Measurement of the charge distribution deposited by an annular plasma synthetic jet actuator over a target surface

Direct and indirect non-thermal atmospheric-pressure air plasma treatments have been growing considerably in last twenty years. The ability of these treatments to sterilize for several typologies of pathogens has been shown [1–3]. Additionally, the possibility to create a selective tool able to kill cancer cells, maintaining alive healthy ones, has been demonstrated [4–6]. A plasma treatment exposes the surface of pathogens or cells to reactive oxygen and nitrogen species (RNOS), radicals, and a significant flux of charged particles, including electrons, and positive and negative ions [7, 8]. These species are those mainly responsible for the beneficial effects against pathogens and cancer cells [1, 3, 5].

The clinical potential of the charged particles has been largely ignored in plasma treatments considered up to now, and only recently their sterilization effect has been investigated by considering a direct and indirect treatment [9]. Nevertheless, to treat irregular and large surfaces, as an apple to decontaminate it, or a scaffold to modify its surface properties, direct treatment can be limiting. Indirect plasma treatments are generally used when samples characterized by complex geometries must be treated or when direct contact with plasma filaments must be avoided. Moreover, in this operating condition, the plasma reactor is developed and optimized without considering the target to be treated. Usually in these kind of treatments, only neutral long-life reactive species are considered.

In a recent work [10], our research group developed plasma synthetic jet actuators (PSJAs) able to produce an ionic wind normal to the surface where the dielectric barrier discharge (DBD) is ignited. This jet propagates for several centimetres at a velocity of several meters per second. The production of the ionic wind is due to the electro hydro dynamic (EHD) interaction. In a surface DBD (SDBD), ions are produced and then accelerated by the electric field. These charged particles collide with the surrounding neutral ones, releasing momentum to them, and producing a unidirectional body force parallel to the actuator surface [11–14]. Actuators featuring annular geometries have shown the ability to produce a jet normal to the actuator surface [15]. These fluid-dynamics plasma actuators are nowadays extensively studied for several applications in aeronautics and astronautics [16–20], and for turbine blade applications [21, 22]. The optimization of the induced EHD interaction reported in [10] has demonstrated the possibility to strongly enhance the delivery of reactive species, like ozone, onto a target to be treated [23].

Basics physics studies on linear aerodynamics plasma actuators showed the presence of charges deposited onto the actuator surface, at a distance up to several centimetres downstream of the plasma region. These charges are produced within the plasma volume and subsequently advected by the tangential wall jet [24, 25].

This paper shows that PSJAs can deliver charged particles up to several centimetres away from the actuator surface where the discharge is ignited. The potential distributions induced by charges deposited on an insulating target surface perpendicularly to the flow have been measured by varying several parameters, such as actuator dielectric material, target distance from the actuator surface, time interval of plasma on and potential of the exposed electrode of the actuator. Spatial charge distribution was calculated starting from electrostatic potential measurements. These distributions allowed the estimation of the charge flux towards the target surface.

A first series of measurements has been performed by using the PCB annular actuator described in [10]. On a PCB slab of thickness of 1.6 mm, an annular electrode of copper (35 µm thick, 5 mm width) with an inner diameter of 30 mm is placed (figure 1(a)). A sinusoidal supply system constituting of a push–pull high voltage transformer controlled by Arduino has been utilized to feed the discharge. It allows us to change both the voltage and frequency in the ranges 0–20 kVp and 15–50 kHz, respectively.

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A second series of experiments has been performed by using a PSJA utilizing PVC and glass as dielectric material, and a copper exposed electrode with an octagonal geometry (figure 1(b)). An electrode thickness of 35 µm and a width of 5 mm is used. The octagon side inner length is 12 mm. By doing this, the octagon perimeter and the circumference length of the annular PCB actuator are about the same. In this way, the average power feeding the discharge is about the same, and, as a consequence, induces the jet results to be similar.

The average power delivered to the discharge has been evaluated by using Lissajous figures [26]. The high voltage applied to the electrodes has been measured by means of a Tektronix P6015 capacitively compensated high voltage probe with a bandwidth up to 75 MHz. The voltage across the measuring capacitor C_m of 1 nF (figure 2), used to evaluate the charge flowing within the discharge, has been detected by a Yokogawa low voltage probe with 75 MHz bandwidth. Both signals have been acquired by a Yokogawa DL1740 4-channel, 500 MHz bandwidth, 1 GS s⁻¹ oscilloscope.

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Schlieren diagnostics has been utilized to visualize the induced hot flow produced by PSJAs. A Z-type configuration has been used [27, 28], passing the light beam in the z direction of figure 2.

