ones in Fig. 29 is not influenced by the meniscus force [6] and, since the interaction force is very much smaller and the tip does not need to indent the surface, it is also not influenced by the density of the sample. 

In the spectroscopy mode of AFM force-distance curves F(z) are recorded at one or more scan points after the z piezo has been adjusted to the force setpoint (contact mode). The classical shape, as illustrated in Section 5.3.4 and Figure 63, is obtained mostly with hard materials or simpler molecules. In air a meniscus of water is formed at the jump-to-contact. Due to the meniscus force the jump-to- and jump-oflf-contact separations differ largely and the area of the hysteresis loop becomes quite large. Force-distance curves can have various appearances. An overview is given in Reference [233]. Spectroscopy is rarely employed in the dynamic mode because with an oscillating probe the tip-sample separation is never well-defined. On the other hand the snap-on is avoided and the complete interaction potential can be inferred from the measured frequency versus distance curve employing simulations [234,235]. 

Fig. 13. Gold nanoparticles arranged in circular indentations (arranged on a square grid with 3 gm pitch) in a thin film by the meniscus force method.

In order to obtain released, uncoated micromachine devices for friction measurement, some devices were dried in supercritical CO2. In this process, the devices were transferred to ethyl alcohol after the oxide release etch and water rinse. The devices were then placed in the supercritical CO2 process chamber while covered with alcohol. The alcohol was then dissolved in CO2 above its triple point, avoiding the meniscus forces that pull structures together. 

Allemandeta/., LongdsDNA Vinyl SAM, polystyrene, poly-L-lysine substrate estimations of meniscus force on the DNA molecule. Variation of combing properties vidth surfece treatment and solution pH... 

The DNA-SWCNTs or DNA deposited on the surface are exposed to meniscus and fluid forces. As suggested by the fact that in experiments alignment is perpendicular to the meniscus, not parallel to fluid flow, the alignment is due to forces applied by the meniscus on DNA-SWCNT rods, not due to fluid friction. One can estimate the fluid and friction forces on the object to see if indeed the meniscus force is much greater. 

In addition to household and conventional products, possible use of roughness-or heterogeneity-induced superhydrophobicity in nano- and biotechnology applications is often discussed. This includes, for example, nonsticky surfaces for the components of micro/nanoelectromechanical systems (NEMS/MEMS). Since adhesion plays an important role for small devices, the so-called stiction of two component surfaces is a significant problem in that industry, which may lead to device failure. Making a surface hydrophobic can reduce meniscus force and stiction. 

Tapping mode measures topography by tapping the sample surface with the cantilever near or at its resonance frequency with typical amplitudes between 20 and 100 nm (Zhong et al, 1993). Because the cantilever taps the surface for a very small fraction of its oscillation period it is prevented from being trapped by adhesive meniscus forces from the contaminant layer (Ktihle et al, 1997,1998 Knoll et al., 2001). Since the contact time... 

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