Khalid Eid

Topography-Based Surface Tension Gradients to Facilitate Water Droplet Movement on Laser-Etched Copper Substrates

This paper describes a method for creating a topography-based gradient on a metallic surface to help mitigate problems associated with condensate retention. The gradient was designed to promote water droplet migration toward a specified region on the surface which would serve as the primary conduit for drainage using only the roughness of the surface to facilitate the movement of the droplets. In this work, parallel microchannels having a fixed land width but variable spacing were etched into copper substrates to create a surface tension gradient along the surface of the copper. The surfaces were fabricated using a 355 nm Nd:YVO4 laser system and then characterized using spray testing techniques and water droplet (2-10 μL) injection via microsyringe. The distances that individual droplets traveled on the gradient surface were also measured using a goniometer and CCD camera and were found to be between 0.5 and 1.5 mm for surfaces in a horizontal orientation. Droplet movement was spontaneous and did not require the use of chemical coatings. The theoretical design and construction of surface tension gradients were also explored in this work by calculating the minimum gradient needed for droplet movement on a horizontal surface using Wenzels model of wetting. The results of this study suggest that microstructural patterning could be used to help reduce condensate retention on metallic fins such as those used in heat exchangers in heating, ventilation, air-conditioning, and refrigeration (HVAC&R) applications.

Correlation between Scattering Properties of Silver Particle Arrays and Fluorescence Enhancement

We report on the nanofabrication of patterned silver particle arrays using electron-beam lithography and the evaluation of their optical properties using backscattering and fluorescence spectroscopy. The silver particles varied in size from 100 to 250 nm and were in the shape of circles, squares, and triangles. Three inter-particle separations, 40, 65, and 90 nm as measured from the side of one particle to the side of the next particle, were used. We observed distinctive patterns of backscattering and fluorescence intensity depending on the particle size, inter-particle spacing, and excitation/emission wavelength used. Our approach allows for a study of the correlation between the backscattering intensities and fluorescence enhancement of silver particle arrays, which can be used to optimize the arrays for multi-fluorophore configuration for advanced sensing designs.

Spontaneous movement of water droplets on patterned Cu and Al surfaces with wedge-shaped gradients

We report a simple technique for moving water droplets on a hydrophilic aluminum surface containing a hydrophobic copper background. A surface tension gradient due to a triangular-shaped wedge moves the droplets towards the end of the wedge which contains more Al surface area. Movement on both horizontal surfaces and surfaces oriented vertically against gravity has been observed. The mode of droplet motion was found to depend on the wedge angle, and the speed was found to depend on both the wedge angle and the droplet contact angles on the Al and Cu.

CPP magnetoresistance of magnetic multilayers : mean-free-path is not the culprit

Abstract The series resistor and Valet–Fert models widely used to describe the current-perpendicular-to-plane (CPP) magnetoresistances of ferromagnetic/non-magnetic (F/N) metal multilayers were recently claimed to apply only for mean-free paths shorter than layer thicknesses; otherwise the mean-free-path was claimed to be an important length scale in the CPP-MR. Contradicting these claims, we show that the behaviors on which they were based do not change when the mean-free path in the N-layer is reduced from about five times to one-fifth its layer thickness. Part of these behaviors can be ascribed to finite spin-memory-loss (spin-flipping) in the F and N metals and the rest might be due to spin flipping at F/N interfaces.

Exchange bias and asymmetric magnetization reversal in ultrathin Fe films grown on GaAs (001) substrates

Magnetization measurements on a series of Fe films grown by molecular beam epitaxy on GaAs (001) substrates and capped with a thin Au layer reveal interesting exchange bias (EB) properties at low temperatures. The observed exchange bias decreases rapidly with increasing temperature, and completely disappears above 30 K. While the Fe samples were not grown with an intentionally deposited antiferromagnetic (AFM) layer, X-ray reflectometry, X-ray absorption near-edge spectroscopy carried out near the L-edge of Fe, and comparison with similar Fe/GaAs samples capped with Al, which do not show exchange bias, suggest that the exchange bias in the GaAs/Fe/Au multilayers is caused by an AFM Fe oxide at the Fe/Au interface formed by penetration of oxygen through the Au capping layer. The observed exchange bias is accompanied by a strikingly asymmetric magnetization reversal of the Fe films occurring when the magnetic field is applied at angles away from the easy axis of the film. The observed asymmetry can be inter...

Conductance quantization: A laboratory experiment in a senior-level nanoscale science and technology course

We describe a simple, inexpensive, and robust undergraduate lab experiment that demonstrates the emergence of quantized conductance as a macroscopic gold wire is broken and unbroken. The experiment utilizes a mechanically controlled break junction and demonstrates how conductance quantization can be used to understand the importance of quantum mechanics at the nanoscale. Such an experiment can be integrated into the curriculum of a course on nanoscale science or contemporary physics at the junior and senior levels.

Interfacial Spin Filtering and Temperature Variation of Copper/GaMnAs Contact Resistance

We studied the temperature dependence of the specific contact resistance (AR_C) of the copper/GaMnAs interface in the range from 15 K to 290 K. This range includes the Curie temperature of the GaMnAs film of about 145 K. AR_C is typically as low as 1×10^{- 7}Ωcm², and decreases slowly with decreasing temperature T. However, as T approaches Curie temperature (T_C), AR_C jumps to about double its value. We suggest that this behavior arises from the suppression of one of the two spin conduction channels, resulting in substantial spin polarization. This might offer a convenient scheme to estimate the spin polarization at a single ferromagnetic/nonferromagnetic (F/N) interface using all-electrical measurements.

Hot probe measurements of n-type conduction in Sb-doped ZnO microwires

The charge carriers type in antimony-doped ZnO (ZnO:Sb) microwires was studied using the hot probe technique. The wires were grown by a simple thermal evaporation method. Contrary to the expected p-type behavior reported for Sb doped ZnO thin films and nanowires, our hot probe measurements of representative single Sb-doped ZnO wires show a stable n-type behavior. The hot probe technique is a simple and efficient way to determine the charge carrier type from thermoelectric measurements on a single semiconductor wire and could offer an alternative to Hall effect measurements. The technique relies on creating a temperature gradient across the wire (i.e., heating one side of the wire relative to the other) and monitoring the resulting open-circuit voltage between the two ends. We also performed Energy Dispersive X-ray Spectroscopy measurements to identify and monitor the elemental composition in these ZnO:Sb wires.

Contact resistance as a probe of near-interface ferromagnetism in GaMnAs/Cu bilayers

We used contact resistance measurements as a sensitive probe of near-surface magnetism in GaMnAs by studying the temperature dependence of the contact resistance at Cu/GaMnAs interfaces. The specific contact resistance (ARC) has a peak that is clearly shifted towards lower temperature than that seen in GaMnAs resistivity. This shift suggests that the magnetization in the GaMnAs film is suppressed near the Cu interface. Furthermore, we show that when a native oxide layer is present between GaMnAs and Cu, the behavior of ARC is dramatically different, due to the thicker tunnel barrier at the interface.

Further evidence against mean-free-path effects in the CPP-MR

Mean-free-paths were recently claimed to be scaling lengths in the current-perpendicular-to-plane magnetoresistance when they are longer than the layer thicknesses of magnetic multilayers. We provide two new sets of evidence against such a claim; one involves reducing mean-free-paths by alloying, the other inserting a source of strong spin flipping.