Adsorption repulsion

These bonding interactions just described are frequently considered to be representative of the major types of forces which exist between species, although there is some disagreement about their nature and magnitude involved. It is probable that combined or hybrid forces come into play in real material interactions. It is also probable that multiple types of attractive and repulsive adsorptive forces are operative [158,160-162]. 

Proposed Gel Friction Mechanism A Repulsion-Adsorption Model... 

Fig. 12 Repulsion-adsorption model for the gel friction on a smooth solid substrate. P normal...

On strongly adhesive substrates (Fig. 14b, c), friction increases with the substrate hydrophobicity in the low-velocity region, showing a weak velocity-strengthening, and a dramatic friction transition at around v = 10 m/s. This value is one order lower than the characteristic velocity of the polymer chain Vf = /Tf. The friction behavior is satisfactorily described by the repulsion-adsorption model below the transition region The friction transition is explained in terms of the elastic... 

According to the repulsion-adsorption model, a solvent layer is formed at the interface when the gel-substrate interaction is repulsive, and the frictional shear stress arises from the viscous flow of the solvent layer. Therefore, the frictional stress should increase with the increase in the sliding velocity, i.e., a ° v. 

When the molecules of the adsorbate approach those of the adsorbent surface, the above forces of attraction are opposed by forces of repulsion. Adsorption and desorption continue till an equilibrium is reached. A molecule being adsorbed is not associated with a single centre. Rather it interacts with many neighbouring centres (see Figure 7.5). 

PROPOSED CEL FRICTION MECHANISM REPULSION-ADSORPTION MODEL... 

Based on the above complex frictional behavior of polymer gels, Gong and cowoik-ers have proposed a repulsion-adsorption model from the viewpoint of polymer-solid interfacial interactions to describe the friction behavior of gels on a smooth... 

FIG U RE 11.8 Schematic illustration of the repulsion-adsorption model for gel friction on a solid substrate. 

The repulsion-adsorption model also predicts that (1) when the substrate is repulsive, the frictional force is lower than the attractive case, and it linearly increases with the normal pressure and velocity when the compressive strain (normal load) is not very high, and (2) when the substrate is attractive, the frictional force increases with the attraction strength. For weak attraction, the pressure dependence of the frictional force is much weaker than in the repulsive case. It becomes stronger when the attraction strength increases [15],... 

In the case of electrostatic repulsion (adsorption against an electrostatic barrier), the particles will preferably adsorb under the perpendicular orientation. The same is true for the oblate spheroid adsorption [15]. 

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