A first attempt to measure advected charges directly within the induced jet has been made by using an electrostatic probe constituting of an enamelled wire with a diameter of 1 mm. One side of the probe was moved within the induced flow. The other side was connected to a copper square strip 1  ×  1 cm. The electrostatic 341B 20 kV TREK voltage probe was positioned over this strip, at a height of 2 mm (the y direction in figure 2). When the discharge was ignited, the wire was moved from the outer part of the induced flow towards the inner core of it (the x direction in figure 2). Unfortunately, due to plasma electromagnetic noise, the probe was always measuring voltages different from zero, even well outside the induced jet. Signals have been measured even positioning the probe under the actuator. These signals were always positive. There was the possibility that positive charges could be repelled away from the actuator due to electrostatic interaction. This behaviour deserves a deeper investigation and will be examined in future experiments.

To overcome the problem of the electromagnetic noise, charge distribution within the induced jet has been measured with an indirect method. A 2 mm Plexiglass slab was placed at a distance from the actuator surface, parallel to it. Several distances from 1 to 5 cm have been used. These are typical distances for indirect treatments. The discharge was ignited for defined times and, after its switching off, the electrostatic voltage probe scanned the surface. In this way, the surface potential induced by charges advected by the flow and deposited over the dielectric surface was measured (figure 2). Under the Plexiglass surface, a 150  ×  150 mm copper tape was attached and grounded. In this way, a zero-potential reference was always present for the electrostatic probe. Electrostatic simulations preformed with FEMM software [29] demonstrate that this reference electrode does not influence the electric field distribution within the plasma formation region. This setup allows us to evaluate the way charges are deposited over the surface to be treated. All measurements were performed inside a 50  ×  50  ×  40 cm sealed insulating box, maintained at a constant air temperature of 25 °C. Despite the use of atmospheric pressure air, a quite repeatable degree of relative humidity was maintained. Values of 40%  ±  5% were measured.

The experimental procedure is performed through the following steps.

• 1.  
  The sealed box was opened.
• 2.  
  The Plexiglass surface was wiped by using a wet rag and then heated for 10 s with a hot gun to eliminate possible traces of humidity.
• 3.  
  The sealed box was closed, and the electrostatic probe was moved over the Plexiglass surface, checking the presence of a zero-voltage signal. If the zero condition was not achieved, steps 1 and 2 were repeated.
• 4.  
  Discharge was ignited for a defined time interval. Suddenly, after the switching off of the discharge, the electrostatic probe was moved over the Plexiglass plate, in the region where the induced jet hits the surface. The induced potential signal was acquired.

Steps 1–4 were repeated for discharge ignition time intervals from 5 ms up to 20 s, for distances of the Plexiglass surface of 1, 2 and 5 cm and with the actuator exposed electrode connected either to the high voltage or the grounded terminal. For each condition, five measurements have been performed, leading to standard deviations within 5%. The electrostatic probe was firstly calibrated by measuring the voltage over an electrode connected to a constant DC potential. The probe has been positioned close to the electrode surface and then moved away. Results have been compared with simulations carried out by the electrostatic FEMM solver. Experimental results and numerical simulations were in very good agreement. Subsequently, the zero-condition of the probe was set at the beginning of each series of experiments.

A first series of experiments has been made by moving the electrostatic probe in different radial directions to verify the axisymmetric geometry of the induced flow hitting the target surface. This is important for the octagonal actuator where eight tangential jets collide together. Measurements performed both in the radial direction corresponding to that which is perpendicular to two opposite sides of the octagon and both corresponding to the radial direction between two vertices have been carried out. Differences are within the standard deviation, demonstrating the axisymmetric nature of the induced jet for both geometries, at least for distances from the actuator surface higher than 1 cm.

Charge distribution has been evaluated from surface potential measurements by using the numerical approach described in [24].

A first series of experiments has been performed using the PCB annular actuator. Unfortunately, surface potentials measured with this actuator have shown weak repeatability. Moreover, a strong decrement in charge quantity has been observed by using the same actuator several times. This ageing effect, together with the weak repeatability, is related to the PCB surface treatment necessary to stick the copper electrode to the insulating layer. Furthermore, to obtain the desired PCB design, an acid treatment must be done. These treatments can modify PCB surface properties, releasing impurities into the plasma and altering charge species production.

