Date

2014

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

Gilchrist, James F.

Other advisers/committee members

Chaudhury, Manoj K.; Cheng, Xuanhong; Snyder, Mark A.

Abstract

Crystalline particle coatings can provide critical enhancement to wide-ranging energy and biomedical device applications. One method by which ordered particle arrays can be assembled is convective deposition. In convective deposition, particles flow to a surface via evaporation-driven convection, then order through capillary interactions. This thesis will serve to investigate convective deposition from fundamental and application-driven perspectives. Motivations for this work include the development of point-of-care diagnostic devices, macroporous membranes, and various energy applications. Immunoaffinity cell capture devices display enhanced diagnostic capabilities with intelligently varied surface roughness in the form of particle coatings. Relatedly, highly crystalline particle coatings can be used to template the fabrication of macroporous polymer membranes. These membranes display highly monodisperse pores at particle contact points. In addition, ordered areas of particles, acting as microlenses, can enhance LED performance by 2.66-fold and DSSC efficiency by 30%. Previous research has targeted the formation of crystalline monolayers of particles. However, much insight can be gleaned from "imperfect" coatings. The analysis of submonolayer coatings, exhibiting significant void spaces, provides insight as to the specific mechanisms and timescales for flow and crystallization. A pair of competing deposition modes, termed ballistic and locally-ordered, enables the intelligent design of experiments and enables significant enhancement in control of resultant thin film morphology. Surface tension-driven particle assembly is subject to a number of native instabilities and macroscale defects that can irreversibly compromise coating uniformity. These include the formation of three-dimensional "streaks," where surface tension-driven flow spurs on the nucleation of large imperfections. These imperfections, once nucleated, exhibit a feedback loop of dramatically enhanced evaporation and resultant flow. In addition, thick nanoparticle coatings, subject to enormous drying stresses, exhibit highly uniform crack formation and spacing in an attempt to minimize system energy. Both these imperfections yield insight on convective deposition as a fundamental phenomenon, and intelligent design of experiments moving forward. Cracking can be suppressed through layer-by-layer particle assembly, whereas streaking can be controlled via several significant process enhancements. Process enhancements include the addition of smaller constituent, as packing aids, to suspension, the application of lateral vibration, and the reversal of relevant surface tension gradients. The transition from unary to binary suspensions represents a significant improvement to convective deposition as a process. Nanoparticles act as packing, and flow, aids, wholly suppress macroscale defects under ideal conditions. A relative deficiency or excess of nanoparticles can generate complex coating morphologies including multilayers and transverse stripes. The application of lateral vibration to convective deposition allows the assembly of monolayer particle coatings under a larger range of operating conditions and at a faster rate. Macroscale defect formation can increased through an enhancement of the natural condition, where evaporative cooling generates a thermal gradient in drying droplets. Conversely, these defects can be suppressed with a reversal of this gradient, which will reverse the direction of surface tension-driven recirculation. These fundamental developments in understanding, and associated process enhancements, are critical in current efforts to scale up convective deposition. As convective deposition evolves from laboratory-scale batch experiments to continuous, large scale, coatings, repeatability and robustness, as well as an ability to controllably change thin film morphology, will be essential.

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