David B. Go

Ionic winds for locally enhanced cooling

Ionic wind engines can be integrated onto surfaces to provide enhanced local cooling. Air ions generated by field-emitted electrons or a corona discharge are pulled by an electric field and exchange momentum with neutral air molecules, causing air flow. In the presence of a bulk flow, ionic winds distort the boundary layer, increasing heat transfer from the wall. Experiments demonstrate the ability of ionic winds to decrease the wall temperature substantially in the presence of a bulk flow over a flat plate, corresponding to local enhancement of the heat transfer coefficient by more than twofold. Multiphysics simulations of the corona and flow describe the ability of the ionic wind to distort a bulk flow boundary layer and confirm the experimentally observed heat transfer enhancement trends.

A mathematical model of the modified Paschen’s curve for breakdown in microscale gaps

Traditionally, Paschen’s curve has been used to describe the breakdown voltage for gaseous ionization between two electrodes. However, experiments have shown that Paschen’s curve, which is based on Townsend effects, is not necessarily accurate in describing breakdown between electrodes spaced less than 15 μm apart. In this regime, electron field emission plays a significant role in the breakdown phenomenon, and recently an alternative mathematical description that accounts for ion-enhanced field emission was proposed to describe the breakdown voltage in small gaps. However, both Paschen’s curve and the small gap equation only work in certain regimes, and neither predicts the transition that occurs between Townsend and field emission effects—the so-called modified Paschen’s curve. In this work, a single, consistent mathematical description of the breakdown voltage is proposed that accounts for both Townsend ionization and ion-enhanced field emission mechanisms. Additionally, microscale breakdown experiment...

Paper-based microfluidic surface acoustic wave sample delivery and ionization source for rapid and sensitive ambient mass spectrometry

A surface acoustic wave-based sample delivery and ionization method that requires minimal to no sample pretreatment and that can operate under ambient conditions is described. This miniaturized technology enables real-time, rapid, and high-throughput analysis of trace compounds in complex mixtures, especially high ionic strength and viscous samples that can be challenging for conventional ionization techniques such as electrospray ionization. This technique takes advantage of high order surface acoustic wave (SAW) vibrations that both manipulate small volumes of liquid mixtures containing trace analyte compounds and seamlessly transfers analytes from the liquid sample into gas phase ions for mass spectrometry (MS) analysis. Drugs in human whole blood and plasma and heavy metals in tap water have been successfully detected at nanomolar concentrations by coupling a SAW atomization and ionization device with an inexpensive, paper-based sample delivery system and mass spectrometer. The miniaturized SAW ionization unit requires only a modest operating power of 3 to 4 W and, therefore, provides a viable and efficient ionization platform for the real-time analysis of a wide range of compounds.

Thermal Transport in Graphene Oxide – From Ballistic Extreme to Amorphous Limit

Graphene oxide is being used in energy, optical, electronic and sensor devices due to its unique properties. However, unlike its counterpart – graphene – the thermal transport properties of graphene oxide remain unknown. In this work, we use large-scale molecular dynamics simulations with reactive potentials to systematically study the role of oxygen adatoms on the thermal transport in graphene oxide. For pristine graphene, highly ballistic thermal transport is observed. As the oxygen coverage increases, the thermal conductivity is significantly reduced. An oxygen coverage of 5% can reduce the graphene thermal conductivity by ~90% and a coverage of 20% lower it to ~8.8 W/mK. This value is even lower than the calculated amorphous limit (~11.6 W/mK for graphene), which is usually regarded as the minimal possible thermal conductivity of a solid. Analyses show that the large reduction in thermal conductivity is due to the significantly enhanced phonon scattering induced by the oxygen defects which introduce dramatic structural deformations. These results provide important insight to the thermal transport physics in graphene oxide and offer valuable information for the design of graphene oxide-based materials and devices.

The solvation of electrons by an atmospheric-pressure plasma

Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H+, NO2−, NO3− and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

An analytical formulation for the modified Paschen’s curve

The modified Paschen’s curve describes the gaseous breakdown potential (voltage) in microscale gaps, when deviations from the traditional Paschen’s curve occur [F. Paschen, Ann. Phys. 273, 69 (1889)]. The deviation is due to ion-enhanced field emission that occurs in the high electric field of microgaps and acts as an additional cathode electron source. The present work derives an analytical formulation for the effect of ion-enhanced field emission and the modified Paschen’s curve that uses a consistent, single breakdown criterion. The proposed model does not require the fitting factor required in prior models and constitutes a single analytical equation for microscale breakdown and the modified Paschen’s curve.

