Chapter 27 Liquid bridges in granules

Abstract

In many industries, granulation processes are often applied to mixtures of powders in which the component solids do not exhibit the same surface properties. Large discrepancies in surface energy create problems during granulation because powders can be selectively wet at the expense of others. In the pharmaceutical industry, most drugs have a low surface energy and therefore are poorly wet by common granulation liquids. However, the behavior of pendular liquid bridges when the liquid-to-solid contact angle is large has not been extensively covered in the literature. The majority of models dealing with pendular liquid bridges either assume zero, small, or fixed contact angles. When the wetting hysteresis is important, the solid–liquid interfaces at the boundaries of the liquid bridge can remain constant with particle separation until a critical separation is reached, at which point the bridge liquid recedes from one particle surface and the corresponding solid–liquid interface reduces. The reduction of the solid–liquid interfacial area with particle separation is accentuated by low wetting hysteresis that drastically alters the shape of pendular liquid bridges, their rupture distance, and post-rupture liquid volume distribution on the solid particles. In this chapter, the current theory on liquid bridges between pairs of particles is presented, followed by a detailed review of the work on the modeling of liquid bridges and the relationship between the micro- and macro-scale granule behavior, developed from direct measurements and observations of liquid bridges among smooth spheres and between real pharmaceutical powders and binders.