Christophe Leys

Non-thermal plasmas in and in contact with liquids

During the last two decades atmospheric (or high) pressure non-thermal plasmas in and in contact with liquids have received a lot of attention in view of their considerable environmental and medical applications. The simultaneous generation of intense UV radiation, shock waves and active radicals makes these discharges particularly suitable for decontamination, sterilization and purification purposes. This paper reviews the current status of research on atmospheric pressure non-thermal discharges in and in contact with liquids. The emphasis is on their generation mechanisms and their physical characteristics.

Nonthermal Plasma Technology as a Versatile Strategy for Polymeric Biomaterials Surface Modification: A Review

In modern technology, there is a constant need to solve very complex problems and to fine-tune existing solutions. This is definitely the case in modern medicine with emerging fields such as regenerative medicine and tissue engineering. The problems, which are studied in these fields, set very high demands on the applied materials. In most cases, it is impossible to find a single material that meets all demands such as biocompatibility, mechanical strength, biodegradability (if required), and promotion of cell-adhesion, proliferation, and differentiation. A common strategy to circumvent this problem is the application of composite materials, which combine the properties of the different constituents. Another possible strategy is to selectively modify the surface of a material using different modification techniques. In the past decade, the use of nonthermal plasmas for selective surface modification has been a rapidly growing research field. This will be the highlight of this review. In a first part of this paper, a general introduction in the field of surface engineering will be given. Thereafter, we will focus on plasma-based strategies for surface modification. The purpose of the present review is twofold. First, we wish to provide a tutorial-type review that allows a fast introduction for researchers into the field. Second, we aim to give a comprehensive overview of recent work on surface modification of polymeric biomaterials, with a focus on plasma-based strategies. Some recent trends will be exemplified. On the basis of this literature study, we will conclude with some future trends for research.

Non-thermal plasmas for non-catalytic and catalytic VOC abatement

This paper reviews recent achievements and the current status of non-thermal plasma (NTP) technology for the abatement of volatile organic compounds (VOCs). Many reactor configurations have been developed to generate a NTP at atmospheric pressure. Therefore in this review article, the principles of generating NTPs are outlined. Further on, this paper is divided in two equally important parts: plasma-alone and plasma-catalytic systems. Combination of NTP with heterogeneous catalysis has attracted increased attention in order to overcome the weaknesses of plasma-alone systems. An overview is given of the present understanding of the mechanisms involved in plasma-catalytic processes. In both parts (plasma-alone systems and plasma-catalysis), literature on the abatement of VOCs is reviewed in close detail. Special attention is given to the influence of critical process parameters on the removal process.

Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics

Currently, there is a strong tendency to replace rigid electronic assemblies by mechanically flexible and stretchable equivalents. This emerging technology can be applied for biomedical electronics, such as implantable devices and electronics on skin. In the first step of the production process of stretchable electronics, electronic interconnections and components are encapsulated into a thin layer of polydimethylsiloxane (PDMS). Afterwards, the electronic structures are completely embedded by placing another PDMS layer on top. It is very important that the metals inside the electronic circuit do not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, resulting in poor adhesion properties. Therefore, in this paper, PDMS films are plasma treated with a dielectric barrier discharge (DBD) operating in air at medium pressure (5.0 kPa). Contact angle and XPS measurements reveal that plasma treatment increases the hydrophilicity of the PDMS films due to the incorporation of silanol groups at the expense of methyl groups. T-peel tests show that plasma treatment rapidly imparts adhesion enhancement, but only when both PDMS layers are plasma treated. Results also reveal that it is very important to bond the plasma-treated PDMS films immediately after treatment. In this case, an excellent adhesion is maintained several days after treatment. The ageing behaviour of the plasma-treated PDMS films is also studied in detail: contact angle measurements show that the contact angle increases during storage in air and angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS species to the surface.

Dc excited glow discharges in atmospheric pressure air in pin-to-water electrode systems

Electrical and optical emission properties of non-equilibrium atmospheric air discharges between a metal pin and a tap water anode/cathode are presented. With a water anode the discharges are of the glow type as is derived from short-exposure time plasma imaging and electrical characteristics. Additionally, the validity of extrapolated scaling laws of low pressure glow discharge supports these findings.In the case of a water cathode the plasma is filamentary in nature at the water surface. In the case of a water anode, the plasma is diffuse down to 10 ns. The timescales on which the filaments are visible in the near water cathode region and estimates of the electrical field in the cathode layer are consistent with the assumption that these filaments occur due to the electrical instability of the water surface.Spatially resolved rotational temperature measurements and dependence of the rotational temperature on current are discussed in detail. The rotational temperatures of OH and N2 in the positive column of the plasma are identical and equal to 3250 ± 250 K. A 2500 K temperature drop in the near anode region clearly shows that the water anode acts as an effective heat sink for the discharge. This indicates that apart from the electrical stabilization of the discharge by the water electrode due to its distributed resistance, a water anode also thermally stabilizes the discharge. The rotational temperature of nitrogen near the metal anode is typically two times smaller.

