Colloidal systems optical microscopy

Two techniques for overcoming the limitations of optical microscopy are of particular value in the study of colloidal systems. They are electron microscopy36-37, in which the limit of resolution is greatly extended, and dark-field microscopy, in which the minimum observable contrast is greatly reduced. 

Khlebtsov et al. (1991) have theoretically and experimentally studied the dispersion effect of the refractive indices of particles and dispersion medium in the turbidity spectrum method. A new approach is put forward to estimate the optical dispersion of the components of a colloidal system and to consider it when the system s parameters are determined from the wavelength exponent. The method has been verified on PS lattices with the particle diameter from 80 to 800 nm. The elaborated version of the turbidity spectrum method can be used as a metrological test for particle sizes, not inferior to electron microscopy in accuracy. 

Solving the nucleation mechanism is important to understand structures and physical properties of any materials. To our best knowledge, no one has succeeded in observing directly the nucleation from the melt, because the number density of small nuclei on the order of nanometers (which we will henceforth call a nano-nucleus ) is too small to detect [4, 5]. Hence, only alternative experimental studies have been performed on macroscopic crystals (macro-crystals) by means of optical microscopy (OM) or bubble chamber [2]. Recent simulation studies performed on colloid systems [6, 7] also fail to provide a direct observation of nano-nucleation because the thermal fluctuation of nano-nuclei should be much more significant than that of macro-crystals or macro-colloids. This chapter introduces CNT, describes experimental approaches, and discusses the results of direct observation of nano-nucleation. 

There has been some research using synthetic polymers, as well. Owen, et al. (90) employed optical tweezers and video microscopy. Here, poly(ethylene oxide), a water soluble polymer, was adsorbed onto 1.1 m diameter sihca microspheres in aqueous media, and the pair interaction potential was examined. This modeled the stabilization of colloidal matter by adsorbed polymer. Figure 14.25 (90) provides a schematic of the system. Four molecular weights of poly (ethylene oxide) were examined, ranging from 4.52 x 10 to 1.58 x 10 g/mol. 

In addition, well-characterized polymer colloids will continue to serve as tools and models in various optical techniques (atomic force microscopy, optical tweezers, photonic force microscopy etc.) for investigating direct measurements of colloidal forces and studying physical properties of biological membranes and vesicles. All these techniques should be useful in different fields of applications, especially in microfluidics systems. 

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