Page 72 - Nanotechnology an introduction

the order of one day, to cure. In the case of microspheres, it is important that the crack intersects and fractures them rather than propagating
between them; this will depend on local elastic conditions at the crack tip.
If successful repair can be demonstrated to have high structural integrity, it is envisaged that these composites will be used in aircraft structures and
in other applications where high structural integrity in safety-critical structures is required. It is difficult to envisage how these concepts can be
extended into metallic structures, but they could be used in civil concrete structures. Presently the technology is in its infancy. The approaches used
so far are very primitive, and there needs to be substantial further development work before they could enter service. In particular, nondestructive
techniques for evaluating the strength of the repair are required, together with engineering data on the delamination resistance, the fatigue
resistance and the environmental resistance of the repair.

The concept of dye bleeding for damage detection works reasonably well, but an obvious disadvantage is the lack of capability to distinguish
damage needing attention from damage that can be left in service. Current synthetic realizations are a very long way from the type of self-repair
schemes used in biological structures, which show vastly better flaw tolerance (equation 2.25).

6.5.4. Nanofluids for Thermal Transport

Nanofluids are suspensions of nanoparticles or nanofibers in a liquid medium. The addition of nanoparticles should substantially enhance the heat
capacity of a fluid, and hence nanofluids have been proposed for heat transfer fluids. The reported enhancements of critical heat flux are especially
interesting in boiling heat transfer. Applications are found wherever cooling is important: in microelectronics, transportation, solid-state lighting, and
manufacturing. Thermal interface materials (TIM) are essentially very viscous nanofluids used to eliminate asperity between a surface and a heat
sink. Some reports of the degree of thermal conductivity enhancement are at variance with established theoretical predictions, however: both the
experiments and the theory need to be scrutinized carefully. Current theory cannot explain the strong measured temperature dependence of the
thermal conductivity of the nanofluids [167].
The nanoparticles may be fabricated in dry form and then dispersed in the fluid, or they may be produced in situ in the liquid. The latter route was
established in the Soviet Union some decades ago. About 1% by volume of copper nanoparticles or carbon nanotubes dispersed in ethylene glycol
or oil increases the thermal conductivity by 40% (copper) and 150% (carbon nanotubes). Similar enhancements using ordinary particles would
require more than 10% by volume. The nanofluids are also much more stable than suspensions of conventional particles.
6.5.5. Alternating Polyelectrolyte Deposition

If the substratum is electrified (via the Gouy–Chapman mechanism) and the dissolved molecule is a polyion with an electrostatic charge of opposite
sign, then it will adsorb on the surface and invert the charge; the strong correlations within the polymeric ion render the Gouy–Chapman mechanism
invalid [65]. The polyion-coated substratum can then be exposed to a different polyion of opposite sign, which will in turn be adsorbed and again
invert the charge; the process can be repeated ad libitum to assemble thick films [143].
This method of alternating polyelectrolyte deposition (APED), invented by Iler [84], appears to have immense potential as a simple, robust method
of surface modification. It requires the substrate to be electrostatically charged when immersed in water. It is then dipped into an aqueous solution
of a polyelectrolyte of opposite charge, with which it rapidly decomes coated. Any excess is then washed off, and the coated substrate is dipped
into a polyelectrolyte of the opposite charge, with which it now becomes coated, and whose excess is again washed off, and so on (Figure 6.10).

Figure 6.10 Upper panel: deposition of a polycation onto a negatively charged substrate followed by a polyanion. Lower panel: deposition of a polycation followed by a negatively charged nanoparticle onto a negatively charged substrate
[112].
There are few restrictions on the choices of polyelectrolytes. Much early work was done with polyallylamine as the polycation and polystyrene
sulfonate as the polyanion. The essential feature of the technique is that at each dipping stage the substrate charge is not only neutralized but
reversed (“overcharging”), hence allowing the deposition to be repeated indefinitely. This phenomenon contradicts the predictions of the mean-field
theories—Gouy–Chapman and Debye–Hückel—of the distribution of ions in the vicinity of charged surfaces (“electrified interfaces”). The
discrepancy arises because the charges of the polyions are correlated. Imagine a polyion approaching a surface already covered with its
congeners. The new arrival will repel the already adsorbed ones, creating a correlation hole (i.e., a negative image) permitting attraction (Figure
6.11) [65]. Since the gaps between already adsorbed polyions may be smaller than the size of one polyion, adsorption results in charged tails,
ensuring overcharging (Figure 6.12).