Dynamics of Mesoscale Order Formation by Colloidal Nanoparticles Adsorbing onto a Solid Surface

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Ordered arrays of nanoparticles often exhibit unique characteristics, which are attracting much attention recently to be utilized as high-functional materials. For the development of such materials and devices, one has to know effects of particle-particle and particle-substrate interactions, e.g., onto the established structure. Thus a simulation method for predicting the dynamics of the order-formation process must be pressing need. This study has been aiming at developing a mesoscale simulation method for colloidal adsorption system, which would stand between the molecular dynamics and the discrete element method. The conclusions are as follows.1. Theoretical study: Based on the Langevin equation, we successfully developed a Brownian dynamics simulation code. Extensive simulations were conducted for a electrostatically stabilized colloidal nanoparticles onto a substrate with counter charge. The findings through the analysis of the simulation results are the following three items. 1) … More The determinant factor for order formation was NOT the average interaction potential energies between adsorbed particles, which was proposed in the past and has been believed to be valid so far. Instead, the factor was found to be what we call "one-directional average force". 2) Based on the above found determinant factor, the ordered structure can be made up with a controlled spacing between adsorbed particles. 3) The order formation process was proven to be essentially a stochastic process, and a model to estimate the probability of order formation against time was successfully developed.2. Experimental study: The substrate must possess a counter charge to have particles adsorbed on it. Then a sapphire plate was used for silica particles, while mica plate was used with polystylene particle with positive charge. Adsorption processes were observed by ex-situ SEM observation, and by in-situ AFM observation. As a result, we clarified the rate process of adsorption, and the importance of the mobility of particles on the surface that severely affects the onset of the order formation.Summarizing, a mesoscale simulation method for colloidal adsorption system was successfully developed, which can give a large amount of detailed information on the adsorption and order formation process. Further, both in theoretical and experimental aspects, many important process and phenomena in colloidal adsorption system were modeled successfully. Less