Net interaction force

What it is concerned with in the physics of polymer systems is not their physical properties for individual polymer conformations but those averaged over the ensemble n of such conformations under given conditions of polymer and solvent. For flexible polymers the averages depend primarily on the strength of the net interaction force F + F and the number of segments contained in the chain. Knowledge of F and F is essential for the understanding of polymers in solution. 

The selection of the type of particle collector is somewhat arbitrary, usually limited by practical considerations. The stream lines of fluid flow around the collectors should be similar to those in a porous medium and yet simple to calculate. As a particle carried by the fluid comes into the proximity of the collector and experiences the net interaction force between itself and the collector, the trajectory of the particle deviates from that of the fluid streamlines. The actual path of the particle is... 

Typical force and current curves for the FcCnSH SAM in bicyclohexyl solvent under a fixed-tip bias of -2.8 V are shown in Figure 20. It should be noted that strictly, the net interaction force between the tip and the sample is the vectorial sum of the loading force and any contributing adhesion forces. However, when performing force curve measurements under solvent, adhesion force due to the former becomes negligible. For the electrical conduction measurements performed, bias magnitudes less than 0.25 V were applied and therefore any... 

Determine the net DLVO interaction (electrostatic plus dispersion forces) for two large colloidal spheres having a surface potential 0 = 51.4 mV and a Hamaker constant of 3 x 10 erg in a 0.002Af solution of 1 1 electrolyte at 25°C. Plot U(x) as a function of x for the individual electrostatic and dispersion interactions as well as the net interaction. 

Interfacial Forces. Neighboring bubbles in a foam interact through a variety of forces which depend on the composition and thickness of Hquid between them, and on the physical chemistry of their Hquid—vapor interfaces. For a foam to be relatively stable, the net interaction must be sufficiently repulsive at short distances to maintain a significant layer of Hquid in between neighboring bubbles. Otherwise two bubbles could approach so closely as to expel all the Hquid and fuse into one larger bubble. Repulsive interactions typically become important only for bubble separations smaller than a few hundredths of a micrometer, a length small in comparison with typical bubble sizes. Thus attention can be restricted to the vapor—Hquid—vapor film stmcture formed between neighboring bubbles, and this stmcture can be considered essentially flat. 

Fig. 10-5 Sketch of (a) current vectors with depth characteristic of an Eckman spiral (b) relationship between wind, surface current, and net water movement vectors and (c) production of circular gyres from the net interaction of the Coriolis force and Eckman transport.

Tsuchida and Abe [2] presented a simple scheme which assumes that the occurrence of complexation depends on the competition of the interaction forces and Egp, where E j denotes the force between a pair of proton-accepting and proton-donating polymers and Egp the forces between solvent and polymer. The net driving force for polymer-polymer complexation is thus written as ... 

For An = A22, A2l2 = 0, and =0, from the viewpoint of van der Waals forces, this condition corresponds to no net interaction between particles. By using experimental criteria to identify this state of affairs, it is possible to vary the medium in a disperse system until this condition is met and then use the surface tension of the medium (via Equation (67)) to evaluate Au and, therefore 422. Going further, Equation (67) can be applied again to estimate y22 for the dispersed particles. This strategy implies that suitable values for d0 are available. 

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). 

VAN DER WAALS FORCES. Interatomic or intermolecular forces of attraction due to the interaction between fluctuating dipole moments associated with molecules not possessing permanent dipole moments. These dipoles result from momentary dissymmetry in the positive and negative charges of the atom or molecule, and on neighboring atoms or molecules. These dipoles tend to align in antiparallel direction and thus result in a net attractive force. This force varies inversely as the seventh power of the distance between ions. 

We have used osmotic stress (4) to examine the effect of Mg2 and Ca2 on the interactions between dioleoylphospha-tidylcholine or dipalmitoyl phosphatidyl choline bilayers. From the net repulsive forces between bilayers we are able to infer electrostatic potentials and charge densities at the site of ion binding these quantities are sensitive to bilayer separation. We find that at any particular bilayer separation dioleoyl phosphatidyl choline bilayers (melted hydrocarbon chains) adsorb less charge than dipalmitoyl phosphatidyl choline bilayers (frozen hydrocarbon chains) and that the binding of Ca2 is greater than that of Mg2 for both kinds of bilayers. 

When the particles are attracted by the plates through dispersion forces (/ <()), the net interaction between plates is repulsive, while when the interactions between particles and plates are repulsive (B>0), the plates are attracted to each other. 

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