arXiv subjects

Harold McQuaid

Publications and source records attributed to Harold McQuaid.

3 recordsLinked to original sources

arXiv2025

Negative charge acquisition by isolated single microdroplets from plasma exposure at atmospheric pressure

Charged liquid microdroplets have generated significant interest recently due to the observation of chemical reaction rate enhancement by orders of magnitude. Droplet charging by plasma irradiation has been observed along with significantly enhanced reaction rates in liquid. In this paper we measure the charge on fixed size (47 $μ$m) individual droplets, exposed for approximately 4 ms to a low temperature RF-driven plasma operated at atmospheric pressure. The measured charge values (0.8 - 2.0 $\times 10^{7}$ electrons) approached the Rayleigh limit for the given droplet diameter. Results were compared with finite element simulations of plasma charging which provided estimates of surface electric fields and charge fluxes to the droplet surface and helps advance the development of a theoretical framework for plasma-charged microreactors.

physics.plasm-ph↗

arXiv2025

Free electron charging of microdroplets in a plasma at atmospheric pressure

Gas-phase microdroplets have recently demonstrated exceptional chemical properties via suspected mechanisms such as contact electrification and surface charge pinning, producing high surface electric fields. Here, microdroplets are injected into a low-temperature atmospheric-pressure plasma and exposed to an excess free-electron flux. We report the first measurements of droplet charge in a fully collisional plasma, with averages up to 2.5 $\times 10^{5}$ electrons for 15 $μ$m droplets, dependent on absorbed power. Simulations indicate solid particles under similar conditions acquire $\sim$40\% less charge, likely due to low-mobility water cluster ion formation around evaporating droplets, and predict surface electric fields up to $10^{7}$ V m$^{-1}$ for the smallest droplets (3 $μ$m). Plasma exposure also produces H$_{2}$O$_{2}$ in the liquid, with concentrations up to 33 mM, corresponding to remarkable generation rates exceeding 275 M s$^{-1}$. The system involves complex, not yet fully elucidated mechanisms. A controlled plasma environment enables defined charge levels and exposure times, allowing systematic study and enhancing droplet properties.

physics.plasm-ph↗

arXiv2022

Generation and delivery of free hydroxyl radicals using a remote plasma

We demonstrate a new gas-based OH generation source using a low power RF-driven atmospheric pressure plasma configured to deliver the radical flux into the far effluent region, well away from interference from other plasma factors such as electric fields, currents, and UV radiation. Using He-H2O gas chemistry isolated from the laboratory air, the plasma generated flux consists of H2O2 and OH, varying with H2O vapour content and absorbed power density. Peak flux values were 2.3 nmol s-1 and 0.23 nmol s-1 for H2O2 and OH respectively at a distance of 50 mm from the plasma, with 790 ppmv H2O and a power density of 1E8 W m-3. The maximum OH flux density was 4.5 x 1E19 m-2 s-1 falling to 1.7 x 1E19 m-2 s-1 at 110 mm, equivalent to generation rates of 74 uM s-1 and 28 uM s-1. Despite high OH recombination rates at the plasma exit, the escaping flux is still significant, indicating a viable delivery capability to downstream targets. Its performance with regard to OH generation rates compares well with traditional OH generation techniques such as radiolysis, advanced oxidation processes and enhanced Fenton-chemistry approaches where OH production rates are sub-uM s-1. Delivering precisely quantifiable OH fluxes provides new opportunities for scientific studies and technological opportunities in cell biology, atmospheric chemistry, protein unfolding and systematic dose studies for plasma-based and other OH related potential medical treatments.

physics.plasm-ph↗