In order to overcome this issue, PSJAs made of PVC and glass have been built. As far as it was not possible to create a perfect annular exposed electrode by using copper tape, an octagonal geometry was utilized. On one side, this geometry is quite easy to be built with a repeatable process. On the other side, it guarantees an induced tubular flow similar to that produced by the PCB annular actuator. This behaviour has been obtained by creating an octagon with a perimeter equal to the circumference length of the annular configuration and supplying the discharge with the same average power. The last condition has been obtained by slightly changing the supplying voltage. The supplying voltage has also been varied to account for different dielectric constants between PVC and glass. In this way, the same average power of 14 W was delivered to the discharge in all of the cases investigated. For all actuators, the applied voltage frequency was fixed to 31 kHz and the peak voltage was set to 5 kV for the actuator made of glass, 5.4 kV for the one made of PCB and 6 kV for that made of PVC. As far as the electric field within the plasma formation region is inversely proportional to the dielectric constant of the dielectric material, to maintain the average power feeding the discharge at a constant value, the higher the dielectric constant value, the lower the applied voltage. As a matter of fact, the relative dielectric constant is about 6 for the glass, 4.4 for the PCB (FR4 material) and 3 for PVC. Lissajous figures of the applied voltage and of the transferred charge for the three dielectrics are shown in figure 3. It is possible to notice that the figures are characterized by the same area. This means that the same energy is delivered to the discharge. As far as the supplying frequency is the same, the average power for the three conditions is the same too.

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A comparison between Schlieren images acquired after increasing time intervals from discharge ignition, for the annular PCB and octagonal PVC actuators is reported in figure 4. The figure shows that induced jets for the two actuator geometries are quite similar. The flow generated by the annular actuator is a tubular axisymmetric flow, at least in the first 2 cm. Subsequently, turbulences appear (figure 3(a)). The octagonal actuator produces eight tangential jets colliding in the centre of the actuator itself. After they merge together, the normal flow is generated. During their collision, tangential jets are subjected to a mixing and the resulting normal jet presents a quasi-axisymmetric geometry. In the annular configuration (left hand side), the pinch effect is stronger and more effective because it is produced by a uniform distribution of tangential jets. Consequently, the tubular jet is tighter and propagates slightly faster.

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As already mentioned, surface potential measurements have been done by using both PVC and glass actuators. The change in the actuator dielectric material slightly influences the amount of charges advected by the flow. As a matter of fact, differences were within the standard deviation. For this reason, in the following, only results obtained by using PVC actuators will be shown.

In figure 5, surface potential and charge density distributions obtained over the Plexiglass plate placed at 10 mm from the actuator surface and for increasing switching-on time of the discharge are shown. In figure 5(a), the exposed electrode connected to the grounded potential is used. In figure 5(b), the exposed electrode is connected to the high voltage terminal. The 'probe position' equal to zero refers to the centre of the jet. In both graphs, charges deposited over the Plexiglass surface always induce positive potentials. This is in agreement with [24] and suggests that charges advected by the flow are always positive. A possible candidate is the H₃O⁺ ion that is produced in relatively high amounts in air discharges even if the humidity degree is quite low [30]. Moreover, this ion is quite stable even at atmospheric pressure. Qualitative behaviours of induced potentials as a function of switching-on time of the discharge are similar in both conditions. Up to several tens of milliseconds of discharge ignition, the surface potential distribution presents an M-shaped behaviour. This could be related to the mushroom-shaped tubular flow that is produced at the ignition of the discharge (figure 4) and subsequently hits the Plexiglass surface. The recirculation region at the upper part of the propagating flow increases the amount of charges in an annular region around the inner core of the flow itself. By leaving the discharge ignited for longer time intervals (hundreds of milliseconds), the amount of charges deposited over the surface increases, filling the inner space within the 'M' too, and creating a bell-shaped distribution. This behaviour is shown by the potential distribution of figure 5(a) at 1000 ms, and more pronounced in figure 5(b) at 500 ms. A further increase in the discharge on-time leads to a decrement in the potential distribution peak, and an increment in the potential values in the wings of the distribution. This behaviour can be related to additional charges advected by the incoming flow, and is able to push to the side of the slab the charges already deposited over the Plexiglass surface. These charges are then accumulated far away from the region where the jet core hits the surface by the flow spreading.

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The overall charge accumulation circular region has a diameter of about 10 cm. This dimension is the same as reached by the incoming flow, after it hits the surface and then spreads on it. This behaviour is shown by Schlieren images of figure 6 where a half of the jet flow hitting the Plexiglass slab, placed at 1 cm from the actuator surface, after increasing plasma-on time, is depicted. In the first image, after 3 ms, the induced jet approaches the Plexiglass slab. After 20 ms, the induced jet starts to spread over the target surface. After 100 ms, the jet already spreads over the target surface at its maximum extension. This configuration is about the same even after 500 ms. The extension range is about 5 cm, and this is about half of the distance reached by the charges deposited on the Plexiglass surface (figure 5). This agreement between deposited charge extension and jet spreading distance, suggests that, as already mentioned, charges are advected by the flow and then they are deposited over the target surface under the action of the spreading jet.

