Off-axis chemical crosstalk in an atmospheric pressure microplasma jet array

Developing arrays of parallel microplasma jets is an attractive route to scaling the area available for the treatment of surfaces with low temperature plasma. Increasing the packing density of the arrays may lead to electrical and gas kinetic jet-jet interactions, but previous work has focused almost exclusively on electrostatic coupling between the jets. Chemical interactions ('crosstalk') have received considerably less attention. We report here the results of an investigation of chemical crosstalk in 4 x 4 arrays of microplasma jets, produced in flowing helium at atmospheric pressure. Oxidation damage to an Escherichia coli lawn serves as a diagnostic of the spatial distribution of molecular radicals and other reactive plasma species, produced at the plasma jet/ambient background interface or between the jets, and incident on the surface. Spatial maps of bacterial inactivation by the microplasma jet array for 20 s show the destruction of E. coli at distances as large as 2.7 jet diameter from the nearest plasma perimeter, compared to typically less than 0.5 jet diameter in the single jet case. Extending to 30 s of plasma exposure leads to destruction of the entire bacterial sample. This 'action at a distance' effect, the production of long-lived species such as O, O-2(a(1)Delta(g)) and O-3 that are responsible for bacterial deactivation, peaks along a line bisecting columns and rows of plasma jets. The data illustrate the synergistic effect of adjacent jets on off-axis formation of reactive species, and show that the chemical and biological impact of an array cannot be inferred from the plasma chemistry of a single jet.