Influence of a DC, point-to-plane, low-pressure discharge in nitrogen on polystyrene thin films

Issue		Eur. Phys. J. Appl. Phys. Volume 22, Number 1, April 2003


Page(s)		51 - 60
Section		Physics and Mechanics of Fluids
DOI		10.1051/epjap:2003016
Published online		25 February 2003

Eur. Phys. J. AP 22, 51-60 (2003)
DOI: 10.1051/epjap:2003016

Influence of a DC, point-to-plane, low-pressure discharge in nitrogen on polystyrene thin films

P. Svarnas¹, N. Spyrou¹ and B. Held²

¹ Electrotechnic Materials Laboratory, Electrical and Computer Engineering Department, University of Patras, 26500 Patras-Rio, Greece
² Laboratoire d'Électronique des Gaz et des Plasmas, Université de Pau et des Pays de l'Adour, CURS, avenue de l'Université, 64000 Pau, France

spyrou@ee.upatras.gr

(Received: 3 September 2002 / Received in final form: 5 December 2002 / Accepted: 13 December 2002 Published online: 25 February 2003)

Abstract
In the first part of this work the regime of the DC electrical discharge in nitrogen, in absence and in presence of polystyrene thin films in the cold plasma reactor, for a constant gap length d=1 cm and for variable pressures ( $1~{\rm ~mbar} < p < 10~{\rm ~mbar}$ ) is characterized, while the gas flow rate is maintained at Q=1 sccm. V- I curves underline that in any case a "normal glow-like" discharge is established. Experimental results bring out that the presence of the films in the volume of the produced plasma seems to modify the structure and some of the discharge characteristics, like the cathode secondary emission effects. V- I curves ( $I <200~\mu$ A), discharge repetitive current impulses, discharge repetitive emitted light impulses along the discharge symmetry axis and photos of the discharge, in presence and in absence of the films, are presented. In the second part of this paper results of polystyrene thin films treatment under the above DC "normal glow-like" discharge regime in nitrogen for improving the wettability of the films are presented. The effect of discharge main parameters (pressure p and mean current I through the discharge) on the treatment is studied under certain experimental conditions. The wettability is characterized by contact angle measurements using deionized water drops and the evolution versus the treatment time $t_{\rm tr}$ of the $\Delta \theta/\theta_{\rm i}=(\theta_{\rm i}-\theta_{\rm f})/\theta_{\rm i}$ ratio ( $\theta_{\rm i}$ : the contact angle before the treatment and $\theta_{\rm f}$ : the contact angle after the treatment) is estimated. It is deduced that optimizing main parameters of the DC electrical discharge in nitrogen it is possible to obtain films with very good wettability (suggestively $\Delta\theta/\theta_{\rm i}=0.951$ corresponding to $\theta_{\rm f}=4^{\circ}$ , at p=4 mbar, $I=200~\mu$ A, $t_{\rm tr}\cong 600$ s). A rapid change of the films surface nature ( $0~{\rm s} < t_{\rm tr} < 60$ s, suggestively) seems to be followed by a slower one and finally by a saturation. The gas pressure and the discharge mean current seem to play a very important role for the treatment. The experimental results allow us to start a discussion about the possible role of some active particles produced in the plasma to the treatment of the films and especially the role of the neutrals and the metastables.

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