Number Density Distribution of Small Particles around a Large Particle: Structural Analysis of a Colloidal Suspension

Some colloidal suspensions contain two types of particles-small and large particles-to improve the lubricating ability, light absorptivity, and so forth. Structural and chemical analyses of such colloidal suspensions are often performed to understand their properties. In a structural analysis study, the observation of the number density distribution of small particles around a large particle (g_LS) is difficult because these particles are randomly moving within the colloidal suspension by Brownian motion. We obtain g_LS using the data from a line optical tweezer (LOT) that can measure the potential of mean force between two large colloidal particles (Φ_LL). We propose a theory that transforms Φ_LL into g_LS. The transform theory is explained in detail and tested. We demonstrate for the first time that LOT can be used for the structural analysis of a colloidal suspension. LOT combined with the transform theory will facilitate structural analyses of the colloidal suspensions, which is important for both understanding colloidal properties and developing colloidal products.