Adhesion capillary condensation

Still another area where chemical and physical interactions can occur involves the enhancement of particle adhesion due to capillary condensation [69]. However, for the purposes of the present discussion, let us limit ourselves to dry particles. 

An example of interaction stiffness and force curves for a Si surface with a native oxide at 60% relative humidity (RH) is shown in Fig. 12 [104]. The stiffness and force data show an adhesive interaction between the tip and substrate. The hysteresis on retraction is due to a real change in contact area from surface oxide deformation and is not an experimental artifact. The adhesive force observed during retraction was consistent with capillary condensation and the surface energy measured from the adhesive force was close to that of water. 

Including capillary condensation with the Hertz approximation, as considered by Fogden and White [20], introduces pressure outside the contact area i.e., adhesion enters the problem nonenergetically through the tensile normal stress exerted by the condensate in an annulus around the contact circle. The resulting equations cannot be solved analytically however, their asymptotic analysis may be summarized as follows. 

In the limit of large, softer solids in vapor pressure closer to the value marking the onset of capillary condensation, the generalized Hertz and the original JKR theories are found to be qualitatively identical. However, the contact area for zero applied load will in general be different, since it is dependent upon the nature of the source of adhesion ... 

The same approach is employed to describe shear-induced transport of soot particles. Based on limited amount of experimental information for such phenomena in the literature we have established a flow cell where soot entrainment from the surface of preloaded filters from the engine exhaust can be studied. Preliminary experiments at ambient conditions reveal that no soot entrainment is observed up to relevant shear rates at the entrance of DPFs. We attribute this to the moisture content in ambient conditions of the soot deposits that due to capillary condensation increases adhesive forces between the particles. In the future experiments at high temperatures are planned to evaluate experimentally the shear-entrained fluxes for soot and ash deposits. 

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. ... 

Figure 9.5 schematically shows the capillary effect and the resulting effect on the particle adhesion. A liquid bridge forms between the particle and the substrate and holds the particle to the surface by surface tension. The liquid film could be a result of capillary condensation in high relative humidity or of a retention of liquid during substrate removal from the liquid bath. The attractive force is composed of the force caused by surface ten-... 

Iwamatsu, M., and K. Horii. 1996. Capillary condensation and adhesion of two wetter surfaces. J. Colloid Interface Sci. 182 400—4-06. 

The applications of the SFM include force measurement between surfaces in liquid and vapor, adhesion between similar or dissimilar materials, contact deformation, wetting and capillary condensation, viscosity in thin films, forces between surfactant and polymer-coated surfaces, and surface chemistry. Fluid-electrolyte interactions between conductive surfaces can also be measured [Smith, et. al., 1988]. A typical microforce of 10 nN can be detected over separation distances to a resolution of 0.1 nm with optical interoferometry between reflective surfaces. With electrostatic forces, relatively large separation are measured 1-100 nm, whereas, short range forces such as van der Waals forces take place over distances of less than 3.0 nm. Ultrasmooth and electrically conductive surfaces can be formed by the deposition of a metal film (40 nm thickness) such as Pt on a smooth substrate of mica [Smith, et. al., 1988]. The separation distance between the two surfaces is controlled by a... 

A very general adhesive effect which stems from such mechanisms as those above is the dwell-time phenomenon. Two surfaces are brought into contact and left for a time. The adhesion is then found to have increased. Further time of contact leads to further increase as shown in Fig. 8.5. This effect could, for example, be a result of the capillary condensation described above. When the surfaces are first in contact, the adhesion is low because roughness inhibits the short-range attractions. But as condensation occurs in the gaps, the adhesion rises with time. 

Relationship between Adhesive Forces and Air Humidity. Experiments reported in [143] indicate that capillary condensation begins to appear at air relative humidities above 50%. It was found that for particles with sizes of 20-30 and 40-60 ixm, the adhesion number remained approximately the same when the air humidity was increased from 5 to 50%. When the air humidity was increased from 50 to 65%, an increase in the adhesion numbers was observed from 40 to 90% for spherical glass particles with a diameter of 40-60 /xm and a detaching force of 2.25 10" dyn, and from 18 to 80% for particles with a diameter of 20-30 jjim and a detaching force of 2.81 lO" dyn. The increase in adhesion number is evidence for an increase in adhesive interaction. ... 

