www.acsprf.org

This research continues from the previous year the studies of a microhollow cathode discharge (MHCD) based plasma microjet operated at direct current (DC) with air, helium or helium/oxygen mixture as working gas and its interaction with water (or contaminated water).

The PMJ device comprises two metal tubes separated from each other by a third insulating tube. The key dimensions are the inter-electrode distance and the diameter of the exit nozzle, which are 0.5 mm and 0.8 mm, respectively. Compressed air, helium/ oxygen mixture were used as the working gas. Gas flow rate usually ranges from 0.3 to 5 standard liters per minute. The discharge sustaining voltage varies in the range of 230 ¨C 600 V (depending on the working gas used and its flow rate) with an operating current in the 3 ¨C 40 mA range. A schematic diagram of the PMJ device as well as a picture of the plasma sustained in liquid can be found in the previous annual report.  This report focuses on the interaction of PMJ with liquid media when PMJ is sustained in a quasi-steady gas cavity in liquid.

Figure 1. (a) A schematic diagram of the PMJ generated in a quasi-steady gas cavity in liquid  and (b) pictures of the PMJ (with compressed air as the feed gas) working in deionized water with Rhodamine WT .

When the tip of the device is immersed in liquid media, the continuous gas flow through the orifice maintains a quasi-steady gas cavity, which protects the powered electrode and subsequently keeps the gas discharge plasma sustained (as demonstrated in figure 1). The plasma activated species (such as ions, excited species, and electrons) is directly injected into the liquid via the opening. Chemical reaction of plasma activated species with liquid solution happens effectively at the surface of the gas cavity and more importantly, on the surfaces of micro-liquid droplets that exist within the gas cavity (as denoted by the little circles within the gas cavity in figure 1 (a)).

In one series of experiments, 200 ppb Rhodamine WT was used to purposely contaminate 200 ml de-ionized water and subsequently treated with PMJ for one hour. The concentration of Rhodamine WT was degraded to ~ 80 ppb. Exemplary pictures of the initial liquid and the treated liquid are shown in figure 2, qualitatively demonstrating the change of the concentration fo Rhodamine WT.

Figure 2. A picture of deionized water with Rhodamine WT contamination before and after 1 hour PMJ treatment (totally treatment volume is 200 ml and with compressed air as working gas).

Figure 3. Jet length as a function of discharge current at different oxygen volume concentration (flow rate: 2 standard liters per minute)

No matter what gas is used as the working gas and at what flow rate or discharge current, it is important that a stable PMJ has to be generated in air before the interaction with liquid. In the case of helium/oxygen mixture as working gas, a general trend observed is that with the increase of oxygen concentration, the overall voltage needed to sustain the plasma is increased. The visible length of the jet in general increases with the increase of the discharge current (at oxygen concentration below 1%). This phenomenon is not as obvious when oxygen concentration is increased to above 1%. At a fixed oxygen concentration (<2%), the jet length also increases with the flow rate until it reaches a plateau when entering the turbulent mode (as shown in figure 4).

Figure 4. Pictures of a He/O₂ (2%) plasma microjet (a) at flow rates from 0.5 to 5 slm (diameter of the end cap opening: 800 mm), Discharge current: 30 mA

The transition from laminar flow to turbulent flow happens between a flow rate of 2.5 slm and 3.0 slm which correspond to Reynolds number between 603 and 724.

Atomic oxygen is considered as one of the key components in the plasma generated species as they can directly participate in the oxidation process of organic and inorganic species in air and in liquid, as well as serving as the precursor of other reactive oxygen species (ROS). Atomic oxygen (with optical emission at 777.4 nm) was monitored via visible optical emission spectroscopy (OES) when different volume percent of oxygen was added into helium carrying gas. It is interesting to note that relative oxygen emission intensity reaches a peak value at around 0.1% oxygen concentration (as shown in figure 5 (a)). This oxygen emission can originate from the O₂ added to the helium stream as well as the air entrainment. Emission monitored from side-on of the jet at a distance 3 mm away from the exit nozzle revealed similar trend.

Figure 5. Relative optical emission of oxygen at 777.4 nm at (a) different oxygen volume percent from end-on and side-on at 3 mm and (b) different flow rate

When fixed at a certain oxygen concentration (0.1%), the optical emission of oxygen peaks at a flow rate of 4 slm. It is very likely that at a higher flow rate, mixing of surrounding air exceeds the optimal oxygen volume concentration that can lead to the maximum oxygen emission.

Figur 6. Optical Emission of OH (306-309 nm) and Cu (325 nm and 327 nm) at different oxygen concentrations

Meanwhile, optical emission of OH and Cu at different oxygen concentrations are also monitored and shown in figure 6. OH emission peaks when no oxygen is added into the working gas, while Cu emission increases with the increase of oxygen concentration. It is rather difficult to monitor light emission when the PMJ is sustained inside water. Therefore no emission spectra are recorded in those cases.

This project has given me a chance to work with students from high school level all the way up to senior college students. In an undergraduate college, this experience is invaluable. I am glad to see many of them graduate from high school and move on to college/university, pursuing a science career because their experience in my lab.