Some Classical and Emerging Experimental Tools

In this section, we present a few examples of instruments available for visual observation and imaging of colloids and surfaces, for measurement of sizes and for surface force measurements. Such a presentation can hardly be comprehensive in fact, that is not our purpose here. Throughout the book, we discuss numerous other techniques such as osmotic pressure measurements, light and other radiation scattering techniques, surface tension measurements, 

The main objective of the present section, however, is to begin with a very standard technique such as optical microscopy and to use it to illustrate why colloids are difficult to see and what modern developments have emerged in recent years to allow us to see and do things that were considered impossible until a decade ago. We also use this opportunity to review briefly some new techniques that are currently available to measure interaction forces between particles directly. We appeal to some of these techniques in other chapters when we discuss colloidal forces. 

Because of the particle sizes involved, classically the optical microscope has been the instrument of choice especially for lyophobic colloids. Excellent books and manuals are available (Bradbury 1991 Cherry 1991 Schaeffer 1953) on the numerous variations of optical microscopy, and we do not go into all the details. Our purpose here is merely to point out some very elementary principles that make this method ideally suited for direct examination of colloids. We also use this introduction as a first step in pointing out modern techniques that fall under the class of microscopy but use principles (e.g., electron tunneling see Vignette 1.8) and radiation (e.g., electron or x-ray) other than those used in optical microscopy. 

19 Basic optical principle governing the operation of an optical microscope (a) the geometry on which the resolving power d of a microscope is based (b) detail showing how light from both sources must be intercepted by the lens to become part of the image. 

Equation (21) is called the Bragg equation after the father-and-son team of W. H. and W. L. Bragg (Nobel Prize, 1915) it is the underlying relationship for all diffraction phenomena. We encounter the Bragg equation again in Chapter 9 when we discuss the diffraction of low-energy electrons by surface atoms (see Section 9.8b). 

---