Nanoscience and Microsystems ETDs

Publication Date

Spring 5-15-2019

Abstract

Many biological photonic structures in nature exhibit a significant degree of disorder within their periodic framework that enhances their optical properties. However, how such disorder contributes to the unique photonic characteristics is not yet fully understood. To facilitate studies on this topic, we investigated self-assembly of microspheres as a method to controllably introduce randomness to photonic structures. Specifically, we examined Langmuir-Blodgett assembly, a layer-by-layer fabrication technique. We developed and experimentally verified a model for the process and determined a condition of surface pressure and substrate pulling speed that corresponds to a maximum structural order in a layer. Along the trajectory described by this condition, disorder can be controllably introduced by increasing the pulling speed. Our model also describes a condition for maximum structural order for multilayer assembly: as the number of layers increases, the surface pressure should also increase at a fixed pulling speed. Overall, we have demonstrated that by carefully choosing assembly parameters along the optimal trajectory, disorders within Langmuir-Blodgett films can be systematically introduced.

To further demonstrate usefulness of disordered photonic structures fabricated from self-assembly methods, we investigated radiative cooling performance of microsphere-based disordered materials under direct sunlight. Radiative cooling is a process in which an object passively loses heat via radiation and thus has a potential to reduce consumption of electricity used for thermal management. Toward a goal of making radiative cooling technology more accessible, we investigated two scalable, and inexpensive methods for fabricating microsphere-based structures that can achieve efficient radiation cooling. Specifically, colloidal sedimentation method and spray coating were employed to create coatings that consist of randomly arranged microspheres. With a systematic study of light scattering in microsphere-based disordered media, we showed how structural parameters influence radiative cooling performance. By combining this understanding with the two facile fabrication methods, we demonstrated that black substrates coated with our microsphere-based materials achieved substantial cooling below ambient temperature even under direct sunlight exposure. Our coatings also outperformed commercially available paints designed for daytime cooling, without use of sophisticated fabrication process or expensive materials. We demonstrated further that cooling capability of our microsphere-based structures was improved by using hollow microspheres instead of solid particles and that mechanical durability was enhanced when the hollow microspheres were embedded in a silicone matrix. Overall, this work provides a path toward wider applications of radiative cooling achieved by microsphere-based disordered systems.

Keywords

Disordered photonics, Langmuir-Blodgett assembly, radiative cooling, microspheres

Document Type

Dissertation

Language

English

Degree Name

Nanoscience and Microsystems

Level of Degree

Doctoral

Department Name

Nanoscience and Microsystems

First Committee Member (Chair)

Sang Eon Han

Second Committee Member

Sang M. Han

Third Committee Member

Andrew Shreve

Fourth Committee Member

Terefe Habteyes

Comments

Chapter 4 is fixed.

Download

Share

COinS