Hydrodynamic adhesion force

The hydrodynamic adhesion force that expresses the resistance occurring when microparticles latch onto deposition elements. It is caused by the liquid that must be extracted out of the space between two particles when both microparticles adhere. This force allows the slowing down of microparticles adhesion and offers a possibility for drowning it in the flowing suspension. 

Regardless of the nature of the adhesive force (Krupp, 1967 Ranada, 1986) holding the lines onto the surface of the coarse particle, an analysis of force balance on the fine particle between adhesion, hydrodynamic drag, and gravity (Kwauk, 1984,1986 Xia and Kwauk, 1985) lent support to the phenomena enumerated above on the varying degree of surface coverage of the coarse particle by the fine particles. 

Cellular adhesion FFF combines the controllable hydrodynamic shear forces of FFF and the selective adhesion of AC. The hydrodynamic shear is used to detach selectively and evenly adhered cells or particles from the surface and allows an estimation of the differences in cell/surface adhesion forces. The channel for cellular adhesion FFF is constructed in the same way as that used for Gr-FFF but is smaller in size and with the modification that the accumulation wall consists of either bare or polymer-coated surfaces. After the cell suspension is filled into the channel allowing sufficient time for cell adhesion, the flow is applied and fractions are collected. Despite the collection of fractions, cellular adhesion FFF can also be used as a tool to study rapid kinetics of cell surface adhesion, a largely unexamined area [332]. 

As noted earlier the removal forces such as those associated with centrifugal or vibrational methods vary as (see Section 9.1), and those associated with hydrodynamic drag vary as On the other hand, all adhesive forces vary as R. Thus the ratio of adhesion force to mechanical force varies as or / , and with reduced particle diameter, more mechanical energy is required to remove particles physisorbed on surfaces. 

A qualitative model for lubrication of the condensed alcohol layer is not fully developed yet. We present here the best qualitative model based on the literature. As the applied pressure is very large and the scanning speed is very low in AFM, one can rule out the hydrodynamic lubrication even in the presence of condensed alcohol layer on the substrate. The AFM tip and substrate interface must be in the boundary lubrication regime. Figs. 7-10 show that the adhesion force reduction upon alcohol adsorption is accompanied by reduction of the friction force. Part of the adhesion force reduction is because of the surface tension decrease of the water layer on the substrate. When alcohol is dissolved in water, the surface tension decreases significantly because of segregation of alcohol molecules to the liquid-air interface. ... 

The number of particles adsorbed on the metal surface depended on hydrodynamic shear forces. These forces were simulated by a rotating disc electrode and then compared with experiments studying polystyrene particle co-deposition with copper. One result was a calculation of the adhesion force of the particles on the copper surface as a function of the current density showing a maximum between 1 and 2 A dm (Figurel2.10). 

After adsorption the adhesion forces between particle and surface will keep the particle adsorbed on the surface until hydrodynamic forces or thermal motion are strong enough to overcome the adsorption barrier. This can be described by the equation... 

Although the mechanism for hydrodynamic detachment is poorly understood, it is clear that the hydro-dynamic drag force required to detach a particle is proportional to the interparticle adhesion force ... 

Here we observe that the adhesive force (or detaching force) varies inversely with the time during which the dust-covered surface is held in the liquid medium (or the time during which the detaching force is applied). Such a relationship, hoever, persists only over a limited period of time, subsequently breaking down. This means that the time required to establish the equilibrium thickness of the liquid interlayer depends not only on the hydrodynamic factor, which involves influx and efflux of liquid in the gap between bodies, but also on other factors. 

Hydrodynamic Factor. In order to discover the reason for the dependence of the adhesive forces on contact time, let us consider the hydrodynamic phenomena taking place when the bodies approach or recede from each other. For this purpose it is customary to employ adhesion-simulating methods (see 8), in particular the method of plane-parallel discs. The hydrodynamic factor due [88] to the motion of the liquid in the gap between the contiguous surfaces, determining the change in adhesion with contact time for the interaction of plane-parallel discs, may be represented by the Stefan—Reynolds equation... 

In this case, rising temperature leads to a fall in the viscosity of the liquid and in the time during which the hydrodynamic processes are taking place, which may even reduce the adhesive force. 

Both adhesive and hydrodynamic forces depend on the size of the particles. The two forces were calculated for CaC03 fillers of various particle sizes homogenized in a PP matrix. The results are presented in Fig. 3. At a certain particle size adhesion exceeds shear forces, aggregation of the particles takes place in the melt. Since commercial fillers have a relatively broad particle size distribution, most fillers show some degree of aggregation and the exact determination of the particle size, or other filler characteristics where aggregation appears, is difficult. Experiments carried out with 11 different CaC03 showed this limit to be around 6 m /g specific surface area [25]. 

Other forces existing between particles often operate against hydrodynamic drag that produces fluidization. Thus, when particles touch one another, there exists a London-van der Waals force of a molecular nature at the point of contact. This force looms in proportion when gravity and drag forces diminish as a particle becomes smaller. For a small particle, the surface force of adhesion may often be thousands of times greater than its weight. 

The adhesion of these cells to the tumor vessel wall occurs when the force between the adhesion molecules on the surfaces of endothelium and effector cell is greater than the hydrodynamic force exerted by blood flow. The deformability of these cells also plays an important role in this process, since it can alter the surface area of contact (Sasaki et al., 1989 Melder and Jain, 1992). Measurement of forces exerted by various adhesion molecules as well as cell deformability in vitro and in vivo is an active area of research in many laboratories, including our own (Ohkubo et al., 1991 Munn et al., 1994). 

The radial hydrodynamic component (y component) of the force is denoted by fj, and represents the net externally applied hydrodynamic force on the particle resulting from the particle being driven toward (or away from) the collector by the external flow (undisturbed or disturbed) plus any negative resistive lubrication force arising from a close approach of the particle to the collector. The attractive molecular London force acting along the line of centers is denoted by (Ad denotes adhesion). Because of the linearity of the Stokes-Oseen equation, the velocity fields and associated forces may be superposed. 

Here, is a dimensionless number termed the adhesion group. Clearly the larger the value of the adhesion group the more dominant is the London attractive force for a fixed gap width and particle location. On the other hand, for smaller values of hydrodynamic interactions dominate. 

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