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The comparison between figures 5(a) and (b) leads to a further consideration. The potential distribution evolutions from M-shaped to bell-shaped are faster when the exposed electrode is at the high voltage figure 5(b). This result can be related to the higher induced jet speed in the latter case [31]. Schlieren images in figure 7 show the induced jet when the exposed electrode is grounded (left hand side) or at high voltage (right hand side), by increasing the plasma-on time. Images clearly show that induced jet propagates faster when the exposed electrode is connected to the high voltage terminal.

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A second set of surface potential measurements has been performed with the Plexiglass plate positioned at 2 cm from the actuator surface. The results obtained for both grounded (a) and high voltage (b) exposed electrodes are shown in figure 8.

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The behaviours of surface potentials are comparable to those showed with a target distance of 10 mm (figure 5). In this case, by also increasing the plasma-on time, a bell-shaped distribution replaces the M-shaped one. Again, this behaviour is faster when the exposed electrode is at high voltage (figure 8(b)).

The presence of charges advected by the flow at a distance of 50 mm from the actuator surface is shown in figure 9. Induced potential values are about one half of those measured with the target at 10 mm (figure 5) In this case, the longer distance and time to reach the surface allows for a portion of the charges to recombine. Potential distribution evolutions follow the same behaviour already obtained for shorter target distances.

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Time evolution of the charge density on the target allows us to estimate the flux of positive particles hitting the surface Φ⁺:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle {{\Phi }^{+}}=\frac{Q}{\Delta te}~\left( {\rm particle}\,{\rm c}{{{\rm m}}^{2}}\,{{{\rm s}}^{-1}} \right)\nonumber \end{align} \tag{ 1 }$$

where Q is the charge density obtained by surface potential measurements, Δt is the time interval in which the charge build-up effect takes place, and e is the charge of the electron equal to 1.602 $\times$ 10⁻¹⁹ C.

By considering the smallest target distance of 10 mm (figure 5) and the charge build-up effect obtained after 5 ms of plasma-on time, an average flux of charges Φ⁺ of about 10¹¹ particle (cm² s)⁻¹ can be estimated. The distribution after 5 ms of plasma-on has been chosen as it represents a condition in which the target surface is almost free of deposited charges. By increasing the plasma-on time, additional charges approaching the surface are partially attached to it and partially repelled away due to the electrostatic interaction. In this case, an underestimation of the charge flux would be realized.

In this work, the capability of PSJAs to deliver charged particles far away from the surface where the discharge is ignited has been investigated. Actuators made with FR4 (PCB), PVC and glass have been used. All actuators have been supplied with a sinusoidal voltage at 31 kHz and the same average power feeding the discharge. A jet with a nearly axisymmetric tubular flow is induced and is about the same for all the dielectric materials used.

An insulating target has been placed perpendicularly to the jet at distances from the plasma actuator from 1 to 5 cm. The charge distribution over the target surface has been evaluated from surface potential measurements. Two actuator configurations with the exposed electrode either at high voltage and grounded have been utilized. Surface potentials have been measured by igniting the discharge at time intervals of plasma-on from 5 ms up to 20 s.

PCB actuator showed a quite important ageing effect and poor repeatability in charge measurements. This is probably due to due to the surface treatments to which this material is subjected during its construction. For this reason, only actuators made of PVC and glass have been utilized showing comparable results.

A first result is that advected charges are always positive, independently from the potential of the exposed electrode. A possible candidate for the positive charges is the H₃O⁺ ion, which is produced in relatively high amounts in humid air discharges, even when the humidity degree is quite low. Moreover, this ion is quite stable even at atmospheric pressure. When igniting the discharge for time intervals of tens of ms, the charge distribution showed an M-shaped function probably due to the recirculation region existing in the tubular flow 'mushroom' top. After hundreds of ms, the charge value peak increases and a bell-shaped distribution is produced. Leaving the plasma ignited for longer times, a decrease in the bell-shaped peak value is measured and an increase of the charges on the lateral sides are observed. This is due to the effect of the incoming charges that push away the charges already stored in the core of the jet. These charges subsequently are spread onto the target surface together with the flow. When the exposed electrode is connected to the high voltage potential, the charge build-up mechanism measured on the target surface takes place in shorter times. This can be related to a more intense EHD interaction obtained when supplying the actuator with the exposed electrode connected to the high voltage terminal. Schlieren images confirm this assumption. Lastly, charge evaluation allows us to estimate a charge flux of about 10¹¹ particle (cm² s)⁻¹.

These results show that, when these plasma actuators are used, the presence of charged particles could be an important factor to be accounted for, for indirect treatment purposes, even at distances up to several centimeters. The current investigation demonstrates an unequivocal effect of biological inactivation of this plasma source when charges are left free to reach the sample.

As far as advected positive charges could be associated to the H₃O⁺ ion, humidity could play a key factor in the production of charges. The effect of humidity onto charge distribution deserves further and accurate studies.