Natural phenomena, molecular causes

As mentioned several times before, the natural adhesion forces (see Section 5.1.1), caused, for example, by molecular (e.g. van-der-Waals) or electrical forces (e.g. due to asymmetric molecular structures), may become much larger than the separation forces which are mass and shape related. Therefore, if collisions or, generally speaking, contact between ultrafine particles occur, a rather strong bond will develop. This phenomenon is also responsible for the fact that most nano-sized particles do not exist as individuals but as assemblies of many particles (Fig. 10.41) this might be a problem in those applications where ultrafine particles must be deposited individually or in monolayers (see Chapter 12). For the effective separation of such particles from gases, however, agglomeration is desired and must be promoted. 

This suggests that near the critical point a fluid displays unusual behavior. The behavior is unusual because natural fluctuations are not completely suppressed, as they are when Kt- is bounded and positive, but neither are fluctuations able to grow so as to force a phase change, as they can when Kj is negative. Such fluctuations cause the observable phenomenon known as critical opalescence moreover, critical fluctuations are independent of molecular constitution, so that near their critical points all fluids have certain traits in common. Descriptions of critical phenomena are beyond the scope of this book see instead [6]. 

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