Capillary condensation takes place over a certain interval of time hence, the effect of capillary forces is not manifested immediately after contact of dust particles with a solid substrate. According to experimental data reported in [143], the increase in adhesive force is completed within approximately 30 min after contact of SO-lOO-zmi glass particles with a substrate in air at a relative humidity near 100%. 

Under conditions of capillary condensation, adhesive forces are determined entirely by capillary forces since they are greater than all other components of the adhesive force. The differences between capillary components and other components of adhesive force is particularly great for particles larger than 10 lim in diameter. 

In an investigation of the influence of air humidity on adhesion, hysteresis phenomena have been observed [143]. In Fig. IV.IO we show the force of adhesion of spherical glass particles 50 2 /im in diameter as a function of the relative humidity of the air. The lower branch of the hysteresis loop shows the way in which adhesion increases as the air relative humidity is increased the lower branch shows the way in which adhesion drops off as the air humidity is reduced. The failure of the two branches of the adhesion-humidity curve to coincide indicates that the processes of capillary condensation and evaporation of mois-... 

The increase in adhesive interaction with increasing contact time between particles and surface in air, by analogy with this sort of process in a liquid medium, is termed aging [89]. There may be several causes of aging an increase in contact area between particle and surface as a result of deformation or as a result of the influence of various contaminants adsorption processes and capillary condensation may take place in the contact zone, so that capillary forces are created. 

Characteristics of Capillary Forces in Adhesion of Cylindrical Particles. As a result of capillary condensation in the gap between a cylindrical particle and a surface, a liquid layer is formed, and this layer affects the adhesive interaction. The capillary component of the adhesive forces of cylindrical particles will depend on the properties of the contiguous surfaces and their relative positions [161]. Since the quantity of liquid between the contiguous bodies will depend on the position of these bodies, the adhesive interaction is different for vertical surfaces (Fig. V. 12.a and V.12.b) and horizontal surfaces (Fig. V.12.c). 

It is possible that a capillary condensed water bridge existed between the silicon asperity and the probe plane. When the Laplace pressure is considered, the adhesion force caused by this type of meniscus between a sphere of radius and a flat sur-... 

The pull-off force was proportional to the radii of curvature of the asperity peaks and was almost independent of the microsurface roughness of the asperities. The adhesion energy agreed well with the Laplace pressure due to capillary condensed water. These findings indicate that the Laplace pressure was a dominant factor in the adhesion force. 

Relation Between the Adhesive Forces and Air Humidity. Experiments [149] show that capillary condensation starts appearing at a relatively low air humidity (above 65%). 

Capillary condensation takes place over a certain period of time, so that the effects of capillary forces are not felt immediately after the dust particle comes into contact with the solid surface. According to the experimental data obtained in [149], the rise in the adhesive forces ends about 30 min after glass particles 80-100 /i in diameter come into contact with the substrate in air at a relative humidity of about 100%. 

III.14). The failure of the adhesive force/humidity curves to coincide indicates that the processes of the capillary condensation and evaporation of moisture in the gap between the contiguous surfaces possess certain peculiarities characteristic of thin layers of liquid. 

The simplest surface force measurement that can be made with any of the surface force techniques, or the JKR instrument, is the force required to separate two like surfaces in adhesive contact - this is usually referred to as the pull-off force, F(0). From this value, it is possible to deduce the work of adhesion and thus the surface energy providing that there is no capillary condensate around the contact area. However to do this, an understanding of the contact mechanics of the two surfaces must be achieved. For a sphere on flat geometry, the deformation of the sphere under adhesive and applied load has been calculated by several authors. The simplest case (for that where there is no adhesion) is that of Hertz (64) where the contact radius a is given by the following ... 

One can wonder about considering forces or energies. As in the present case, the slope in the deflection versus piezo displacement is equal to unity during tip retraction is obvious, to a first approximation, that the adhesion force (proportional to pull-off deflection) and energy of contact disruption (integral work of contact disruption) vary in proportion. Work will be done on forces. The capillary condensation induces the formation of a meniscus... 

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