Monitoring particle adsorption and thin film formation by laser reflectometry near the critical angle

Reflection and transmission of the light in a random medium are composed by coherent and incoherent waves. The coherent one can be modeled as interacting with a medium with effective optical coefficients. In a random dilute suspension, the coherent wave travels in a medium with an effective index of refraction given by the van de Hulst formula. This effective index is, in general, complex. The imaginary part takes into account the loss of the coherent wave due to scattering. Internal reflection, due to random particles in suspension defines a critical angle determined by the effective index of refraction of the particles in suspension. The curve of reflectivity is smoothed near the critical angle by the imaginary part of the effective index of refraction. One can show that the diffuse component of the reflection tends to zero at the critical angle. In this work, laser reflectometry near the critical angle is used to study particle adsorption on a flat surface. We monitored the adsorption of polystyrene particles with positive and negative charge in suspension. This method allows the direct measuring of reflectivity and its angle derivative on the prism surface where is formed the film.