Text

theses

ETD doctoral

Electrospray deposition (ESD) utilizes the balance of electrostatic forces and surface tension within a charged spray to produce charged micro droplets with narrow dispersion in size. In ESD, each droplet carries a small quantity of suspended material to a target substrate. Past ESD falls into major categories: (1) continuous spray of films onto conducting substrates and (2) spray of isolated droplets onto insulating substrates. A cross over regime, or a self-limited spray, has only been limitedly observed in the spray of insulating materials onto conductive substrates. In such sprays, a limiting thickness emerges where the accumulation of charge repels further spray. In this dissertation, we present a systematic investigation on the self-limiting electrospray deposition (SLED), including (1) fundamental mechanism of thickness-limited spray, (2) geometry limits of SLED, (3) compositional limits of SLED and (4) morphological control through homogenous gelation in SLED. (1) We have demonstrated that SLED will occur when spraying insulating glassy materials from volatile solvents, with the substrate temperature below its glass transition temperature. This leads to charge accumulation on the coatings surface that slows the growth of the film thickness. (2) We employed model macroscale substrates to quantitatively study the technique’s limits with regard to geometry and scale. Further, to optimize the fabrication process for micro-coatings by SLED, we combined SLED with pre-existing polymer films, optionally patterned, to study SLED’s fundamental behavior in a bilayer geometry. The results reveal that the pre-deposited coatings can affect the coating thickness by SLED and the selection of polymer coatings determines the magnitude of this effect. (3) We have investigated the compositional limits of self-limiting and non-self-limiting solute blends through SLED. The results show that composited polymers still exhibit self-limiting ability through SLED and their mechanical properties could be tuned effectively within compositional ranges. (4) The morphology of coatings created by ESD can be generally divided into three categories: wire mats, particles, and films. We have demonstrated that the ESD of a fibril gel former, methylcellulose (MC) in water:ethanol, leads to gelation enhanced by shear and surface charge that electrospins on a drop-by-drop basis to create forests of individual nanowires.

ETD_11390

Ph.D.

Includes bibliographical references

doi:10.7282/t3-m4te-7550

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