Wetting and spreading phenomena affect a wide variety of fine-particle processing operations such as the preparation of particle dispersions, the dissolution of solids, wet separation processes for particulate materials, and the drying of solids. This review attempts to highlight the current state of understanding of wetting and spreading phenomena applicable to particulate systems. Aspects of wetting of individual particles, and larger collections of particles are addressed. Following a brief introduction, there view is organized into five main sections.
Section 2 describes the general features of wetting and spreading phenomena and introduces the vocabulary associated with the description of wetting. Most of the original analysis and current investigations of wetting dealt with the contacting of liquids onto well defined solid surfaces. Although wetting of particles is far different from this idealized picture, the phenomena that contribute to the wettability of particles are the same. Thus, the results presented in Section 2 form the foundation for the understanding of the wettability of particles and collections of particles.
Section 3 discusses the relationship between wettability and the molecular features of both the solid and the contacting fluid. In general, there are both physical and chemical contributions to wettability. Understanding of these effects can lead to useful predictions of wetting tendencies, and also offer insight into how wetting can be modified through the use of dispersants or wetting aids to compatibiliie the solids and liquids.
Section 4 reviews the state of understanding of the wetting of individual isolated particles. e various methods that have been developed over the years for the assessment of the wettability of particles are discussed and the difficulties associated with these measurement techniques are presented. Quantification of the influence of wetting on the dispersibility and stability of individual particles are also described.
Section 5 reviews wetting processes for large quantities of particles or individual agglomerates of particles. Critical conditions for the spontaneous wetting of collections of particles are discussed. The various stages associated with the wetting of collections of particles are also presented. The problems associated with contacting liquids with large quantities of particles are outlined. The influence of wetting on cohesivity of particle compacts and agglomerates is also described.
Section 6 provides a brief review of some practical aspects of wetting phenomena for particle processing technologies. A specific issue addressed is the influence of wetting on the dispersibility of agglomerates.
The summary section of the review identifies some unresolved problems pertinent to particle processing that are recommended for additional research.
The principle conclusion of the review is that while there has been much study of the wetting of particles and large quantities of particles, significant difficulties remain in characterizing the important phenomena. Although much theoretical and experimental effort has been spent in an attempt to quantify wetting phenomena or to predict wetting tendencies in particulate systems, these studies are often thwarted by the lack of readily characterizable surface features of particles at all of the relevant length scales. These important features include both the morphology of the particle surface and the chemical makeup of the particle interface with the fluid.
Since the same physical and chemical phenomena govern the wetting of both ideal (flat, rigid, chemically homogeneous) solids and particle surfaces, it is understandable that the study of the wetting of particles has borrowed concepts originally developed by surface scientists for the study of fluids with idealized surfaces. However, particle technologists must contend with impreciesely defined parameters such as apparent contact angle and inferring wetting behavior based on assumptions about the structure and homogeneity of the particle surface. While the lack of uniformity and detailed knowledge of the particle sufiace may prevent quantitative a priori predictions about wetting, the theories do allow predictions of relative wetting behavior and experimental methods can be used to assess the effectiveness of treatment strategies designed to improve wettability of particle systems.