Positive and negative DC glow discharges: a comparative study to characterize self-organized patterns on water surface

Self-organized patterns (SOP) in plasma discharges arise from the complex interplay of electric field, reactive species and charged particles, driven by non-linear plasma dynamics. While studies have explored SOP formation in various configurations, no systematic comparison of positive and negative DC glow discharges has been conducted to explain why SOP form exclusively when polarization is negative. This study aims to analyze SOP formation mechanisms by comparing the electrical, optical and spectral properties of positive and negative DC glow discharges interacting with a grounded water surface. Key differences in gas temperature, electric field and reactive species distribution are hence identified. For positive DC glow discharges (PGD), the gas temperature remains in the 350-370 K range, while the reduced electric field remains below 100 Td across the gap. The plasma is dominated by OH• and N₂* species, whose excitation results from direct electron impact and energy transfer in a low-field environment. The absence of strong ionization and electric field gradients leads to a spatially homogeneous emission layer on the liquid surface, resulting in a circular uniform plasma (CUP) pattern without self-organization. In contrast, SOP emerge exclusively under negative DC glow discharges (NGD) at currents above 15 mA. These discharges are characterized by a non-linear reduced electric field, peaking at 485 Td at 1mm from the cathode pin, dropping below 100 Td in the central gap and rising to 460 Td near the water surface. There, the plasma layer still contains OH• and N2* species but also N2+ ions, the latter being critical for SOP formation. SOP morphology evolves with gap size: at 7 mm, patterns transition from specks to filaments, with pattern diameters and thickness as high as 5.5 mm and 210 μm respectively. Lowering water surface tension with surfactants reduces SOP size and modifies pattern morphology. Our results deepen understanding of plasma self-organization mechanisms, particularly the role of polarity and liquid surface dynamics等离子体放电中的自组织图案（Self-organized patterns, SOP）源于电场、活性物种与带电粒子间的复杂相互作用，由非线性等离子体动力学驱动。尽管已有研究在多种放电构型中探索了SOP的形成过程，但目前尚未有针对正、负直流辉光放电的系统性对比研究，以解释为何仅当电极极化呈负极性时才会形成自组织图案。本研究旨在通过对比与接地水面相互作用的正、负直流辉光放电的电学、光学及光谱特性，解析SOP的形成机制，由此明确了气体温度、电场与活性物种分布的关键差异。对于正直流辉光放电（Positive DC glow discharges, PGD），其气体温度维持在350-370 K区间，且放电间隙内的约化电场始终低于100 Td。该等离子体以OH•与N₂*物种为主，这些物种的激发源于低场环境中的直接电子碰撞与能量转移过程。由于不存在强电离与电场梯度，液体表面形成了空间均匀的发射层，最终呈现出无自组织现象的圆形均匀等离子体（circular uniform plasma, CUP）图案。与之形成鲜明对比的是，负直流辉光放电（Negative DC glow discharges, NGD）仅在电流高于15 mA时才会产生自组织图案。此类放电具有非线性约化电场分布：在距阴极针1 mm处峰值可达485 Td，在放电间隙中部降至100 Td以下，而在靠近水面处回升至460 Td。此时等离子体层中除OH•与N₂*物种外，还存在N₂+离子，后者是驱动SOP形成的关键因素。SOP的形貌随放电间隙尺寸发生演化：当间隙为7 mm时，图案从斑点状过渡为丝状，其直径与厚度分别可达5.5 mm与210 μm。通过表面活性剂降低水面表面张力，可减小SOP的尺寸并改变其形貌。本研究结果深化了对等离子体自组织机制的理解，尤其是电极极性与液面动力学的作用。