Optical emission spectroscopy as a diagnostic for plasmas in liquids: opportunities and pitfalls

In this contribution, optical emission spectroscopy is evaluated and thoroughly analysed as a diagnostic to characterize plasmas in and in contact with liquids. One of the specific properties of plasmas in and in contact with liquids is the strong emission of OH(A?X) and of hydrogen lines. As an example a 600?ns pulsed dc excited discharge in Ar, He and O2 bubbles in water is investigated by time resolved optical emission spectroscopy. It is shown that the production processes of excited species and the plasma kinetics strongly influence the emission spectrum. This complicates the interpretation of the spectra but provides the opportunity to derive production mechanisms from the time resolved emission. The importance of recombination processes compared with direct electron excitation processes in the production of excited states of the water fragments in plasmas with high electron densities is shown. The OH(A?X) emission spectrum illustrates that even in these highly collisional atmospheric pressure discharges the rotational population distribution deviates from equilibrium. A two-temperature fit of the OH rotational population distribution leads to realistic gas temperatures for the temperature parameter corresponding to small rotational numbers. The H? and H? lines are fitted with two component profiles corresponding to two different electron densities. The obtained electron density is in the range 1021?1023?m?3. Possible complications in the interpretation of obtained temperatures and electron densities are discussed.

Characteristics of atmospheric pressure air discharges with a liquid cathode and a metal anode

Electrical and optical emission properties of a burning plasma between a liquid cathode and a metal anode are presented in this paper. The plasma has constricted contact points at the liquid cathode and is clearly filamentary in nature near the water surface.The cathode voltage drop depends on conductivity rather than pH and is significantly different for distilled water and electrolyte solutions. An acidification of the liquid due to the plasma is always observed.The rotational temperature of OH and N2 in the bulk of the plasma is, respectively, in the range 3200?3750?K and 2500?2750?K. The rotational temperature of nitrogen near the metal anode is typically two times smaller. Electron densities near the cathode measured by Stark broadening of H? are in the range (5.5?8.0) ? 1014?cm?3, the atomic excitation temperatures in the range 5750?7250?K. Differences in electrical and optical emission properties between the cases when distilled water and electrolyte solutions are used as cathode are discussed in detail.

Surface modification with a remote atmospheric pressure plasma: dc glow discharge and surface streamer regime

A remote atmospheric pressure discharge working with ambient air is used for the near room temperature treatment of polymer foils and textiles of varying thickness. The envisaged plasma effect is an increase in the surface energy of the treated material, leading, e.g., to a better wettability or adhesion. Changes in wettability are examined by measuring the contact angle or the liquid absorptive capacity. Two regimes of the remote atmospheric pressure discharge are investigated: the glow regime and the streamer regime. These regimes differ mainly in power density and in the details of the electrode design. The results show that this kind of discharge makes up a convenient non-thermal plasma source to be integrated into a treatment installation working at atmospheric pressure.

Surface Modification of Non-woven Textiles using a Dielectric Barrier Discharge Operating in Air, Helium and Argon at Medium Pressure

In this paper, polyethylene terephthalate (PET) and polypropylene (PP) non-wovens were modified by a dielectric barrier discharge in air, helium and argon at medium pressure (5.0 kPa). The helium and argon discharges contained a fraction of air smaller than 0.1 %. Surface analysis and characterization were performed using X-ray photoelectron spectroscopy, liquid absorptive capacity measurements and scanning electron microscopy (SEM). The non-wovens, modified in air, helium and argon, showed a significant increase in liquid absorptive capacity due to the incorporation of oxygen-containing groups, such as C—O, O—C=O and C=O. It was shown that an air plasma was more efficient in incorporating oxygen functionalities than an argon plasma, which was more efficient than a helium plasma. SEM pictures of the plasma-treated nonwovens showed that the hydrophilicity of the nonwovens could be increased to a saturation value without causing physical degradation of the surface. The ageing behavior of the plasma-treated textiles after storage in air was also studied. It was shown that during the ageing process, the induced oxygen-containing groups re-orientated into the bulk of the material. This ageing effect was the smallest for the argon-plasma treated non-wovens, followed by the helium-plasma treated non-wovens, while the air-plasma treated non-wovens showed the largest ageing effect.

Water surface deformation in strong electrical fields and its influence on electrical breakdown in a metal pin-water electrode system

Electrical breakdown and water surface deformation in a metal pin?water electrode system with dc applied voltages is studied for small inter-electrode distances (2?12?mm). The radius of curvature of the metal pin is 0.5?cm to exclude corona before breakdown at these small inter-electrode spacings. Calculations of the water surface deformation as a function of the applied voltage and initial inter-electrode spacing are compared with measurements of the water elevation. For distances smaller than 7?mm the calculated stability limit of the water surface corresponds with the experimentally obtained breakdown voltage. It is proved with fast CCD images and calculations of the electrical field distribution that the water surface instability triggers the electrical breakdown in this case. The images show that at breakdown the water surface has a Taylor cone-like shape. At inter-electrode distance of 7?mm and larger the breakdown voltage is well below the water stability limit and the conductive channel at breakdown is formed between the pin electrode and the static water surface. Both cases are discussed and compared.