Contact interactions compressive force

As discussed in detail in Chapter 3.1, the advantage of bioinert materials is that they do not release any toxic constituents to the human body environment. However, on the downside they do not show positive interaction with living tissue. Instead, the body usually responds to these materials by forming a non-adherent fibrous capsule of connective tissue around the bioinert material that in the case of bone remodelling manifests itself by a shape-mediated contact osteogenesis. Consequently only compressive forces will be transmitted through the bone-biomaterial interfaces ( bony on-growth ). 

The methods considered for determining the forces of interaction between particles are very cumbersome, and the apparatus required is ad hoc and complex. In addition to this, results obtained by different authors for the forces of interaction between the same surfaces differ. This is because the preliminary compressive force bringing the bodies into contact with one another (which determines the true contact area, and naturally affects the force of interaction between the particles) differs for different authors (although generally remaining constant for any particular set of experimental conditions), and is indeed not usually quoted. 

The forces of interaction (attraction and repulsion) depend not only on the properties of the bodies in contact and the layer separating these, but also on the external applied force. This force determines the thickness of the gap between the bodies. If the compressing force is no greater than Fj ax (height of the force barrier) the adhesive force will be relatively small and equal to F nin H the compressive force exceeds F the adhesive force will be equal to F nin ... 

The quality of solvent (i.e., solvent-polymer interactions) can also clearly affect the interaction forces. In a good solvent, polymer segments favor contacts with the solvent. Since the compression of the polymer layer by an approaching surface tends to squeeze out the solvent and force segment-segment interactions, the net result is a repulsion. In contrast, poor solvents produce an opposite effect, and a net attraction is possible for certain range of compression (see Section 13.6b). 

In Fig. 17.2, we give some results of the calculation of the interaction force Pe(h) acting between two compressed bmsh layers as a function of reduced separation kIi for several values of ZNJzn at scaled brush thickness Kdp = 10. Calculation was made with the help of Eq. (17.12) for h<2dp and Eq. (17.1) for h>2dp. We see that the interaction force is always repulsive before and after the two brushes come into contact. 

The duration of the actual particle-particle interactions taking place in real flow situations in process vessels is however limited and may vary considerably in time and space. The net force which compresses the fluid particle must thus act for a sufficient time to ensure that the intervening film drains to the critical thickness so that film rupture and coalescence take place. In an early view it was postulated that for these processes to occur, the actual particle-particle collision (contact) time interval Atcoi must exceed the coalescence time interval Zitcoai of the coalescence processes, Z fcoi > fcoai- The probability of coalescence was thus generally defined as a function of the ratio... 

Figure 9.24 Force curve obtained when extending chains ofPOEA from the chrome surface in water with frequency of jumps < 1 pm s I no tip-POEA interactions II film being compressed lll/IV repulsive contact with the substrate V pull-of force due to surface adhesion VI cantilever pulls off the surface VII attractive peak due to stretching of POEA chains. (Reprinted with permission from Microscopy and Microanaiysis, Atomic Force Spectroscopy on Poly(o-ethoxyaniline) Nanostructured Films Sensing Nonspecific Interactions byF.L. Leite, C.E. Borato, W.T.L. da Silva, P.S.P. Herrmann, O.N. Oliveira Jr., L.H.C. Mattoso, 13, 304 (Copyright 2007).

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