Date on Master's Thesis/Doctoral Dissertation

12-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Willing, Gerold A.

Committee Member

Williams, Stuart J.

Committee Member

Jaeger, Vance

Committee Member

Watters, James

Author's Keywords

Nanoparticle halo; stabilization; silsesquioxane; temperature; molecular dynamics; umbrella sampling

Abstract

One of the methods of assembling colloids into 3D crystal structures is through the use of nanoparticle haloing. Nanoparticle haloing is a stabilization mechanism in binary particle suspensions possessing both a size and charge asymmetry, with which the nanoparticles aid in the bulk suspension’s stability. By altering the volume fractions of nanoparticles, it is possible to control the effective repulsion between the microparticles. Understanding the colloidal interactions and aggregate crystallinity as a function of nanoparticle concentration, temperature, and time are key challenges in developing future materials and designing crystalized 3D colloidal systems. In this study, we investigated the effect of temperature and nanoparticle volume fraction on the aggregation size using experimental techniques and molecular dynamics simulations. Gravity settling results showed a rapid aggregation in the absence of the nanoparticles due to the van der Waals interactions. However, by adding the nanoparticles to the system, the rate of gravity settling and aggregation significantly decreased due to the effective potential barrier that arises from the nanoparticle halo formation. The effect of temperature on the aggregation of the nanoparticle haloing systems was investigated using a confocal microscopy. By applying a temperature shock to the binary suspensions, the average colloid aggregates' size increased while the systems' coarseness decreased. The average aggregate size growth was more significant at the higher temperatures and the lower nanoparticle volume fractions. Overall, applying the temperature shock resulted in a more idealized structure with higher crystallinity. Molecular dynamics simulations were employed to determine the repulsive barrier between colloidal particles induced by the nanoparticles as a function of nanoparticle volume fraction. Results showed that the induced repulsive barrier between the microparticles increases with increasing the volume fractions of nanoparticles, and it reaches 6.5 kBT at the highest nanoparticle volume fraction of 10-3. This potential barrier was strong enough to prevent aggregation gelation and increase the stability of the suspension, which was in agreement with the experimental results.

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