Graduate Studies
First Advisor
Stephen A. Morin
Date of this Version
5-2024
Document Type
Article
Citation
A thesis presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Chemistry
Under the supervision of Professor Stephen A. Morin
Lincoln, Nebraska, May 2024
Abstract
This research explores ways for engineering elastomeric surfaces for programmed wettability enabling controlled droplet motion for microfluidic applications. Chemical modifications combined with the electric field were explored as stimuli to pattern wettability profiles to illustrate compartmentalized and series reactions on a single substrate and enhance control over the rate of reaction. PDSM was selected for this work because of the well-developed protocols of surface modification techniques and use as dielectric material.
Optimizing the parameters for dielectric layer thickness and required voltage for inducing sufficient dielectrophoretic forces driving droplets motion, and by patterning electric field, control over speed, directionality and distance could be achieved. After chemical modifications, it was observed that higher operating voltages were required because of the increase in dielectric constant. Possible reasons were voltage-induced enhanced hydrophobic recovery on activated PDMS surface occurring on the scale of minutes over the entire dielectric layer, and the impregnation of DTS molecules in the porous dielectric layer changing the porosity of network (as inferred from the optical modelling to explain change in refractive
index in Raman Spectroscopy based studies). To minimize this effect, monomer extraction was done but it increased surface roughness offering more adhesion force to the surface resulting in increasing required voltage. Therefore, to eliminate the risks associated with the use of higher voltage, mechanical strain induced hydrophobic recovery was proposed as a potential alternative technique to program surface wettability.
Additionally, this research presents a Raman Spectroscopy-based probe for detecting surface chemical states on elastomeric surfaces. DTS-based chemical gradients on surfaces exhibited no change in peak shift and width along the direction gradient, only the Raman intensity of methyl groups varied positively with increasing hydrophilicity. Raman Interference effect stemming from the change in refractive index along the gradient because of changing concentration of impregnated DTS molecules in the porous network and reduced per unit volume connectivity of Si-O bond, was attributed as the cause of this correlation. These findings suggest the potential use of Raman Spectroscopy and optical modeling method used in detecting surface modifications on complex surfaces, integral for transparent electronics, and understanding the behavior of polymer-based dielectrics.
Advisor: Stephen A. Morin
Comments
Copyright 2024, Nabeeha Malik. Used by permission