In this chapter, we review the applications of optical coherence tomography (OCT) on the nondestructive characterization of the drying processes of colloidal droplets and latex coatings. Employing time-lapse, high-speed imaging, OCT can be used to monitor the dynamic process of drying colloidal droplets. With the aid of high-scattering, micron-sized tracer particles, fluid flows have been captured; phase boundaries are also visible in liquid crystal droplets; and the speckle contrast analysis differentiates the dynamics of particles, showing the packing process and the coffee ring phenomenon. In a waterborne latex coat, time-lapse OCT imaging reveals spatial changes of microstructures, i.e., detachment of latex, cracks, and shear bands; with speckle contrast analysis, 1D and 2D particles' packing process that is initiated from latex/air interface can also be monitored over time. OCT can serve as an experimental platform for fundamental studies of drying colloidal systems. In the future, OCT can also be employed as an in-line quality control tool of polymer coatings and paints for industrial applications.
The optical properties of nanoparticles and their array are closely related to their surface plasmon resonance of the particle and periodic structure parameters. In this paper, optical response features of single Ag nanosphere and periodical two-dimensional structure arrays are theoretically studied. The Mie theories and the multipole resonance theory are employed in the simulation. For Ag spheres each with a radius of less than 40 nm, one extinction peak can be observed and attributed to electric dipole resonance. When the radius of Ag sphere is more than 40 nm, apart from the peak contributed by the electric dipole, there is a peak of extinction at short wavelength, caused by resonance of the electric quadrupole. Generally, the frequency of multipole resonance decreases with increasing particle radius. The simulated results are in accord with the experimental data. For an infinite two-dimensional Ag-nanosphere arrays, two resonance peaks come from the dipole resonance of single particle and the Wood-Rayleigh anomalous diffraction. The frequency of multipole resonance can be controlled by tuning the size and the periodicity distribution of arrays. This paper provides a significant method to design advanced nanostructures with particular optical properties.
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