Solids roughening

J. Krug, H. Spohn. Kinetic roughening of growing surfaces. In C. Godreche, ed. Solids Far from Equilibrium. Cambridge Cambridge University Press, 1991. 

Solid metal electrodes with a crystalline structure are different. The crystal faces forming the surface of these electrodes are not ideal planes but always contain steps (Fig. 5.24). Although equilibrium thermal roughening corresponds to temperatures relatively close to the melting point, steps are a common phenomenon, even at room temperature. A kink half-crystal position—Fig. 5.24c) is formed at the point where one step ends and the... 

Figure 2.56 Oxidation -reduction cycle for the roughening of an Ag electrode, After R.K. Chang and B.L. Laubc in CRC Critical Reviews in Solid State Materials Science, Vol. 12, pp, 1 73, CRC Press Inc., Boca Raton, Florida (1984).

Determination of friction sensitivity is applicable to solids, pastes, and gel-type substances. To determine the friction sensitivity, a thin sample is placed under a load between two roughened surfaces, and the surfaces are then rubbed together in a controlled manner. The load can be varied. Results from this action, such as smoke, cracking, or discoloration, are observed. Examples of apparatus of this type are the BAM friction apparatus, shown in Figure 2.30, the rotary friction test, and the ABL friction test. 

The roughening transition has also been studied by computer simulation methods . Figure 42 shows characteristic configurations of a f.c.c. (100) surface in the simple solid-on-solid (SOS) model, calculated by Gilmer . The roughening temperature in this model corresponds to a parameter k T/ = 0.6. 

For liquid-vapor interfaces, the correlation length in the bulk is of t he order of atomic distance unless one is close to the critical point Hence the concept of local equilibrium is well justified in most practical circumstances For. solid surfaces above the roughening temperature, the concept also makes sense. Since the surface is rough adding (or removing) an atom to a particular part of the surface docs not disturb the local equilibrium state very much, and this sampling procedure can be used to determine the local chemical potential. This is the essence of the Gibbs-Thomson relation (1). 

The quantity under the square root in (5) becomes negative at small values of P5x, and P5, this happens at temperatures above the roughening temperature of the (110) surface where the solid-on-solid approximation for the step free energy f ( ) is not positive definite and the simple theory considered here breaks down. 

The viscoelastic samples to be tested by this method may be in different forms. The simplest to work with is a soft or liquid-like viscoelastic material such as mayonnaise or other food emulsions. These are easy samples to work with terms of sample loading. More solid-like samples such as cheese or food gels are more difficult to load onto the instrument in a consistent matter. The degree of compression of soft samples should ideally be controlled using a normal force measure or force rebalance system. Slippage is also a concern and roughened plates or even adhesives may be needed if slip is an issue. As this protocol is a general one, it is assumed that the sample is already loaded on the rheometer and has achieved equilibrium in terms of temperature and viscoelastic stmcture (time-dependent behavior). 

Once the solid particles enter the back end of the screw, they are carried forward and compressed by the rotation of the screw and the friction between the solids and the barrel. Friction can be increased by roughening the surface of the barrel, and particularly by grooving the internal surface of the barrel. 

It is difficult, however, to accept that the performance increase is caused by a surface roughening of the metal alloy crystallite as proposed by Los Alamos and Berkeley, since it would be expected that the surface rearrangements at these temperatures would be very fast, particularly for the nano-dimensions involved. It is a worthwhile exercise to translate the dimensions for solid state diffusion of a metal atom in a metal matrix from the usual m2hour to nmV (i.e., 1 m2hour is 1.6 x 1016 nm2 s 1). It is clear that atomic rearrangements on a nanometer dimensioned metal surface occur very rapidly, especially with the added inducements of... 

In order to investigate molecules adsorbed at the solid-liquid interface roughened electrode surfaces or metal colloids in solution (sols) are prepared. For investigations of the solid-gas or solid-vapour interface several methods are available to produce metal island films on SERS-active substrates. 

The situation inside a pit at a later stage may be represented by Fig. 20M(c). The walls of the pit are roughened by corrosion. The electrolyte in the pit is saturated with respect to AlCl, and there is a great deal of solid AlCl inside the pit, slowing down the movement of ions and allowing the pH to decrease. Eventually a stable pH is reached inside the pit. Its numerical value depends on the equilibrium constant in Eq. 55M or similar equations corresponding to other ions, depending... 

Colloid Mill Colloid mills are rotor-stator systems that can be used to reduce the particle size distribution of both liquid dispersions (emulsions) and solid dispersions (suspensions). The emulsion or suspension is pumped through a narrow gap that is formed by the rotating inner cone and the stationary outer cone. The width of the annulus can be adjusted by changing the relative position of the two cones. The principal size reduction in colloid mills is due to the high shear forces that are caused by the velocity difference between the rotor and the stator surfaces. To increase wall friction and reduce slip, surfaces are usually not smooth but are roughened or toothed, which, in turn, changes the flow conditions from laminar to turbulent, thereby increasing the shear forces in the annulus. 

---