Date of Degree

2-2016

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Alan M Lyons

Committee Members

Shuiqin Zhou

Alexander Greer

Marc Hodes

Subject Categories

Chemistry | Materials Science and Engineering | Polymer Chemistry

Keywords

Dropwise Condensation, Biomimetic Surface, Heat Transfer, Protein Repellent, Biocompatible, Wetting, Hydrophobic, 3D-Printing, Silica Nanoparticles, Surface Modification

Abstract

This thesis describes the study of protein adsorption and water condensation on hybrid superhydrophobic-hydrophilic surfaces for various technological applications such as diagnostics, artificial organs and medical devices, water collection, and heat transfer.

In the chapter 1, a general introduction to wetting theories, superhydrophobic surface, and hybrid surfaces is given. In chapter 2, design and fabrication of a hybrid superhydrophobic surface for studying dropwise condensation and heat transfer is discussed. Effect of surface chemistry and wettability on protein adsorption is discussed in the chapter 3. Finally, in chapter 4, the protein adsorption study on hybrid superhydrophobic surfaces made by 3D-printing and subsequent coating of the surface with modified silica nanoparticles to understand the effect of surface roughness and wettability on protein adsorption is described.

Condensation of water vapor is an essential process in power generation, water collection, and thermal management. Because of the high surface energy of the metal surfaces, filmwise condensation of water vapor occurs, forming a static, thermally insulating film. Numerous efforts have been made to create surfaces that promote dropwise condensation; however these efforts result in thermally insulating layers or degrade over time. Nature provides an alternative approach. The Namib beetle (Stenocara gracilipes) has a carapace that collects water by promoting dropwise condensation on raised hydrophilic regions which then roll off and slide along the hydrophobic surface. We designed and fabricated a hybrid superhydrophobic-hydrophilic surface to mimic, and improve upon, this behavior. An array of hydrophilic needles, thermally connected to a heat sink, was forced through a robust superhydrophobic polymer film. Condensation occurs preferentially on the needle surface due to differences in wettability and temperature. As the droplet grows, the liquid drop on the needle remains in the Cassie state and does not wet the underlying superhydrophobic surface after 5 days of testing. The water collection rate on this surface was studied using different surface tilt angles, needle array pitch values and needle heights. Water condensation rates on the hybrid surface were shown to be 4 times greater than for a planar copper surface and twice as large for hydrophobic silicon or superhydrophobic surfaces without hydrophilic features. A convection-conduction heat transfer model was developed; predicted water condensation rates were in good agreement with experimental observations. This type of hybrid superhydrophobic-hydrophilic surface with a larger array of needles could be used for heat transfer and water collection applications.

In chapter 3, we study the effect of surface chemistry and wettability on the adsorption of proteins with the goal of designing protein repellent, biocompatible, surfaces for medical devices, diagnostics, pharmaceutical and food processing applications. To this purpose oxidized silicon wafers were modified to render them either hydrophilic or hydrophobic. Hydrophilic surfaces with different surface chemistry were prepared by attaching: a) polyethylene glycol silane (PEG) or b) a zwitterionic siloxane (sulfobetaine siloxane; SBS) to the oxide surface. Hydrophobic surfaces were prepared by coating the silicon oxide surface with: a) methyl terminated polydimethyl siloxane (PDMS), or b) a chemical vapor deposition of dimethyl dichlorosilane (DMDCS). Prepared surfaces were characterized by contact angle goniometry and X-ray photoelectron spectroscopy (XPS). Surfaces were exposed to protein solutions of bovine serum albumin (BSA), fibrinogen, and lysozyme. The protein adsorption on the prepared surfaces were studied as a function of time using confocal fluorescence microscopy (CFM), XPS and contact angle (CA) measurement techniques. Maximum protein adsorption was observed on clean, unmodified hydrophilic silica surfaces (CA ~ 4°). Almost no protein was observed on PEG- and SBS-coated hydrophilic surfaces as indicated by CFM and XPS. Intermediate amounts of protein adsorption were observed on hydrophobic surfaces. Protein absorption was shown to change the wettability of the surface as measured by changes in CA. PEG- and SBS-modified surfaces showed almost no change in CA, which indicated little or no protein adsorption. Coating of a hydrophilic silicon dioxide surface with PEG or SBS was shown to be an effective approach to designing protein repellent surfaces.

This study examines the idea how the surface chemistry and wettability affect the protein adsorption on a flat surface (without any roughness). In the next chapter (Chapter 4), these findings and techniques are applied to study protein adsorption in a complex rough surface to study the effect of surface roughness and wettability on protein adsorption. Hopefully, that will help us to design an effective protein repellent surface for application in diagnostics and other applications where less non-specific protein adsorption is desired.

Superhydrophobic surfaces that show high water contact angles (˃150°)and low slip angles (˂10°)have been explored for various applications due to their excellent self-cleaning and low adhesion properties.Proteins and cells can adhere to hydrophobic surfaces, however, due to interactions between the hydrophobic portion of the protein and the hydrophobic surface, resulting in surface fouling. As a result of this change in surface wetting, the hydrophobic properties of the surfaces are lost. In the previous chapter, the hydrophilic coating with polyethylene glycol or zwitterionic molecules on the flat surfaces has been shown to be especially effective at preventing protein adsorption. In this chapter I have prepared superhydrophobic surfaces using hydrophilic nanoparticles to make protein-resistant surfaces. The effect of surface chemistry and roughness on the wetting behavior of superhydrophobic silica/silicone hybrid surfaces was also studied. In addition, the absorption of proteins on these surfaces was investigated with BSA as model protein with the goal of designing biocompatible super repellent surfaces for biomedical applications. Superhydrophobic surfaces made with TS530 were shown to adsorb minimum amount of protein.

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