Surface instability interactions

In a force-displacement curve, the tip and sample surfaces are brought close to one another, and interact via an attractive potential. This potential is governed by intermolecular and surface forces [18] and contains both attractive and repulsive terms. How well the shape of the measured force-displacement curve reproduces the true potential depends largely on the cantilever spring constant and tip radius. If the spring constant is very low (typical), the tip will experience a mechanical instability when the interaction force gradient (dF/dD) exceeds the... 

Fig. 1. Schematic diagram illustrating the mechanical instability for (a) a weak spring (spring constant k) a distance D from the surface, experiencing an arbitrary surface force (after [19]) and (b) the experimentally observed force-distance curve relative to the AFM sample position (piezo displacement) for the same interaction.

The channel-to-channel interactions may affect pressure drop between the inlet and the outlet manifold, as well as associated temperature of the fluid in the outlet manifold and temperature of the heater. The frequency and the phase are the same for all these fluctuations. They increase at a constant value of mass flux with increasing heat flux. The large heated wall temperature fluctuations are associated with the CHF. As the heat flux approached CHF, the parallel-channel instability, which was moderate over a wide range of heat fluxes, became quite intense and should be associated with maximum temperature fluctuation of the heated surface. 

The studies on adhesion are mostly concerned on predictions and measurements of adhesion forces, but this section is written from a different standpoint. The author intends to present a dynamic analysis of adhesion which has been recently published [7], with the emphasis on the mechanism of energy dissipation. When two solids are brought into contact, or inversely separated apart by applied forces, the process will never go smoothly enough—the surfaces will always jump into and out of contact, no matter how slowly the forces are applied. We will show later that this is originated from the inherent mechanical instability of the system in which two solid bodies of certain stiffness interact through a distance dependent on potential energy. 

The friction from the repulsive pinning center is of particular interest because it is contrary to the common belief that friction must result from attractive interactions between sliding surfaces. The results presented in Fig. 17(a) demonstrate that friction can be created by purely repulsive interactions. What really matters is the instability of the sliding body and energy dissipation, rather than the attractive or repulsive nature of interactions. This may also shed a light on the efforts to explore the correlation between friction and adhesion. 

For solid surfaces interacting in air, the adhesion forces mainly result from van der Waals interaction and capillary force, but the effects of electrostatic forces due to the formation of an electrical double-layer have to be included for analyzing adhesion in solutions. Besides, adhesion has to be studied as a dynamic process in which the approach and separation of two surfaces are always accompanied by unstable motions, jump in and out, attributing to the instability of sliding system. 

If the polymer layers increases the stability of the dispersion, it is denoted steric stabilisation. The polymer must fulfil two key criteria (i) the polymer needs to be of sufficient coverage to coat all the particle surfaces with a dense polymer layer, and (ii) the polymer layer is firmly attached to the surface. How this is engineered is beyond the scope of this article, but the consequences of not satisfying these criteria are informative in understanding the effect that polymers have on the overall interparticle interaction. Since complete or incomplete coverage of the particles results in very different properties (i.e stability or instability), this is clearly one way in which minimal change in initial conditions can lead to major differences in product. 

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

The role of instabilities involving confined impurity atoms has been investigated by Mtiser using a model in which two one-dimensional (1-D) or 2-D surfaces were separated by a very low concentration of confined atoms and slid past one another.25 The motion of the confined atoms was simulated with Langevin dynamics where the interactions between these atoms were neglected and the atom-wall interactions were described by... 

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