Ubiquity of Colloids

The second extraordinary feature of colloids is the problem that colloids should not really exist according to the first law of adhesion. Just as the clouds should not stay up in the atmosphere, because water is much denser than air, so the droplets should not remain separate but should coalesce under van der Waals 

Each droplet is spherical because it strives to minimize its surface area. In the same way, the droplets should aggregate to reduce the overall surface and form a single large sphere, going from Fig. 10.2(a) to (b). The energies involved in this coalescence are known from experiments like the Wilhelmy plate shown in Fig. 10.2(c). A thin wetted plate, width L, dips into the water and is sucked in by 

When the water droplets coalesce, to reduce the total surface energy, the surface energy y is released as heat and it is evident that the temperature rise of the water is inversely proportional to the droplet diameter. Thus, Fig. 10.3 shows that 1 nm diameter water drops can be boiled by the coalescence phenomenon, illustrating the thermodynamic driving force for aggregation. 

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