Description
Dual-frequency atmospheric pressure plasma jets (APPJs) represent a promising method to manipulate the ion transportation and the plasma propagation on a surface [1]. Despite the potential shown by these unique devices, the fundamental mechanisms through which such systems interact with a liquid substrate and govern the induced liquid flow have not been investigated yet. In this work, we implemented a coaxial dual-frequency configuration employing an upstream high-frequency electrode (HF) at 17kHz and a downstream radio-frequency (RF) electrode at 27.12 MHz [1].
Within this configuration, the plasma discharge is mainly sustained by the RF Ω-mode ionisation regime at twice the RF oscillation frequency (54 MHz). Concomitantly, the HF high-voltage electrode modulates the thickness of the sheath at the liquid interface, triggering the RF γ-regime through secondary electron emission. Time-resolved measurements of the electrode voltage, surface charge on the substrate and optical emission above the liquid substrate demonstrated the transition between the two RF ionisation modes.
To quantify the electrohydrodynamic forces (EHD) within the liquid phase, Particle image velocimetry (PIV) was performed using graphene oxide (GO) as tracers in solutions of varying conductivity.
A significant finding in our work has been revealed by comparing different operational modes. Pulsed HF signal combined with continuous RF excitation led to a discharge with electrical features dominated by the HF component, while the upward induced flow observed is consistently similar to the RF-only operation. These flows remain lower compared to the non-pulsed dual-frequency condition. Moreover, evaporation rate measurements exhibit a strong correlation with the applied RF power, opening possibilities for a precise flow control by reducing the applied RF power combined with micro- or nanosecond pulse generation techniques.
[1] Patelli A, Scaltriti S G, Popoli A, Martines E and Cristofolini A 2025 Plasma-substrate interaction in a dual frequency APPJ Plasma Sources Sci. Technol. 34 025010