Goal: To clarify fundamental physics and chemistry processes at plasma-material interfaces
Background: The dynamics of multiphase systems containing gas, plasma and liquid or solid states are characterized by vastly different intrinsic time scales: a slow liquid or solid motion time scale (usually milliseconds), a fast electron time scale (nanoseconds) and an intermediate ion time scale (microseconds). This work leverages our expertise on the presence of solid microparticles in plasmas, “dusty plasmas”, to a new regime of “misty plasmas” that describe a gaseous plasma within which liquid micro-droplets levitate and can absorb component plasma species, evaporate, and break up electrostatically, drastically changing plasma properties. Plasma is often generated in liquids within voids and gas bubbles. However, bubble-free liquid plasma has been also generated experimentally by applying high-voltage sub-nanosecond pulses to electrodes immersed in liquid. No theory currently exists for this new physical object called a “liquid plasma”, but its formation has similarities with ultrafast laser ablation.
Challenges: The physics and chemistry at plasma-material interfaces and multi-phase plasmas are poorly understood. Background: Plasma interactions with surfaces are incredibly complex and often require embedding quantum models to describe electron emission processes at surfaces. The dynamics of multiphase systems containing gas, plasma and liquid or solid states are characterized by vastly different intrinsic time scales: a slow liquid or solid motion time scale (usually milliseconds), a fast electron time scale (nanoseconds) and an intermediate ion time scale (microseconds). This work leverages our expertise on the presence of solid microparticles in plasmas, “dusty plasmas”, to a new regime of “misty plasmas” that describe a gaseous plasma within which liquid micro-droplets levitate and can absorb component plasma species, evaporate, and break up electrostatically, drastically changing plasma properties. Plasma is often generated in liquids within voids and gas bubbles. However, bubble-free liquid plasma has been also generated experimentally by applying high-voltage sub-nanosecond pulses to electrodes immersed in liquid. No theory currently exists for this new physical object called a “liquid plasma”, but its formation has similarities with ultrafast laser ablation.
Proposed Research: We study gas plasma interactions with solid and liquid surfaces, and the properties of “liquid plasma.” Elements of quantum mechanics are integrated into plasma physics to describe heterogeneous chemical reactions, electron transport through material interfaces, and plasma self-organization at gas-liquid interfaces and within liquids. We develop multi-fluid, multi-temperature plasma models to describe multi-phase reactive plasmas. The AMAR framework is being extended to conduct fundamental studies of transport and reactivity of misty plasma to understand liquid droplet charging, heating, evaporation and electrostatic break up within the plasma. We adapt our Boltzmann-Fokker-Planck kinetic solvers to simulate liquid breakdown and bubble-free plasma formation in liquids. This work provide foundation to TT1 – 4 by clarifying the physicochemical reactions observed in LTP. The research will clarify: a) Fundamental interactions of plasmas with micro-droplets and plasma-liquid
surfaces; b) Dominant mechanisms of solid dust or droplet charging (diffusion, field, and photo) in
different plasma environments (magnetized, atmospheric-pressure, space plasma); c) Effects of plasma
reactivity, selectivity, and liquid droplets size on surface processing control; d) How to select specific
plasma source and operation conditions for specific applications; e) Effects of dust-plasma interactions on nano- and micro-particle charging and transport in laboratory and space plasmas.
Impact: Plasma-activated water and mist are used for biomedical and agriculture applications. The work clarifies the physics of gaseous plasma interfaces, electric surface charging, electron emissions, and nanoparticle synthesis, which has immediate applications to designing electronic structures, metamaterials, plasma-aided printing and additive manufacturing.
FR1.4a: Models for multi-phase plasmas
We conduct experiments and develop models to understand self-organization and pattern formation at
plasma interfaces with solids and liquids. Explore pattern formation with liquid-anode discharges and
droplet ejection from liquid cathodes in contact with plasma.
Image 1 represents the changing anode patterns with increasing the liquid conductivity from 0.1 mS (left) to 4.8 mS (right).
FR1.4b: Misty plasma
Explore properties of gaseous plasma with levitating liquid micro-droplets: “Misty plasmas” describe a gaseous plasma within which liquid micro-droplets levitate and can absorb component plasma species, evaporate, and break up electrostatically, drastically changing plasma properties. The initial study of misty plasmas will include experiments where single water droplets are observed to gravitationally fall through a low temperature plasma column. It is expected that the interplay between charging from the plasma environment and the onset of gravitational instabilities will lead to the droplet breakup and evaporation. These experiments will be supplemented by simulations where a generalized hydrodynamic fluid model will be used to understand how these processes depend on the discharge conditions. The model will be adapted from previous studies which investigated the interaction of a bubble (a localized low-density region) and a droplet (a localized high-density region) in a strongly coupled dusty plasma.
Image 2 shows the time evolution of bubble–droplet density in viscoelastic fluids for fixed viscous term and varying coupling strength arising from changing relaxation parameter in (a), (b) and (c), respectively. The image was adapted from (Dharodi, 2021). This theory will be explored using a Fractional Laplacian Spectral (FLS) approach, which calculates the probability for transport as a function of spatial scale in media with both random disorder and nonlocal interactions. In the FLS model, dimensionless disorder represents deviation from a perfect lattice order. Thus, increasing the disorder can be used to model a highly distorted lattice approaching a liquid state. The model can account for anomalous electron diffusion, which is commonly observed during discharge generation.
Image 3 shows the color stands for calculated probability for transport as a function of spatial lattice scale in the a) super-diffusive, b) diffusive, and c) sub-diffusive regime. Here, red (blue) color represents high (low) probability for transport. All calculations are performed for dimensionless disorder and assuming the range of nonlocal interactions is equal to the spatial scale of interest. In a) and b), regions where the probability for transport transition from positive to vanishing are marked by a white dashed contour. In a) and c), white dotted contours mark regions where the transport seems to deviate from the expected overall behavior. The image was adapted from (Kostadinova, 2020).
FR1.4d: Phase transitions
- We develop a hybrid model for transport processes at liquid/plasma interfaces based on the smoothed particle hydrodynamics (SPH)
- The model describes material removal through ion spattering at plasma/liquid interfaces, keyhole formation, and plasma plume induced by long and short laser pulses
- We study multi-phase plasma plumes induced by explosive fragmentation of material targets by ultrashort laser pulses