Fundamental properties of field emission-driven direct current microdischarges

For half a century, it has been known that the onset of field emission in direct current microdischarges with gap sizes less than 10 μm can lead to breakdown at applied voltages far less than predicted by Paschens law. It is still unclear how field emission affects other fundamental plasma properties at this scale. In this work, a one-dimensional fluid model is used to predict basic scaling laws for fundamental properties including ion density, electric field due to space charge, and current-voltage relations in the pre-breakdown regime. Computational results are compared with approximate analytic solutions. It is shown that field emission provides an abundance of cathode electrons, which in turn create large ion concentrations through ionizing collisions well before Paschens criterion for breakdown is met. Breakdown due to ion-enhanced field emission occurs when the electric field due to space charge becomes comparable to the applied electric field. Simple scaling analysis of the 1D Poisson equation de...

Decoupling Interfacial Reactions between Plasmas and Liquids: Charge Transfer vs Plasma Neutral Reactions

Plasmas (gas discharges) formed at the surface of liquids can promote a complex mixture of reactions in solution. Here, we decouple two classes of reactions, those initiated by electrons (electrolysis) and those initiated by gaseous neutral species, by examining an atmospheric-pressure microplasma formed in different ambients at the surface of aqueous saline (NaCl) solutions. Electrolytic reactions between plasma electrons and aqueous ions yield an excess of hydroxide ions (OH(-)), making the solution more basic, while reactions between reactive neutral species formed in the plasma phase and the solution lead to nitrous acid (HNO2), nitric acid (HNO3), and hydrogen peroxide (H2O2), making the solution more acidic. The relative importance of either reaction path is quantified by pH measurements, and we find that it depends directly on the composition of the ambient background gas. With a background gas of oxygen or argon, electron transfer reactions yielding excess OH(-) dominate, while HNO2 and HNO3 formed in the plasma and by the dissolution of nitrogen oxide (NOx) species dominate in the case of air and nitrogen. For pure nitrogen (N2) gas, we observe a unique coupling between both reactions, where oxygen (O2) gas formed via water electrolysis reacts in the bulk of the plasma to form NOx, HNO2, and HNO3.

Evidence for the electrolysis of water by atmospheric-pressure plasmas formed at the surface of aqueous solutions

The formation of atmospheric-pressure plasmas with liquid electrodes holds great importance for a number of emerging technologies and applications, yet fundamental questions remain about the nature of the interactions at the plasma/liquid interface. In particular, when the liquid serves as the anode, the plasma supplies gas-phase electrons to the liquid surface, and how these electrons interact with the liquid has not been fully explained. In this work, we present experimental evidence that in the case of water, plasma electrons are involved in electrolytic reactions leading to the conversion of protons (H + ) to hydrogen gas. Reactions associated with water electrolysis are indirectly characterized by pH measurements that show qualitatively and quantitatively that the liquid at the plasma interface increases in basicity, consistent with the reduction of protons by plasma electrons. Mass spectrometry measurements confirm the evolution of hydrogen gas, directly providing evidence of water electrolysis. This work highlights the critical role that plasma electrons can play in plasma/liquid interactions with broad implications for any plasma system involving an aqueous electrode, including emerging applications in plasma medicine and plasma‐liquid materials synthesis.

High-Frequency AC Electrospray Ionization Source for Mass Spectrometry of Biomolecules

A novel high-frequency alternating current (AC) electrospray ionization (ESI) source has been developed for applications in mass spectrometry. The AC ESI source operates in a conical meniscus mode, analogous to the cone-jet mode of direct current (DC) electrosprays but with significant physical and mechanistic differences. In this stable conical-meniscus mode at frequencies greater than 50 kHz, the low mobility ions, which can either be cations or anions, are entrained within the liquid cone and ejected as droplets that eventually form molecular ions, thus making AC ESI a viable tool for both negative and positive mode mass spectrometry. The performance of the AC ESI source is qualitatively shown to be frequency-dependent and, for larger bio-molecules, the AC ESI source produced an ion signal intensity that is an order of magnitude higher than its DC counterpart.