Gilad Chaniel

Physical mechanisms of interaction of cold plasma with polymer surfaces

Physical mechanisms of the interaction of cold plasmas with organic surfaces are discussed. Trapping of plasma ions by the CH2 groups of polymer surfaces resulting in their electrical charging is treated. Polyethylene surfaces were exposed to the cold radiofrequency air plasma for different intervals of time. The change in the wettability of these surfaces was registered. The experimentally established characteristic time scales of the interaction of cold plasma with polymer surfaces are inversely proportional to the concentration of ions. The phenomenological kinetic model of the electrical charging of polymer surfaces by plasmas is introduced and analyzed.

Plasma treatment switches the regime of wetting and floating of pepper seeds

Cold radiofrequency plasma treatment modified wetting and floating regimes of pepper seeds. The wetting regime of plasma-treated seeds was switched from the Wenzel-like partial wetting to the complete wetting. No hydrophobic recovery following the plasma treatment was registered. Environmental scanning electron microscopy of the fine structure of the (three-phase) triple line observed with virgin and plasma-treated seeds is reported. Plasma treatment promoted rapid sinking of pepper seeds placed on the water/air interface. Plasma treatment did not influence the surface topography of pepper seeds, while charged them electrically. Electrostatic repulsion of floating plasma-treated seeds was observed. The surface charge density was estimated from the data extracted from floating of charged seeds and independently with the electrostatic pendulum as σ≈1-2μC/m2.

Hydrophilization and hydrophobic recovery in polymers obtained by casting of polymer solutions on water surface.

We demonstrate the possibility of hydrophilization of polymer films in situ under the process of their preparation. The polymer surface is hydrophilized when the polymer solution is spread on the water surface and the solvent is evaporated. Essential hydrophilization of the polymer surface is achieved under this process. We relate the observed hydrophilization of polymer films to the dipole-dipole interaction of the polar moieties of polymer chains with highly polar water molecules. The dipole-dipole interaction between water molecules and polar groups of polymer chains, orienting the polar groups of a polymer, may prevail over the London dispersion forces. The process, reported in the paper, allows to manufacture the films in which the hydrophilic moieties of the polymer molecule are oriented toward the polymer/air interface. It is demonstrated that even such traditionally extremely hydrophobic polymers as polydimethylsiloxane can be markedly hydrophilized. This hydrophilization, however, does not persist forever. After removal from the water surface, hydrophobic recovery was observed, i.e. polymer films restored their hydrophobicity with time. The characteristic time of the hydrophobic recovery is on the order of magnitude of hours.

Superposition of Translational and Rotational Motions under Self-Propulsion of Liquid Marbles Filled by Aqueous Solutions of Camphor

Self-locomotion of liquid marbles, coated with lycopodium or fumed fluorosilica powder, filled with a saturated aqueous solution of camphor and placed on a water/vapor interface is reported. Self-propelled marbles demonstrated a complicated motion, representing a superposition of translational and rotational motions. Oscillations of the velocity of the center of mass and the angular velocity of marbles, occurring in the antiphase, were registered and explained qualitatively. Self-propulsion occurs because of the Marangoni solutocapillary flow inspired by the adsorption of camphor (evaporated from the liquid marble) by the water surface. Scaling laws describing translational and rotational motions are proposed and checked. The rotational motion of marbles arises from the asymmetry of the field of the Marangoni stresses because of the adsorption of camphor evaporated from marbles.

Probing properties of cold radiofrequency plasma with polymer probe

The probe intended for the characterization of cold plasma is introduced. The probe allows estimation of the Debye length of the cold plasma. The probe is based on the pronounced modification of surface properties (wettability) of polymer films by cold plasmas. The probe was tested with the cold radiofrequency inductive air plasma discharge. The Debye length and the concentration of charge carriers were estimated for various gas pressures. The reported results coincide reasonably with the corresponding values established by other methods. The probe makes possible measurement of characteristics of cold plasmas in closed chambers.

Biofilm formation on agricultural waste pretreated with cold low-pressure nitrogen plasma and corona plasma discharges

Agricultural waste (AW) was pretreated with cold low-pressure nitrogen plasma (LPD) and corona atmospheric plasma discharges (CAPD), in an attempt to increase the bacterial attachment and biofilm formation. Biofilm formation was examined in the presence of exogenously added P. putida and B. cereus as well as in a sterile medium where only the indigenous bacteria which grow naturally on the wood surface could form biofilm. The exposure of AW to (LPD) led to a 3.5-fold increase in biofilm formation of the exogenously add P. putida F1 in MMT (minimal medium supplied with toluene) and a 1.6-fold increase in MMG (minimal medium supplied with glucose) compared to the untreated AW. The increase in biofilm formation was also observed with the exogenously added B. cereus or with indigenous bacteria that grow naturally on the AW. The effect of the CAPD on biofilm formation was weak. SEM analysis of the LPD-treated AW showed an increase in surface roughness, which we assume is one of the reasons for the enhancement of the biofilm formation. The apparent contact angle of a sessile drop on the surface of LPD-treated AW as well as on the bacterial layer showed their hydrophilic nature. In conclusion, the increase in biofilm formation of the exogenously added P. putida or B. cereus was due to the LPD treatment. Importance To the best of our knowledge, this is the first study to describe the effect of wood plasma treatment on biofilm formation. This technology can be further implemented for bioremediation of contaminated soils.

Surface charging by the cold plasma discharge of lentil and pepper seeds in comparison with polymers

Cold plasma treatment charges organic surfaces. Surface density of the electrical charge supplied by cold radiofrequency plasma to the pepper and lentil seeds and various polymers: polyethylene, polystyrene, polycarbonate and poly(methyl methacrylate) was established experimentally. The surface charge density for all of studied interfaces, with one exception of polycarbonate was estimated as σ≈0.75-3.7μC/m2; for the polycarbonate it was almost an order of magnitude larger, namely σ≈7-9μC/m2. The kinetics of the surface charge leakage was studied. The characteristic time of temporal change in the surface density of the electrical charge gained by the seeds is markedly larger than that of synthetic polymers.