Gary Tepper

Microscale polymeric helical structures produced by electrospinning

Microscale helical coils consisting of a composite of one conducting and one nonconducting polymer were produced using electrospinning. The nonconducting polymer was poly(ethylene oxide) and the conducting polymer was poly(aniline sulfonic acid). The coil structures were studied over a range of processing conditions and fiber composition. The data suggest that the helical structures are formed due to viscoelastic contraction upon partial neutralization of the charged fibers. Polymeric microcoils may find applications in microelectromechanical systems, advanced optical components, and drug delivery systems.

High surface area polymer coatings for SAW-based chemical sensor applications

Abstract High surface-to-volume ratio coatings consisting of nano-scale polymer particles were applied to surface acoustic wave (SAW) transducers. The resulting sensors were tested upon exposure to analyte vapors and were compared with sensors developed from bulk films of the same polymers. The relative importance of surface adsorption and bulk absorption was investigated and it is shown that high surface area coatings can be used to improve the sensitivity and response time particularly in the case of polymers exhibiting low vapor permeability. Furthermore, because the mean particle diameter is small compared to the spacing of the SAW interdigital electrodes, nano-scale particulate coatings appear acoustically uniform and are therefore, compatible with SAW technology. The high surface-to-volume ratio coatings were deposited using a spray-on technique known as rapid expansion of supercritical solutions (RESS) and both glassy and viscoelastic polymers were studied.

Predicting shape and stability of air–water interface on superhydrophobic surfaces with randomly distributed, dissimilar posts

A mathematical framework developed to calculate the shape of the air–water interface and predict the stability of a microfabricated superhydrophobic surface with randomly distributed posts of dissimilar diameters and heights is presented. Using the Young–Laplace equation, a second-order partial differential equation is derived and solved numerically to obtain the shape of the interface, and to predict the critical hydrostatic pressure at which the superhydrophobicity vanishes in a submersed surface. Two examples are given for demonstration of the method’s capabilities and accuracy.

In situ, noninvasive characterization of superhydrophobic coatings.

Light scattering was used to measure the time-dependent loss of air entrapped within a submerged microporous hydrophobic surface subjected to different environmental conditions. The loss of trapped air resulted in a measurable decrease in surface reflectivity and the kinetics of the process was determined in real time and compared to surface properties, such as porosity and morphology. The light-scattering results were compared with measurements of skin-friction drag, static contact angle, and contact-angle hysteresis. The in situ, noninvasive optical technique was shown to correlate well with the more conventional methods for quantifying surface hydrophobicity, such as flow slip and contact angle.

Effect of fiber orientation on shape and stability of air-water interface on submerged superhydrophobic electrospun thin coatings

To better understand the role of fiber orientation on the stability of superhydrophobic electrospun coatings under hydrostatic pressures, an integro-differential equation is developed from the balance of forces across the air–water interface between the fibers. This equation is solved numerically for a series of superhydrophobic electrospun coatings comprised of random and orthogonal fiber orientations to obtain the exact 3D shape of the air–water interface as a function of hydrostatic pressure. More important, this information is used to predict the pressure at which the coatings start to transition from the Cassie state to the Wenzel state, i.e., the so-called critical transition pressure. Our results indicate that coatings composed of orthogonal fibers can withstand higher elevated hydrostatic pressures than those made up of randomly orientated fibers. Our results also prove that thin superhydrophobic coatings can better resist the elevated pressures. The modeling methodology presented here can be used...

Predicting shape and stability of air–water interface on superhydrophobic surfaces comprised of pores with arbitrary shapes and depths

An integro-differential equation for the three dimensional shape of air–water interface on superhydrophobic surfaces comprised of pores with arbitrary shapes and depths is developed and used to predict the static critical pressure under which such surfaces depart from the non-wetting state. Our equation balances the capillary forces with the pressure of the air entrapped in the pores and that of the water over the interface. Stability of shallow and deep circular, elliptical, and polygonal pores is compared with one another and a general conclusion is drawn for designing pore shapes for superhydrophobic surfaces with maximum stability.

An electrospray-based, ozone-free air purification technology

A zero-pressure-drop, ozone-free air purification technology is reported. Contaminated air was directed into a chamber containing an array of electrospray wick sources. The electrospray sources produce an aerosol of tiny, electrically charged aqueous droplets. Charge was transferred from the droplets onto polar and polarizable species in the contaminated air stream and the charged contaminants were extracted using an electric field and deposited onto a metal surface. Purified air emerged from the other end of the chamber. The very small aqueous electrospray droplets completely evaporate so that the process is essentially dry and no liquid solvent is collected or recirculated. The air purification efficiency was measured as a function of particle size, air flow rate, and specific system design parameters. The results indicate that the electrospray-based air purification system provides high air purification efficiency over a wide range of particle size and, due to the very low power and liquid consumption ...

Modeling resistance of nanofibrous superhydrophobic coatings to hydrostatic pressures: The role of microstructure

In this paper, we present a numerical study devised to investigate the influence of microstructural parameters on the performance of fibrous superhydrophobic coatings manufactured via dc and ac electrospinning. In particular, our study is focused on predicting the resistance of such coatings against elevated hydrostatic pressures, which is of crucial importance for submersible applications. In our study, we generate 3D virtual geometries composed of randomly or orthogonally oriented horizontal fibers with bimodal diameter distributions resembling the microstructure of our electrospun coatings. These virtual geometries are then used as the computational domain for performing full morphology numerical simulations to establish a relationship between the coatings’ critical pressure (pressure beyond which the surface may depart from the Cassie state) and their microstructures. For coatings with ordered microstructures, we have also derived analytical expressions for the critical pressure based on the balance o...

High resolution room temperature ionization chamber xenon gamma radiation detector

Abstract A unique thin walled dual-type gridded ionization chamber gamma radiation detector using ultra pure Xe gas as the detection medium is described. The detector was operated at room temperature and the energy spectra of 60Co, 137Cs, 22Na and 133Ba were obtained. An energy resolution of (16 keV) 2.4% FWHM was determined for the 662 keV 137Cs gamma peak which is substantially better than the resolution of conventional NaI(Tl) detectors. The effect of electric field strength, grid field ratio, gamma energy, and electronegative impurities on energy resolution are discussed.

Microscale electrospinning of polymer nanofiber interconnections

Polymer fiber interconnects were produced between microscale features on a substrate using only electrostatic forces. Electric-field-driven directed growth of nanoscale carboxymethylcellulose fibers was achieved between microscale droplets of a concentrated polymer solution. The fibers were studied using atomic force and scanning electron microscopy and were observed to emerge from the tip of conical protrusions formed at the surface of the droplets. The conical structures appear to be analogous to the characteristic Taylor cones formed in an electrospinning process and the process is interpreted as a microscale version of electrospinning requiring significantly lower driving potentials.