BIOPLASMAS & PLASMAS WITH LIQUIDS - Joint Conference of COST ACTIONS TD1208 “Electrical discharges with liquids for future
applications” & MP1101 Biomedical Applications of Atmospheric Pressure Plasma Technology, Bertinoro, Italy, 13th-17th September 2015
Laser Induced Breakdown at the Air-Water Interface
T. Murakami1, T. J. Morgan2, A. Young2, L. Hüwel2, W. G. Graham3
1
Department of Systems Design Engineering, Seikei University,
3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 1808633, Japan
2
Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA
3
Department of Physics and Astronomy, Queen’s University Belfast, Belfast BT71NN,
United Kingdom
Air-water interfaces (AWI) are ubiquitous yet not well comprehended. Laser interaction at AWI is becoming
increasingly important in biological applications, chemical analysis and medicine. The conversion of laser
radiation energy into thermal, chemical, and mechanical effects needs to be understood in detail. Additionally,
plasma AWI processes introduce electrons and new ionic species and contribute to self-organization at the
interface. Electron and ion behaviour at AWI still has many unanswered questions [1, 2].
Modelling and experiments are underway to investigate the AWI, using laser induced breakdown, by sending
laser light into a water cell parallel to the AWI and focussed either above, at, and below the surface. Additionally,
to gain insight into how exposure to laser radiation changes the AWI, two laser pulsed experiments can be
performed, the first focussed just below the AWI and the second just above it. This allows the study of an AWI
perturbed by a laser induced breakdown (perturb-probe experiments). Preliminary shadowgraphy and video
camera data for single laser breakdown a few hundred microns below the AWI demonstrates that the vapor
region above the surface is drastically modified producing erupted droplets and a higher concentration of water
vapour that slowly falls back to the water surface.
To provide insight into the associated physics and chemistry at work in the evolution of the plasma in the AWI
region, COMSOL Multiphysics modelling has been developed and used to study the plasma evolution just
above the AWI. A zero dimensional model and simulation code is used to solve the rate equations and help
identify the important collision mechanisms [3]. A COMSOL one dimensional simulation is also used in an
analogous electrical discharge between parallel plates of finite separation, s. In the limit, as s0 this model
approaches a zero dimensional energy deposition model, simulating laser breakdown. Experimentally, laser
breakdown is produced by a Nd:Yag laser at 1064 nm (9 ns pulse width, 100-250 mJ/pulse energy) [4]. The
experiment can employ spectroscopic, shadowgraphy and gated CCD imaging techniques.
The overall goal of the work is to gain insight into the diverse characteristics, dynamics and structure of the
AWI, a region that occurs naturally in many biological processes yet is still not well understood (see for example
ref. 5).
References:
[1] F. Uhling et al, J. Phys. Chem. Lett. 4 (2013) 338.
[2] Y. Levin et al, Phys. Rev. Lett. 103 (2009) 257802.
[3] T. Murakami et al, Plasma Sources Sci. Technol. 22(2013)015003/22(2013)015003/23(2014)025005.
[4] S. Khalighi-Monfared et al, Plasma Sources Sci. Technol. 20 (2011) 035001.
[5] Z. Liao et al, Langmuir, 27 (2011) 12775.